Large-scale stratigraphic architecture and sequence analysis of an early_甜梦文库
Large-scale stratigraphic architecture and sequence analysis of an early
Int J Earth Sci (Geol Rundsch) (3C875 DOI 10.-013-0984-3Original PaperLarge-scale stratigraphic architecture and?sequence analysis of?an early Pleistocene submarine canyon fill, Monte Ascensione succession (Peri-Adriatic basin, eastern central Italy)Claudio?Di?Celma?? Riccardo?Teloni?? Andrea?Rustichelli?Received: 18 December 2012 / Accepted: 11 November 2013 / Published online: 11 December 2013 ? Springer-Verlag Berlin Heidelberg 2013Abstract? The Monte Ascensione succession (c. 2.65C 2.1?Ma) is a well-exposed example of an exhumed submarine canyon fill embedded within slope hemipelagic mudstones. This gorge represented a long-lasting pathway for sediment transport and deposition and during the Gelasian delivered Apennine-derived clastic sediment to the adjacent Peri-Adriatic basin. A total of six principal lithofacies types, representing both canyon-confining hemipelagic deposits and canyon-filling turbidity current and masstransport deposits, can be delineated in the studied sedimentary succession. The canyon-fill deposits display a marked cyclic character and the component lithofacies succeed one another to form at least fifteen fining-upward stratal units, which are interpreted to represent high-frequency, unconformity-bounded depositional sequences. Variability in the vertical repetition of constituent lithofacies allows the identification of three basic styles of sequence architecture that can be interpreted in terms of differing positions along a conceptual down-canyon depositional profile. An integrated chronology, based on biostratigraphic data and on palaeomagnetic polarity measurements, strongly supports a one-to-one correlation between the sequence-bounding surfaces and oxygen isotope stages G2C78, suggesting that the most feasible sequence-engendering mechanism is that of orbitally dictated glacio-eustatic changes in sea level, which regulated timing of sediment storage on the shelfC.?Di?Celma?(*)?? R.?Teloni?? A.?Rustichelli? School of?Science and?Technology, University of?Camerino, Camerino, Italy e-mail: claudio.dicelma@unicam.it Present Address: R.?Teloni? School of?Earth and?Environment, University of?Leeds, Leeds, UKand its redistribution beyond the shelf edge. One of the most significant aspects of this study is the demonstration that processes occurring within upper slope canyons can be expected to be strongly influenced by var that is, the erosional and depositional features evident in these deposits can be strongly controlled by allocyclic processes rather than autocyclic or random processes. Keywords? Peri-Adriatic basin?? Gelasian?? Sedimentary architecture?? Turbidity currents?? Submarine channellevees?? Depositional hierarchy?? Deep-water sequence stratigraphyIntroduction Submarine canyons and deep-water slope valleys of varying size are common features in both modern-day and ancient continental margins and are known to be volumetrically significant in trapping and moving coarse-grained terrigenous sediments from nearshore and shelf environments to deeper parts of the adjacent basin (e.g., Shepard 1981; May et?al. 1983). As such, they may form important repositories for reservoir-quality sediments and have proven to be viable exploration targets in several hydrocarbon provinces (e.g., McHargue and Webb 1986; Bruhn and Walker 1995; Samuel et?al. 2003; Mayall et?al. 2006; Porter et?al. 2006). However, while it is clear from high-resolution near surface seismic investigations that the fill of these features would have a high level of stratigraphic complexity (e.g., Deptuck et?al. 2007; Gong et?al. 2011), many heterogeneities are often below the resolution of seismic and well data. To reduce uncertainty associated with these resolution issues, outcropping deep-water depositional systems represent an important source of the fine-scale sedimentologic,13
844Int J Earth Sci (Geol Rundsch) (3C875stratigraphic, and structural data and their comprehensive analysis has proved essential for constraining the reservoir compartmentalization and vertical connectivity commonly encountered within subsurface deep-water systems (e.g., Morris and Busby-Spera 1988; Cronin and Kidd 1998; Cronin et?al. 2005; Satur et?al. 2005; Anderson et?al. 2006; Dykstra and Kneller 2007; Kane et?al. ; Hodgson et?al. 2011; Brunt et?al. 2013a, b). The early Pleistocene (Gelasian), mud-rich clastic succession of the Peri-Adriatic basin crops out extensively along the foothills of the central Apennines and is punctuated by a number of large, coarse-grained submarine canyon-fill successions that tend to be highly variable instratal geometry, size, and sedimentary architecture. These slope systems served as primary conduits for sediment gravity flows into the main axial trough of a piggy-back basin and their fills account for only a part of the significant quantities of sediment that were mobilized from the shelf and transported into the adjacent basin (Fig.?1). The most prominent of these ancient slope systems are exposed at Monte Ascensione (the focus of this paper), Castignano (Di Celma et?al. 2013), Offida (Di Celma 2011), and Colle Montarone (Di Celma et?al. 2010a) and represent outcrop analogues for age-equivalent slope systems described in 3D seismic datasets of the Central Adriatic Basin by Dalla Valle et?al. (2013) and Tinterri and Lipparini (2013).Fig.?1??Generalized tectonic map of eastern central Italy, showing the location of the study area. Cross-section and unconformity-bounded units from Artoni (2007)13Int J Earth Sci (Geol Rundsch) (3C875 845These successions present an ideal opportunity to undertake detailed sedimentological and stratigraphic analysis of the canyon-fill strata at a scale comparable to seismic examples and make the Peri-Adriatic basin one of the few places in the world where it is possible to examine, from an outcrop perspective, how this type of deep-water depositional system evolves and fills during periods of highamplitude, high-frequency sea-level fluctuations. Aside from the overall good outcrop conditions and virtual lack of structural complications, additional remarkable aspects of these coarse-grained successions are the availability of a relatively high-resolution chronostratigraphic framework and independently derived proxies for both climatic (e.g., Bertini 2010) and eustatic sea-level changes (Lisiecki and Raymo 2005) during deposition. The purposes of this paper are threefold: (1) to provide a detailed outcrop analysis of the various lithofacies within the Monte Ascensione turbidite system and trace their distribution and (2) to set the stratigraphic cyclicity evident in the architecture of the Monte Ascensione succession within a sequence str and finally (3) to appraise the possible allogenic controls (tectonics, sediment supply, and glacioeustasy) on cycle development. For sake of clarity, it is important to stress that in this paper the position of the PlioceneCPleistocene boundary follows the recent IUGS ratification of the formerly Pliocene Gelasian Stage (2.588C1.806?Ma) as the oldest stage of the Quaternary system and Pleistocene series (Gibbard et?al. 2010).Geological and?stratigraphic setting The Apennines thrust-and-fold belt and associated foreland basin system (Ricci Lucchi 1986) developed since the late Oligocene in response to the convergence of the European and Adria-African plates (e.g., Malinverno and Ryan 1986; Royden et?al. 1987; Doglioni 1993). The retreating foreland ramp hinge led an eastward shift of the foredeep depocentres and this migration is documented by the temporal distribution of the foredeep clastic wedges, which are progressively younger toward the Adriatic foreland (e.g., Ricci Lucchi 1986). Detailed reconstructions of the tectonic and stratigraphic evolution of the late MiocenePleistocene Central Apenninic foreland basin system in the Marche-Abruzzi sector is provided by Artoni (2013) and Bigi et?al. (2013). This portion of the basin (Fig.?1) assumed its present-day configuration during the late Pliocene and early