Facies Tracts Research Articles

Evolution of a confined gravelly river to a braided-meandering river in the Late Cretaceous Khasi group, southern Shillong plateau, NE India

This paper intends to unravel the depositional condition of the Basal Conglomerate/Jadukata Formation and the Lower Mahadek Formation in southern Shillong Plateau, NE India deposited during the Late Cretaceous period. The studied succession is entirely siliciclastics in composition and was built up on the Precambrian basement rocks. State-of-art facies analysis and palaeocurrent evidence helped us to reconstruct the palaeogeography and palaeoenvironment. The facies tracts reveal that frequent hyperconcentrated mass flow deposits dominated the proximal part of the river channels. Gradually, the river evolved to a sandy braided and finally mud-dominated, meandering system. The river channel changed its pattern through onlapping and progressive elimination of the accommodation space created by the tectonic subsidence/upliftment. Major shifts in palaeocurrent direction at different stages of the river evolution confirm the role of tectonics on the river channel evolution. QFL detrital modes of the sandstones reflecting lithic wacke composition and the recycled orogeny corroborate the interpretation. Rapid lateral and vertical facies transition suggests a pull-apart type basin due to ongoing rifting. Geochemical analyses also show signatures of the overlapping tectonic conditions supporting the pull-apart basin model.

Flow transformations, mud partitioning and the variable stratigraphic architecture of basin-floor fan fringes

Highly efficient sediment gravity flows can bypass mid fan channels and lobes and deposit significant volumes of sand, mud and particulate organic matter in outer fan and basin plain settings. The Serpukhovian to Bashkirian fill to the Shannon Basin, western Ireland, includes deep-water fan deposits (Ross Sandstone Fm) that gradationally overlie basin floor shales (Clare Shale Fm). As part of a broader progradational succession, the upward transition from muddy basin floor to sandy fan preserves the stacked deposits of settings present prior to and outboard of mid-fan channels and lobes. Three fully cored boreholes and associated wireline data constrain the facies tracts in an 18 km long panel orientated oblique to original depositional dip. Two distal successions dominated by hybrid event beds (HEBs) are recognised, separated by a prominent condensed section. The lower Cosheen system includes m-thick, tabular HEBs with prominent linked debrites that pass down dip into much thinner sandstones overlain by sand-speckled mudstone caps that thicken distally before thinning. The latter are interpreted as secondary mudflows released following reconstitution of more thoroughly mixed sections of the up-dip linked debrites. Significant bypass and accumulation of mud by this mechanism helped heal local topography and maintain a relatively flat sea floor promoting an overall tabular geometry for the deposits of larger volume hybrid flows reaching the distal sector of the basin. The overlying distal Ross system fringe is characterised by very fine to fine-grained sandstones and is lateral to compensationally-stacked lobes further to the west. It has a progradational (at least initially) stacking pattern, facies transitions developed over shorter length scales, and includes outsized event beds but these are thinner than those in the Cosheen system. Common banding and evidence for turbulence suppression by dispersed clay rather than entrained mud clasts indicate these were transitional flows. In this case, event beds are inferred to taper distally, with significant mud emplaced by plug flow retained as caps to sandy event beds rather than bypassing down-dip. Different flow transformation mechanisms thus impacted how mud was partitioned across the fringe of the two systems and this influenced bed geometries, larger scale bed stacking patterns and stratigraphy. Whereas the flow efficiency concept stresses the ability of flows to carry sand in a basinward direction, it is also imperative to consider the variable efficiency of mud transport given the operation of clay-induced flow transformations. These can either promote bypass or trigger premature fallout of mud with implications for how systems fill accommodation, bed -scale facies transitions and the burial and preservation of particulate organic carbon fractionated along with the clay in deep-water system fringes.

Paleoclimate and the origin of two 1000 km Lower Mississippian facies tracts in southeastern Laurentia (USA): Cool-humid Famennian and Kinderhookian – warm-arid Osagean

Two lithologically distinctive Mississippian facies tracts (MFTs) are described from Lower and Middle Mississippian strata (Kinderhookian and Osagean North American Stages) in southeast Laurentia (SEL). The MFTs extend from the central Appalachian Basin westward across the Cincinnati Arch, through the Eastern (Illinois) Basin, onto the Burlington shelf in central Missouri and into the Western Interior (Forest City) Basin, USA. The MFTs developed under radically different climatic regimes. Kinderhookian climate cycles ranged from 3rd order humid to 4th and 5th order humid-subhumid alternation. These climate fluctuations controlled Kinderhookian sediment flux, evidenced by fluvial-deltaic sandstones, coal beds, and pro-deltaic terrestrial-organic-matter-enriched marine black shales. A dramatic climate shift coincided with the Kinderhook-Osage boundary, from 3rd order cool-humid to 3rd order warm-arid conditions, causing cessation of deltaic sedimentation and the onset of eolian sedimentation at that boundary. This abrupt climate reorganization is reflected in 3rd, 4th, and 5th order continental and eolo-marine loessites that replaced fluvial-deltaic facies. Eolianites, evaporites, and calcareous protosols indicate Osagean aridity. Consequently, we reject the deltaic depositional paradigm for Osagean siliciclastic facies. Osagean eolian sediment consisted mainly of quartz silt with significant pristine (unaltered) feldspar silt (∼ ≤ 10%). Pristine feldspars are consistent with an arid sedimentary source that lacked significant chemical weathering under an arid paleoclimate. Dissolution of the chemically reactive disordered lattice of eolian-abraded quartz in subarkosic loess served as the predominant source of silica for massive amounts of biotic and abiotic chert in Osagean eolo-marine sediments. The ≤20 μm fraction of quartz dust is particularly susceptible to dissolution and re-precipitation as microcrystalline quartz (chert). We conclude that tectonic, eustatic, and climatic (allocyclic) processes all exerted some control on deposition; tectonics and eustasy controlled accommodation space, whereas paleoclimate changes (cycles), driven predominantly by orbital forcing, were the principal control on sediment supply and lithostratigraphy.

Seismic-based paleoenvironmental analysis of the Paleocene carbonate shelf in Ajdabiya Trough, north-central of Libya

Abstract The results of the seismic and well logs interpretation, together with previous literature, were used to