Pleistocene when, as a result of the progressive propagation of the thrust fronts towards the east, the previous foredeep was fragmented into discrete piggyback basins limited on both sides by active thrust fronts(Ori et?al. 1991). The Peri-Adriatic basin, therefore, hosts the most peripheral extension of the Apennines thrust-andfold belt, which comprises four main systems of easterly directed thrust sheets and related folds striking northCsouth all along the study area. From the inner to the more external (i.e., from west to east), they are the M. Acuto-Montagna dei Fiori (AF), the Roccafinadamo-Strada (RS), the Jesi-Nereto-Zaccheo (JNZ), and the Campomare-Tortoreto (CT) thrust fronts (Artoni 2013). On the whole, these structures appear to propagate in a forward-breaking sequence and, therefore, are increasingly younger from west to east. The most internal structure, the AF thrust front, was mainly active during the Messinian and is now exposed along the Apenninic margin. The other three, RS, JNZ, and CT, are subsurface structures mostly buried beneath Pleistocene cover (Ori et?al. 1991), and were mainly active during the Zanclean, late Zanclean to early Piacenzian, and Gelasian times, respectively (Tozer et?al. 2006; Artoni 2013). Owing to such a complicated tectonic deformation history, the infill of the Peri-Adriatic basin is irregularly punctuated by a series of major unconformities that mark abrupt changes in basin geometry and subsidence regime and define basinwide tectono-stratigraphic units (Ricci Lucchi 1986; Artoni 2007). Northeastward tilting and emergence of the basin fill took place mostly over the last 1.5 My, when the study area was uplifted at an average rate of 0.8C1.0?m ky?1 (Centamore and Nisio 2003; Pizzi 2003). During this phase, the youngest (late Calabrian) portion of the basin fill succession recorded an overall upward-shallowing trend from slope, through shelf, to coastal and alluvial deposits. These sediments have prograded from the western margin basinward and their long-term regressive trend and conspicuous cyclothemic organization were mostly driven by the composite effect of regional uplift and high-frequency glacioeustatic fluctuations of sea level (Cantalamessa and Di Celma 2004).Study area and?field methods The Monte Ascensione strata are exposed over an area of approximately 45?km2 that extend from the Monte Ascensione to the south, where their eroded up-system extent projects into the air, and the village of Montelparo in the north, beyond which the exposure is lost in the subsurface. The studied succession accumulated within a narrow piggyback basin that was bounded by thrust anticlines both to the west and to the east. At the current level of exposure, it consists of a south to north elongated body of conglomerates, sandstones and mudstones, which is approximately 2.5C3?km wide, at least 12?km long in a down-dip direction, and exhibits a cumulative vertical thickness of more than 1,000?m. The canyon fill displays a complex stratigraphic13
846Int J Earth Sci (Geol Rundsch) (3C875architecture that has been detailed through the use of a comprehensive outcrop-derived dataset, including geological mapping at 1:10,000 scale, line drawing over photographic panels where continuously vertical cliffs prevented ground inspection, extensive collection of paleocurrent readings and measurement of vertical stratigraphic sections. These field observations were supplemented by examination of numerous unweathered samples collected for micropaleontological analyses (foraminifera and calcareous nannoplankton). On average, the strata strike between 110 and 140° dipping 10C20° to the northeast as part of a regional monocline, and paleocurrent measurements indicate paleoflow to the north. A key aspect of the study area is that the sedimentary succession is cut by a number of eastCwest-trending river valleys, which provide a series of depositional strike-oriented stratigraphic cross sections that are distributed along different positions of the slope depositional profile and that, from south to north (i.e., from proximal to distal), display progressively younger portions of the turbidite succession.throughout the study area. Multiple and genetically related third-order elements stack into fining-upward, fourth-order architectural elements separated by sharp, canyon-wide contacts. Fourth-order architectural elements cluster to form a composite canyon fill succession (fifth-order element) wholly confined within a large-scale erosional surface having a few hundred metres of topographic relief. Our discussion of the architecture the Monte Ascensione system is based primarily on the more easily recognizable and mappable third- and higher-order surfaces and lithosomes.Lithofacies and?third?order depositional elements A detailed characterization and sedimentological interpretation of dominant lithologies, physical sedimentary structures, bed thickness, and bedding pattern of this composite deep-water succession indicated that an array of subaqueous sediment density flows were responsible for the transport and emplacement of a spectrum of both coarseand fine-grained sediments. Fill within the canyon can be described in terms of five major lithofacies (generally thicker than 5?m), or third-order elements: (1) clast-su (2) amalgamated medium- to t (3) medium- to thick- (4) medium- to very thin-bedded sa and (5) pebbly mudstone and chaotic beds. An additional lithofacies, composed of massive mudstone forms the canyon-confining succession. The descriptive lithofacies scheme, designed to take into account the principal features of the lithologies encountered in the studied succession, is summarized in Table?1 and provided below. To include a greater variety of lithofacies, this scheme is a somewhat expanded version of that initially defined by Cantalamessa et?al. (2009) for the most proximal portion of the Monte Ascensione system. Lithofacies A (LF?A): clast?supported conglomerate Description This lithofacies occurs as internally complex conglomerate bodies that may exceed 50?m in thickness (Fig.?2a). Each sedimentary body consists primarily of medium- to thickbedded, clast-supported conglomerate that is composed of well-rounded, granule- to cobble-sized clasts and interbedded with a minor proportion of thin lenticular sandstone beds and pebbly mudstone. Clast composition reflects a heterogeneous mixture of rock types derived from unroofing of the adjacent Apennines, including Mesozoic and Tertiary carbonate sediments and Messinian siliciclastic sediments (Centamore and Deiana 1986). Clast sorting is generallyHierarchical stratigraphic framework A striking feature of most deep-water successions is their regular, multi-scale cyclic sedimentation pattern that can be systematically described within a hierarchical framework comprising both bounding surfaces and the stratigraphic units they define (e.g., Cronin and Kidd 1998; Beaubouef et?al. 1999; Crane and Lowe 2008; Prélat et?al. 2009; Flint et?al. 2011). To facilitate description, interpretation and organization of the preserved rock record, to assist accurate comparison of processes and stratal patterns across different temporal and spatial scales, and to promote more accurate prediction of reservoir properties, a variety of hierarchical schemes have been developed for the analysis of deep-water clastic systems (e.g., Gardner et?al. 2003; Sprague et?al. 2005; Deptuck et?al. 2008; Prélat et?al. 2009; McHargue et?al. 2011). For the purposes of this study, the outcrop-derived hierarchical ordering scheme proposed by Hickson and Lowe (2002) and Lowe (2004) has proved to be a successful method by which to organize the stratigraphy of the Monte Ascensione canyon-fill succession. Following this approach, the succession is divisible into stratal units that are bundled in a five-tiered hierarchy of bounding surfaces and intervening lithosomes. Secondand first order elements are represented by the sedimentation unit produced by a single gravity-flow depositional event and its component sedimentary structure divisions, respectively. Third-order architectural elements, or lithofa? cies, are comprised of stacks of similar sedimentation units and, typically, correspond to the smallest lithosomes that can be mapped with a relatively high degree of confidence13Table?1??Sedimentologic attributes of lithofacies identified in the Monte Ascensione study areaDescription MA-IV1 From 50 to 15 Up to 45 Up to 45 50 At least 35 7 15 12 Up to 15 MA-IV2 MA-IV3 MA-IV4 MA-IV5 MA-IV6 MA-IV8 MA-IV9 MA-IV13 Poorly sorted, clast-supported, granuleto cobble-size, medium- to thickbedded conglomerates in a coarse sandy matrix variably interbedded with a significantly minor proportion of thin lenticular sandstone beds and pebbly mudstones. Clasts are well-rounded and extrabasinal. Internally, beds are mostly massive or with subordinate inverse grading. The prevalent bedding is tabular, lenticular or, to a lesser extent, cross-stratified. Extensive channelling and scouring have produced complex cross-cutting relationships. The palaeocurrent data indicate a broad dispersion about the mean direction at individual localities MA-IV6 MA-IV7 MA-IV8 MA-IV9 MA-IV11 MA-IV12 MA-IV14 36 35 23 35 60 41 9 This lithofacies consists of medium- to coarse-grained sandstones, and subsidiary pebbly sandstones, mudstoneclast breccia, and lenses of pebble and cobble conglomerate. Sandstone beds are medium- to thick-bedded and display abundant basal scours and amalgamation surfaces. Mudstone drapes between sandstone beds are rare to absent. Internally, sandstone sedimentation units are typically ungraded or crudely normally graded and structureless, preserving only dewatering structures. Some beds show faint to well-developed parallel lamination near the top. Lateral continuity of beds is highly variable and controlled by the presence of erosional cuts. The bedding architecture between succeeding erosional surfaces displays an overall thinning-upward trend Thick-bedded, amalgamated, massive to normally-graded, sandstone beds rich in dewatering structures are commonly interpreted as the result of rapid suspension deposition by collapsing, highly sedimentcharged turbulent flows (S3). The plane-parallel lamination division (Tb) forms via repeated collapse of traction carpets at lower sediment fall out rates. The lenticular mudstone-clast breccia and pebble to cobble conglomerates are bypass lags built out of clasts left behind by through-going, high- density gravity flows Massive and normally graded conglomerate depositional units (R3) suggest that most of the clasts were deposited directly from through sespension sedimentation beneath channelized, gravelly, high-concentration turbidity currents. Sets of large-scale, cross-stratified conglomerate beds (R1) indicate significant bed-load transport of gravel- and cobble-forming bars in the channels and were deposited by largely bypassing turbulent flows Interpretation Occurrence Thickness (m) Depositional element Conglomerate-dominated channelcomplex: The poorly sorted but clastsupported texture of the conglomerates, their high degree of channeling, the multiple truncation surfaces, the variability and noticeable dispersion in the prevailing palaeocurrent directions, all point to suggest that this lithofacies relates to a very active, deep-marine braid plain system characterized by a dense network of shifting, multiple-thread, relatively short-lived channels within a submarine channel beltLithofacies codeLithologyInt J Earth Sci (Geol Rundsch) (3C875 LF-AClast-supported conglomeratesLF-BMedium- to thick-bedded amalgamated sandstonesSandstone-dominated channel complex: Based on the complex internal organization, each of these sandstone bodies consists of the remnants of a series of erosive-based, laterally stacked turbidite channel fills (i.e., they are sand-prone channel complexes). This type of sedimentary architecture records multiple episodes of channel incision, bypass, and filling that, in turn, can be interpreted as the product of repeated cycles of increasing than decreasing flow energy13847Table?1??continuedDescription MA-IV10 50 30 MA-IV15 Medium- to thick-bedded, fine- to very fine-grained, massive to subtly graded sandstones. Some thicker beds are accompanied by plane-parallel laminations in the uppermost parts. Individual sandstone beds are tabular and separated by thin packages of very thin-bedded siltstone and mudstone Sediments of this lithofacies are regarded to reflect rapid suspension sedimentation by collapsing, sand-rich, high-density flows (S3). The plane-parallel lamination division (Tb) forms via repeated collapse of traction carpets at slightly lower suspended-load sedimentation rates. Very thin-bedded siltstone and mudstone (Td and Te divisions) represent deposition from the less energetic, low-density turbidity currents MA-IV2 55 More than 15 As much as 35 50 54 77 100 MA-IV11 MA-IV12 MA-IV13 MA-IV14 47 82 35 37 MA-IV3 MA-IV5 MA-IV7 MA-IV8 MA-IV9 MA-IV10 Interpretation Occurrence Thickness (m) Depositional element Frontal splay: The apparent absence of channelization, the overall tabular nature of the sandstone beds, characterized by absence of intense scouring at the base and preservation of intervening mudstone intervals, and some evidence for flow collapse and mass-dumping of the high-density loads of the flows, suggests that this lithofacies may have been deposited by rapidly expanding flows in a relatively unconfined setting, such as downstream of the mouth of a leveed channel as part of a frontal splay (Internal) Levee-overbank: This lithofacies is interpreted to represent levee-overbank deposits that were emplaced by decelerating, moderate- to low-concentration turbidity flows that spilled out of nearby channels. The channel levee interpretation is consistent with sedimentary processes dominated by traction, the occurrence of laterally adjacent channel fills, and the well-defined fining- and thinning-upward character of these sediments. Within levee-overbank settings, the finely-laminated sandstones and siltstones are interpreted to have been created by pulses in the thickness and grain size composition of the overspilling flows that, in turn, may have been generated by the presence of internal waves within the turbidity currents transiting the channels MA-IV1 MA-IV2 MA-IV3 MA-IV4 30 From 15 to 5 From 10 to 5 From 10 to 5 Mass-transport deposit: Similar chaotic packages are commonly referred to as mass-transport deposits or mass-transport complexes. Based on the abundance of well-rounded extrabasinal clasts, these sediments are interpreted as resulting mostly from mass wasting of the shelf-edge staging area and downslope transport, with minor contribution from local failure of steep canyon walls
84813This lithofacies is made up of fining- and The thicker, structureless (Ta) and planar-parallel laminated (Tb) thinning-upward packages. At the base, sandstone beds represent deposithese packages may comprise tabular, tion from high-density currents. medium-bedded, normally graded The thin- to very thin-bedded, sandstone beds showing a structureless ripple laminated (Tc) sandstones division at base and planar-parallel alternating with mudstone layers lamination on top intercalated with (Te or hemipelagic background packets of thinly interbedded ripplesedimentation) record deposition laminated sandstones and massive from depletive, low- concentramudstones. In the upper part, the tion turbidity currents fining- and thinning-upward packages include thin- to very thin-bedded, ripple laminated, fine-grained sandstones alternating with structureless mudstone layers Folded and distorted thin-bedded mudstones variably interbedded with a poorly sorted mixture of pebble- to cobble-size exstrabasinal clasts floating in a muddy matrix. Typically the folds are tight to isoclinal and may have upright axial planes Deposits from sediment slumps and cohesive debris flowsLithofacies codeLithologyLF-CMedium- to thick-bedded sandstonesLF-DMedium- to very thinbedded sandstones and mudstonesInt J Earth Sci (Geol Rundsch) (3C875LF-EPebbly mudstones and chaotic bedsInt J Earth Sci (Geol Rundsch) (3C875 Slope background hemipelagic deposition849Pale blue-gray, massive or faintly bedded mudstones rich in benthic and planktonic microfauna (foraminifera and nannofossils) and punctuated by rare intercalations of thin, very finegrained sandstone bedspoor, whereas shapes may vary from platy to nearly spherical to elongate. Whole and broken mollusc shells are present locally. Stratification displays a fairly complex lenticularity at scales of tens of metres to a fraction of a metre, with abundant erosional features ranging from simple shallow scours to more complex and laterally cross-cutting surfaces that define concave-upward channel forms (Fig.?2b, c). Zones of metre-scale cross-stratification are common. Single beds of the horizontally stratified conglomerates range up to 8?m thick, do not extend laterally over distances greater than a few tens of metres, and show predominantly a framework-supported texture with a medium- to coarsegrained sandy matrix. In most of the cases, these beds display massive, ungraded to normally graded clast fabrics. A weak upcurrent-dipping clast imbrication is fairly common in the conglomerate beds, despite the general dominance of rod and spheroid-shaped clasts. Intervening sandstones are medium- to fine-grained, structureless or faintly parallel laminated and occur as either individual beds less than a metre thick or couplets associated with subjacent conglomerate. Pebbly mudstone intervals, usually up to 1?m thick and a few ten metres in lateral extent, occur sporadically as discontinuous lenses in the horizontally stratified conglomerates and overall are a relatively minor component of these deposits. Planar cross-stratified conglomerates are present in this lithofacies at some localities and constitute a volumetrically small, yet sedimentologically significant component of the clast-supported conglomerates. They occur in sets up to 1.5?m thick and foresets are well defined by remarkable changes in grain-size and/or by the long axis of platy clasts dipping in the flow direction and oriented parallel to the foreset (pseudo-imbrication). In general, individual foresets range from 10 to 25?cm in thickness, dip at up ~20°, and exhibit significant decrease in grain size obliquely upward. The palaeocurrent data for Lithofacies A are mostly derived from clast imbrication and trends of the channel margins and indicate a broad dispersion about the mean direction at individual localities. Sedimentological processes and?depositional element Massive and normally graded conglomerate depositional units, classified as R3 divisions after Lowe (1982), are interpreted to have accumulated directly from the suspended load of very energetic, high-density turbidity flows carrying a mixed load of mud, sand, and gravel. These flows were partitioned into a highly concentrated gravelly dispersion near the base and an overriding, less concentrated and turbulent suspension that bypassed finer-grained detritus farther down system. Clast composition implies an updip, direct shelfal input of gravels, whereas the presence of shallow-marine molluscs suggests that prior to final deposition and burial inOccurrence Description InterpretationThickness (m)Depositional elementTable?1??continuedLithofacies codeLF-FMassive mudstonesLithologyThis lithofacies indicates slow deposition from suspension fallout of hemipelagic particles and unusual sedimentation events by very dilute, waning, unconfined turbidity currentsCanyonencasing lithology13
850Int J Earth Sci (Geol Rundsch) (3C875Fig.?2??Photographs showing selected attributes of clast-supported conglomerates (LF-A). a Panoramic view of Monte Ascensione. Shown are the prominent cliff-forming conglomerate units (LF-A) at the base of the canyon fill interbedded with mass-transport deposits(recessive-weathering intervals). b Close-up of channelized conglomerates. A 30?cm long rock hammer (lower centre) is used for scale. c Large exposure of LF-A strata showing their complex internal organizationa slope environment these clasts resided for at least some time in the littoral zone. The downstream-dipping interpretation of the large-scale cross-stratified conglomerates is preferred here to that of lateral accretion packages produced on the inner bend of laterally migrating channels (e.g., Winn and Dott 1977; Dykstra and Kneller 2009; Kane et?al. 2009). This notion is substantiated by the dip directions of the cross-beds, which are approximately the same as those of paleocurrent directions determined from clast imbrications in the interstratified horizontally bedded conglomerates. The high-degree of channelization and multistorey character, the prominent dispersion of paleoflow direction, the apparent lack of lateral accretion strata, and some evidence for downstream-accreting bar forms indicate that the conglomerate bodies are not single channel fills, but rather represent braided-like channel belts characterized by a dense network of unstable, multiple-thread, low-sinuosity channels that were typically a few metres deep and some tens of metres wide. Ancient examples of very similar gravel-prone channel belts have been reported previously from outcrops of many different deep-water successions (e.g., Cavazzaand DeCelles 1993; Hickson and Lowe 2002; Satur et?al. 2005; Hubbard et?al. 2008; Kane et?al. 2009). Lithofacies B (LF?B): amalgamated medium? to?thick?bedded sandstone Description Lithofacies B forms packages that are up to 60?m thick and consists almost entirely of medium- to coarse-grained sandstones with subsidiary pebbly sandstone, discontinuous mudstone-clast breccia, and lenses of pebble and cobble conglomerate. Sandstone beds range from 0.5 to 3?m thick and display abundant basal scours and amalgamation surfaces, with sets of amalgamated beds forming massive units up to 5?m thick (Fig.?3a). Centimetre-scale mudstone drapes between sandstone beds are rare to absent. Internally, sandstone sedimentation units are typically ungraded or crudely normally graded and structureless, preserving only dewatering structures (Fig.?3b). Some beds show faint to well-developed parallel lamination near the top. Lateral continuity of beds is highly variable and controlled by the13Int J Earth Sci (Geol Rundsch) (3C875 851Fig.?3??Representative photographs of thick-bedded sandstones (LFB). a Oblique view of amalgamated sandstone beds. Hammer (encir? cled) for scale. b Blow-up of the boxed area shown at the lower left corner of d illustrating a soft-sediment deformation structure recording rapid loading and dewatering of the underlying sandstones. c Blow-up of the boxed area at the right-hand side of d showing an internal concave-upward erosional surface (dotted line) that truncates into older sandstone beds and is directly overlain by a conglomerate lag. d The composite nature of this lithofacies is proven by the presence of lenticular packages that are interpreted as erosional remnantsof channel fills. In this photo, the infill of individual turbidite channels displays a general thinning-upward trend. Rectangles represent the areas shown in detail in Fig.?3b, c. e Detail of the lower portion of a channel fill comprised of a mud-clast breccia at the base and an overlying clast-supported conglomerate lag, both interpreted as the deposit left behind from the flows that cut the channel and continued to transport sand-sized sediment farther downslope. The overlaying thick-bedded sandstones were deposited during the backfill of the channel. Hammer (encircled) for scalepresence of internal erosional cuts that are typically associated with lenses of poorly sorted gravel lags and mud-clast breccias (Fig.?3c). The mud-clast breccia layers consisttypically of interconnected, subangular mudstone clasts, surrounded by a coarse-grained sandstone matrix. The bedding architecture between succeeding erosional surfaces13
852Int J Earth Sci (Geol Rundsch) (3C875displays an overall thinning-upward trend (Fig.?3d) and the gravel lags associated with these surfaces form lenticular, clast-supported beds 0.2C1?m thick that thicken and thin laterally (Fig.?3e). Sedimentological processes and?depositional element Thick-bedded, often amalgamated, massive to normallygraded sandstone beds (S3 division of Lowe 1982) rich in dewatering structures are commonly attributed to very rapid rates of suspended sediment fallout from collapsing, high-density turbidity currents. The very thin mudstone layers that separate the sandstone beds represent turbidite Te divisions and are regarded to represent deposition from less energetic, low-density currents. Based on the complex internal organization, each package of this sandstone-rich lithofacies is interpreted to consist of the remnants of a series of erosionally bounded, laterally stacked turbidite channel fills (i.e., they are channel complexes) similar to those documented by Camacho et?al. (2002), Eschard et?al. (2003), Samuel et?al. (2003), Campion et?al. (2005), and Schwarz and Arnott (2007), among others. In this context, the gravel lags and the mud-clast breccias associated with erosional cuts are interpreted as material left behind in the deepest parts of turbidity-current channels by transiting flows and suggest an early stage of erosion and minor sedimentation. Lithofacies C (LF?C): medium? to?thick?bedded sandstoneSedimentological processes and?depositional element Medium- to thick-bedded, massive to normally-graded, often water-escape structured turbiditic sandstones (S3 division of Lowe 1982) and the overlying plane-parallel lamination division (Tb division of Bouma 1962) are regarded as to reflect rapid sedimentation from rapidly collapsing high-densit S3 intervals form when deposition rates are sufficiently high to suppress near-bed turbulence and the Tb division form via repeated collapse of traction carpets at sediment fall out rates (e.g., Leclair and Arnott 2005; Talling et?al. 2012 and references therein). Ripple cross-laminated intervals (Tc divisions) were deposited by low-density, fully turbulent turbidity currents (Talling et?al. 2012). Individual sandstone beds are separated by thin siltstone and mudstone interbeds comprised of turbidite Td and Te divisions deposited from lowdensity turbidity currents. The apparent absence of channelization, the overall tabular nature of the sedimentation units, and some evidence for flow collapse and mass-dumping of the high-density loads of the flows, suggests that this lithofacies may have been deposited by rapidly expanding flows in a relatively unconfined setting, such as downdip from the mouth of a leveed channel or as part of a frontal splay (Bernhardt et?al. 2011). Lithofacies D (LF?D): medium to?very thin?bedded sandstones and?mudstones DescriptionDescription This lithofacies consists primarily of medium- to thickbedded, non-amalgamated sandstone beds separated by recessive packages of very thin-bedded sandstone and mudstone that, typically, are less than 0.25?m thick (Fig.? 4a). Sandstone beds are fairly laterally continuous and commonly maintain a relatively uniform thickness across the outcrop. Internally, beds consist of wellsorted, medium- to fine-grained sandstone that, typically, is normally graded, they are structureless at their bases and plane-parallel laminated towards the top, where well-developed dewatering convolution of laminae and carbonate-cemented nodular concretions are occasionally present (Fig.?4b). Current-ripple lamination is present, but is not common. Some of the beds contain scattered granule- and pebble-sized mud clasts at their bases and high-concentrations of finely comminuted plant debris (leaves) within the laminated division (Fig.?4c). Lower bed contacts are planar and non-erosional, but a few sole marks, including groove and flute casts, are locally preserved (Fig.?4d). This lithofacies is subdivided into a lower sandier component and an upper finer-grained component that represent two end members of a vertical continuum of facies (Fig.? 5a). The stratigraphic thickness of the lower portion is highly variable, ranging from 2 to 12?m, and in most of the cases it forms a very minor part of this lithofacies. This lower interval is comprised mainly of thin- to medium-bedded tabular sandstone beds interbedded with centimetreto decimetre-thick packages of very thin-bedded siltstone and mudstone. Sandstone beds generally range from 10 to 30?cm thick although beds up to 75?cm thick are observed. They are medium- to fine-grained and laterally persistent across the width of the outcrop, with no discernible change in sediment texture or bed thickness. Their bases are generally flat and non-erosive, though some of the thickest beds exhibit a slightly erosive, stepped base (Fig.?5b). Characteristically, sandstone beds display normal grading and thin, slightly coarser, structureless divisions at their base commonly capped by plane-laminated divisions. Laminae are millimetre-scale thick and characterized by abundant concentrations of land-derived plant fragments (Fig.?5c,13Int J Earth Sci (Geol Rundsch) (3C875 853Fig.?4??Field photographs of medium- to thick-bedded sandstones (LF-C). A compass and a lens cap (6.5?cm in diameter) are used for scale. a Tabular sand-rich turbidite beds in interpreted frontal splay deposits. b Small-scale dewatering convolution of plane-parallel laminae. The resistant masses within the sandstone bed are carbonate-cemented nodular concretions. c Detail of the base of a sandstone bed showing scattered pebble-size, rounded to well-rounded mud clasts and land-derived plant fragments. d Groove casts on the base of a turbidite sandstone bed. Palaeoflow was into photographd). Current-ripple lamination is less common, whereas climbing ripples are virtually absent. At one location, a small sand-filled channel form cuts through these sediments (Fig.?5b). Sandstone beds decrease progressively in both abundance and thickness upward, culminating into an upper portion characterized by thinly interbedded siltstone and mudstone in various ratios and the intercalation of thin, laterally extensive, ripple-laminated, very fine- and finegrained sandstone beds (Fig.?5e). The ripple heights are typically 2C5?cm with wavelengths on average 20?cm. Sedimentological processes and?depositional element The predominance of parallel lamination and non-climbing current ripple-lamination (Tb and Tc Bouma divisions), the fine grain-size, and the thinly interbedded character ofthe stratigraphically higher deposits indicate that Lithofacies D reflects deposition from low-energy, low-density turbidity currents. The gradual thinning- and fining-upward trends suggest increasingly lower depositional energy and more dilute flows. Lack of climbing ripples indicates that fallout rate was never so high as to create a perceptible angle of climb. The abundance of fine plant material is evidence of a fresh-water, terrestrial influence. The structureless mud alternating with the sandstone beds is interpreted either to record hemipelagic background sedimentation or drape deposition from the dilute tail of the flow event (Te division). Sediments of this lithofacies are considered to form from protracted overbank deposition of relatively dilute, moderate- to low-concentration turbidity currents. This interpretation is consistent with sedimentary processes dominated13
854Int J Earth Sci (Geol Rundsch) (3C87513Int J Earth Sci (Geol Rundsch) (3C875 855?Fig.?5??Illustrations of medium- to very thin-bedded sandstones andmudstones (LF-D). a Large exposure showing the overall thinningand fining-upwards trend. Sandstone beds can be traced along the entire length of the exposure without appreciable irregularities or change in thickness. Centimetre- to decimetre-thick recessive packages are interbedded siltstones and mudstones. Geologists (encircled) for scale. b Shown in the upper left-hand side of the photograph is a channel sandstone body that is isolated within thin-bedded deposits and is interpreted as being the infill of a crevasse channel breaching the levee crest. The inset photo is a zoom-in of one of the thickest sandstone beds illustrating a detail of its stepped basal surface. The nearly vertical character of the steps, which are pointed by black arrows, suggests that the scour was formed by the removal of mudstone as rip-up clasts. Paleoflow was into photograph and this bed thins both to the left and to the right. A 30?cm long hammer is used for scale. c Parallel laminations emphasized by concentrations of finely comminuted carbonaceous material along the lamination planes. d When visible on bedding surfaces, the carbonaceous material consists of poorly sorted fragments of leaves that range in size from millimetres to centimetres. Due to current action, these fragments display a preferred orientation of their long axes. Coin for scale is 2.4?cm in diameter. e Thinly interbedded siltstone and mudstones in the upper portion of LF-D. The white arrows point to two slightly thicker, medium- to fine-grained sandstone beds sparsely distributed within this lithofaciescentimetre- to decimetre-scale mudstone clasts in a sandy matrix (Fig.?6b). The breccia contains varying amounts of muddy matrix, so that the fabric ranges from clast-supported to matrix-supported. The highly chaotic component shows remarkable soft-sediment deformation, generally comprising gentle upright and isoclinal recumbent folds, rotation, or incomplete disaggregation of a sediment mass (Fig.? 6c, d). Typically, this lithofacies is poorly exposed and forms vegetated recesses between ledges of conglomerates (LF-A) or medium to very thin-bedded heterolithic deposits (LF-D). Where exposed, the basal contact is sharp and shows little erosional relief, whereas the upper surface is truncated. Sedimentological processes and?depositional element The occurrence of extrabasinal clasts supported by cohesive clay matrix, plus both brittle (e.g., mud clasts) and plastic deformation (e.g., folds) of the original bedding, suggests that the deformed sediments originated from gravity-induced failure events and represent a process and facies continuum from slumps to fully homogenized cohesive debris flows (López-Gamundì 1993; Tripsanas et?al. 2008). Owing to their disorganized character and mudstonedominated composition, these deposits are better interpreted as mass-transport deposits, a general term used to indicate packages of remobilized, unconsolidated-to-semiconsolidated, mud-prone sediments that result from gravitational instability and mass failure of shelf-edge staging areas and/or canyon walls (e.g., López-Gamundì 1993; Posamentier 2003; Pickering and Corregidor 2005; Posamentier and Martinsen 2011). Lithofacies F (LF?F): massive mudstones Descriptionby traction (Walker 1985), the occurrence of laterally adjacent channel fills (LF-A), and the well-defined fining- and thinning-upward character of these sediments (e.g., Mutti 1977; Pickering 1982; King et?al. 1994; Kane et?al. 2007; Navarro et?al. 2007; Campion et?al. 2011), which can be either attributed to progressive deactivation of the feeding channel or to reduction in the spillover processes related to pronounced levee growth (Dennielou et?al. 2006). In such a context, the limited size of the sand-filled channel form cutting through these thin-bedded levees deposits suggest that this feature represents a crevasse channel. Owing to their location within the confines of a larger scale erosional fairway, the levees are comparable to the “internal levees” of Kane and Hodgson (2011). Lithofacies E (LF?E): pebbly mudstones and?chaotic beds Description This lithofacies, which occurs in units that are up to 30?m in thickness, consists of one or more of the following components: (1) mud-matrix sup (2) and (3) partially disaggregated chaotically bedded and distorted packages of sandstones. These components rapidly grade into each other both laterally and vertically, providing a pronounced internal complexity. Mud-matrix supported conglomerates display an unsorted fabric and are composed of varying concentrations of well-rounded, pebble- to cobble-sized extraformational clasts floating in a massive mudstone matrix (Fig.?6a). The breccia is characterized by poorly sorted, sub-rounded to angular,Lithofacies F comprises a monotonous succession of pale blue-gray, massive or faintly bedded mudstones. An abundant microfauna is composed mainly of skeletal parts of planktonic and benthic microfossils, including foraminifera and nannoplankton. Sedimentological processes and?depositional element This lithofacies, representing the bulk of the canyonencasing sedimentary succession, documents a low-energy marine environment of deposition within which the slow suspension fallout from waning silty and muddy clouds and settling of hemipelagic particles through the water column constituted the dominant background sedimentation processes.13
856Int J Earth Sci (Geol Rundsch) (3C875Fig.?6??Outcrop photographs showing the main sedimentological characteristics of the most common components of LF-E. A camera lens cap (6.5?cm in diameter) and a hammer (30?cm long are used for scale). a Close-up view of pebbly mudstones composed of wellrounded extrabasinal pebbles scattered throughout a muddy matrix. b Detail of the chaotic mixture of mud-matrix supported pebble conglomerate and angular mudstone clasts dispersed in a muddy matrix. c Deposit of gravelly mudstone, containing large deformed and par-tially disaggregated sandstone bed. d Panoramic view of mud-matrix supported conglomerates along with a contorted sandstone bed sandwiched between undeformed intervals. The irregular base of the chaotic package is interpret to result from active erosion during transport and suggests that it accumulated by plowing into the underlying sediment. Photographs in a and c reproduced from Cantalamessa et?al. (2009), by permission of SEPMLithofacies architecture of?fourth?order elements Field mapping of the Monte Ascensione outcrop belt reveals that the canyon-fill succession is characterized by vertical repetition of coarse-grained and fine-grained intervals, with each pair of one coarse-grained interval (including LF-A and/or LF-B and LF-C) and one fine-grained interval (LF-D and/or LF-E) forming an erosionally-based, fining-upward fourth-order element. Similar highly ordered sedimentation patterns have been found in many other outcrop studies of deep-water successions (e.g., Ito and Katsura 1993; Cronin and Kidd 1998; Beaubouef et?al. 1999; Hodgson et?al. 2006; King et?al. 2007; Di Celma et?al. 2011) and can be interpreted to represent the product of repeated cycles of increasing than decreasing flowenergy (e.g., Gardner et?al. 2008; McHargue et?al. 2011). In short, an ideal fourth-order element indicates the progression from: (1) an initial phase of canyon-wide erosion and sediment bypass by highly efficient, energetic turbidity currents characterizing the waxing portion of the sedimentation energy cycle (slope above grade); (2) a second phase, marking the beginning of the waning portion of the energy cycle, characterized by deposition of underfit leveed channel belts and frontal splays from high-densit and (3) a final phase, which documents deposition during most of the waning portion of the sedimentation energy cycle (slope below grade), comprising either mass-transport deposits or increasingly mud-rich sediments deposited by slope failure events and low-density turbidity currents, respectively.13Int J Earth Sci (Geol Rundsch) (3C875 857In the study succession, the fourth-order elements have been coded alphanumerically starting with an acronym for the slope system (“MA” for Monte Ascensione) followed by the Roman numeral IV, representing the hierarchical order, and a subscript indicating a particular cycle from the oldest (1) to the youngest (15). Althoughlithofacies stacking is well-ordered and repetitive and the fining-upward trend is regular and predictable through each fourth-order element, type and proportion of component lithofacies may change from cycle to cycle to form three distinct cyclic patterns, termed Motif-1, Motif-2, and Motif3 (Fig.?7), which almost certainly reflect different positionsFig.?7??Idealized lithologic logs contrasting the main features of Motif-1, Motif-2 and Motif-3 fourth-order elements (see text for further details). Dashed and dotted lines represent third- and fourth-order bounding surfaces, respectively13
858Int J Earth Sci (Geol Rundsch) (3C875along a conceptual down-canyon depositional profile. The Motif-1 architecture, well represented in the most proximal reaches of the exhumed canyon fill (MA-IV1 through MA-IV4), is a twofold-lithosome succession comprising a basal channel-levee complex (including LF-A and laterally adjacent LF-D) that is typically overlain by mudprone mass-transport deposits (LF-E). An ideal Motif-2 sequence (MA-IV5 through MA-IV9 and MA-IV11 through MA-IV14) usually includes a coarse-grained channel complex at the base (LF-A and/or LF-B) that is flanked on either side and overlain by thin-bedded deposits (LF-D). Motif-3 sequences (MA-IV10 and MA-IV15) are mostly encountered in the most distal outcrops of the turbidite succession and, generally, include sand-prone frontal splay deposits (LF-C) directly overlain by thinly bedded heterolithic deposits (LF-D). Minor departures from these typical internal organizations may occur due to omission of a certain lithofacies and, therefore, only some fourth-orderelements contain a full complement of depositional components. The boundaries between successive fourth-order elements are expressed by flat to gently concave-up erosional contacts across which a relatively coarse-grained interval is sharply juxtaposed on a relatively fine-grained interval (Fig.?8). Internal architectures of the Monte Ascensione fourthorder elements (Fig.?9) mimic those of other markedly cyclical canyon-fill successions of the Peri-Adriatic basin. In particular, the stratal packaging of the Monte Ascensione Motif-1 and Motif-2 fourth-order elements resembles the Motif-2 and Motif-1 fourth-order architectures documented at Colle Montarone (Di Celma et?al. 2010a), respectively. Similarly, the Monte Ascensione Motif-1fourth-order elements are very similar to those described at Offida by Di Celma (2011). Deepwater sequence motifs sharing characteristics with those described in this study, however, have been documentedFig.?8??Representative photographs of some outcrop expression of fourth-order bounding surfaces. These erosional contacts, across which there is an abrupt replacement of fine-grained deposits by coarse-grained deposits, are candidate sequence-bounding surfaces, whereby the loci of coarser-grained clastic deposition conceivably shift basinward during periods of relative sea level fall. a Concave upward contact separating thick massive sandstones (LF-B) from underlying thinly-bedded sandstones and mudstones (LF-D). b Pano-ramic view of an interpreted sequence boundary across which a gravelly mudstone, containing deformed and partially disaggregated sandstone beds, is truncated by an overlying conglomerate unit. c Sharp contact between pebbly mudstones (LF-E) of sequence MA-IV1 and the overlying thin-bedded turbidite sandstone and mudstone (LF-D) of sequence MA-IV2. d Thick-bedded sandstones (LF-B) resting on thinly-bedded sandstones and mudstones (LF-D) of the preceding cycle13Int J Earth Sci (Geol Rundsch) (3C875 859Fig.?9??Outcrop views towards the south of two-fourth-order architectural elements showing an overall fining-upward lithofacies organization. Dashed and dotted lines highlight third- and fourth-order bounding surfaces, respectively. See Fig.?11b for stratigraphic position of these two outcrops. a This fining-up succession consists of thin-bedded sandstone and mudstone (LF-D) at the base that are capped by mass-transport deposits (LF-E). The red dogleg line indicates locationof the measured section (MSA) presented at left. Arrows in the section correspond to turning points. Backpack (encircled) for scale. b This fining-up succession is composed of a basal conglomerate body (LF-A) resting erosively on mass-transport deposits (LF-E) and overlain by a package of thin-bedded heterolithic deposits (LF-D). The red line indicates location of the measured section (MSB) presented at rightnot only in the Peri-Adriatic basin, but are also known from the Pleistocene Kazusa Group of Japan (Ito and Katsura 1993), the Miocene UrenuiCMount MessengerFormations of New Zealand (King et?al. 2007), and the Paleogene Carmelo Formation of central California (Cronin and Kidd 1998).13
860Int J Earth Sci (Geol Rundsch) (3C87513Int J Earth Sci (Geol Rundsch) (3C875 ?Fig.?10??Outcrop photographs of the most landward exposures of the861canyon fill (Monte Ascensione). Dotted and thick dashed thick lines underscore the system confining (fifth-order) erosional incision and high-frequency (fourth-order) stratigraphic surfaces, respectively. a Photomosaic of Monte Ascensione, looking northwest. The prominent steep scarps on the left-hand side are channelized conglomerates (LF-A) interbedded with the recessive-weathered mass-transport deposits (LF-E). The cyclic sedimentation pattern reflects the intermittent supply of sediment from a significant siliciclastic source. Most of the sand bypassed through this part of the canyon and accumulated farther down the system. The fifth-order, canyon-bounding unconformity can be mapped with relative confidence only in this area, where it cuts deeply into the underlying hemipelagic mudstones (LF-F) and exceeds 2.5C3?km in width. Its full dimensions, however, cannot be given as only one margin of the canyon fill is preserved. b Southeastward, depositional-dip-oriented photomosaic of the most proximal portion of the Monte Ascensione system. Locations of photos shown in Figs.?8b, 10c, d are indicated by boxes. c Enlargement of outcrop shown at the lower left corner of b, illustrating sequence composition at this canyon-margin setting. MA-IV1 includes basal LF-A deposits capped by LF-E deposits, whereas at the base of the overlaying MA-IV2 are heterolithic sediments (LF-D) that are interpreted to represent a levee succession that bounds the conglomerate channels occurring at the base of the same sequence in more axial position. d Closer view of the boxed area shown at the left-hand por? tion of the panorama in b, illustrating the fining-and thinning-upward character of the LF-D package as a whole. Figure?10a, b, and c modified from Cantalamessa et?al. (2009) and reproduced with permission of SEPMChronostratigraphic framework A detailed chronostratigraphic framework for the most proximal and oldest portion of the Monte Ascensione Canyon (from MA-IV1 to MA-IV5) and the encasing slope mudstones has been established by Cantalamessa et?al. (2009) based on integrated magnetostratigraphy and biostratigraphy (both foraminifera and calcareous nannoplankton). In that study, the combination of a number of high-precision age datums (biostratigraphic events and polarity reversals) provided a high-resolution magnetobiochronology time control that permitted the construction of a robust age model for this part of the turbidite system (Fig.?12). According to magnetostratigraphic data, two magnetozones can be identified in the most proximal portion of the Monte Ascensione system. A single normal polarity interval encompasses both the highest part of hemipelagic mudstones underlying the Monte Ascensione system and the canyon fill from the base of MA-IV1 up to the mud-prone interval of MA-IV2. This is followed by a rather long interval of reversed polarity that characterizes the remaining part of the canyon fill from MA-IV3 to MA-IV5. As discussed below, biostratigraphic constraints allow a straightforward interpretation of the magnetic-polarity zones, which are interpreted as the Gauss Chron and the Matuyama Chron of the geomagnetic the normal subchron of Reunion was not reached. The biostratigraphic evidence leading to this interpretation are: (1) the presence of the MPL5 biozone throughou (2) the occurrence in the hemipelagic mudstones immediately underlying the first conglomerate wedge of the calcareous nannofossil Discoaster asymmetricus, which, being distributed up to about the Discoaster tamalisCDiscoaster pentaradiatus zone boundary (Rio et?al. 1990), permitted assigning that interval to the Pliocene Discoaster tamalis Zone (MNN16a); and (3) the first appearance of small and primitive specimens of Bulimina marginata in samples from the fine-grained interval of MA-IV2. In the present study, to achieve a more complete picture of the chronostratigraphic framework of the Monte Ascensione canyon-fill succession, the biostratigraphic analysis has been extended to the entire outcropping succession and additional unweathered bulk samples for analyses of the foraminiferal and nannoplankton assemblages have been taken from MA-IV4 to MA-IV14. Overall, the newly collected samples yield poor microfossil assemblages and are characterized by the co-occurrence of the age-diagnostic calcareous nannofossil Discoaster brouweri and benthic foraminifera Bulimina marginata throughout the entire stratigraphic interval, the presence of the planktonic foraminifera Globorotalia puncticulata only in samples taken from MA-IV6, and the first occurrence of Globoro? talia inflata in samples taken from MA-IV14. According to these biostratigraphic data, this portion of the canyon fillDetailed mapping of the various lithofacies encountered in the study area reveals that the Monte Ascensione succession shows an overall fining-upward trend and that this vertical pattern is accomplished in a stepwise manner through the juxtaposition of fifteen, fining-upward fourthorder elements that are between 40 and 110?m thick. The stratigraphically lowest portion of the canyon fill succession is exposed at the southern end of the outcrop belt (Fig.? 10) and, as suggested by its very coarse turbidite facies and richness in mass-transport deposits, it represents a very proximal depositional setting. Outcrop quality in this area is limited due to extensive vegetation and high vertical cliffs that make access to most of the rock difficult for detailed geological observations. Figure?11a shows a depositional strike stratigraphic cross section of the canyon fill at Monte Ascensione, reconstructed on the basis of the lithological mapping. Palaeocurrent measurements in the conglomerate channel belts, as determined from clast imbrication, strike/dip of inclined stratification, and channel-margin orientations, change from site to site and exhibit a wide scatter, but indicate a general south to north transport direction. The reconstructed stratigraphic cross section in Fig.?11b provides a roughly depositional-strike view of the middle sector of the canyon fill, about 4?km further down the system. The northernmost, and therefore more distal, known outcrops of the Monte Ascensione canyonfill succession are located about 5?km further down system (Fig.?11c).13
862Int J Earth Sci (Geol Rundsch) (3C87513Int J Earth Sci (Geol Rundsch) (3C875 863?Fig.?11??EastCwest-trending (along depositional strike), simplifiedcorrelation panels of the Monte Ascensione succession constructed with summary lithologic logs (palaeoflow is into the page). Rose dia? grams are shown to document the dispersal patterns in the studied area. a Lower portion of the canyon fill at Monte Ascensione. a Middle portion of the canyon fill. This section is stratigraphically younger than that shown in a and is located about 4.5?km farther down-depositional dip. c Upper part of the canyon fill. This section is stratigraphically younger than that shown in b and is located about 5?km farther down-depositional dipspans from the middle part of the MPL5 foraminiferal biozone to the lowermost part of the MPL6 foraminiferal biozone, indicating an early Pleistocene (Gelasian) age.Discussion Possible external controls over?the development of?fourth?order elements The physical stratigraphy documented above shows a high degree of organization of the studied sedimentary succession, which is indicative of systematic changes in the factorscontrolling the timing of sediment storage on the shelf and its redistribution beyond the shelf edge. This section discusses briefly some of the most viable external or “allogenic” controls on the highly regular cyclicity documented in the Monte Ascensione sedimentary succession, particularly regional tectonic activity, sediment supply, and eustatic sea-level changes. Constraints on the possible controls over the development of the Gelasian fourth-order elements involve consideration on their average duration. The tight chronostratigraphic control available on this turbidite system allows placing of mapped fourth-order elements within a highresolution temporal framework and provides the basis to accomplish this task. The stratigraphic interval between the Gauss-Matuyama paleomagnetic boundary (2.588?Ma) and the first occurrence of G. inflata (2.09?Ma) spans 0.498?m.y. and includes twelve of the fifteen fourth-order elements in the succession (from MA-IV3 to MA-IV14), yielding an average duration of 41.5?k.y. per element. High?frequency tectonism The Monte Ascensione succession was deposited adjacent to the Apennines on the western margin of the activelyFig.?11??continued13
864Int J Earth Sci (Geol Rundsch) (3C87513Int J Earth Sci (Geol Rundsch) (3C875 865?Fig.?12??Correlation between the fifteen sequences recognized withinthe Monte Ascensione succession and the late Pliocene and early Pleistocene global oxygen isotope curve. Comparison has been made using the Gauss-Matuyama polarity transition, the last occurrence of G. puncticulata and the first occurrence of G. inflata as correlation tie points. The representative stratigraphic column shows that there is an upward decrease in number and thickness of conglomerate and masstransport deposits, with an associated increase in sandstones and heterolithic lithofacies. The benthic δ18O isotope stack, which is used as a proxy for early Pleistocene sea-level changes, is from Lisiecki and Raymo (2005). Magnetic reversal, stage boundaries age, and chronology of calcareous nannoplankton and planktonic foraminifer datum events are based on ATNTS2004 (Lourens et?al. 2004). Age estimation of selected regional bioevents (*) from Pasini and Colalongo (1994), and Patacca and Scandone (2004) and references therein. Benthic Zones after Colalongo and Sartoni (1979)Climatically?induced variation in?water and?sediment discharge High-frequency changes of continental climate may exert an important control on the amount of precipitation and its regime, as well as on type and extension of vegetation cover, inducing important variations in type and rate of erosion, run-off and transport capacity of rivers and, ultimately, sediment supply from the fluvial systems into the basins (Leeder et?al. 1998). The climatic conditions in central Italy at the time of deep-water deposition within the Monte Ascensione canyon are well documented by pollen records from both continental and marine successions outcropping within the Apennines. They indicate that, since Piacenzian times, significant alternations of drycool and humid-warm phases took place in concert with obliquity-forced glacial-interglacial cycles, and that alternation of steppe-like vegetation and subtropical to warmtemperate deciduous forest marked the overall glacialinterglacial vegetation turnover (Bertini 2010). However, pollen data from the Tiberino basin (central Italy), which spans the interval from Marine Isotope Stage (MIS) 100 (c. 2.51?M.a.) to MIS 82 (c. 2.16?M.a.) and is therefore coeval with the Monte Ascensione section, indicate that during glacial phases rain-demanding coniferous forests thrived along the coastline and that steppe elements were significantly reduced (Pontini and Bertini 2000). The expansion and downward shift of the mountain vegetation during the glacial phases suggests that in central Italy temperature and humidity did not decrease sufficiently and implies the existence of wet conditions during both glacial and interglacial phases, allowing the continuous presence of a forested environment (Pontini and Bertini 2000; Fauquette and Bertini 2003). The persistence throughout both glacial and interglacial phases of a wet precipitation regime and a thick forest cover provided little chances for cyclic variations in erosion rates in the catchment areas and noticeable climatic modulation of sediment flux to the basin, suggesting that during the Gelasian the Peri-Adriatic basin did not suffer significant changes in the rate of climatically controlled sediment supply. From this perspective, it seems very unlikely that the cyclic pattern recognized at Monte Ascensione may have been driven by this mechanism. This conclusion is further corroborated by a series of studies demonstrating that under severe icehouse condi}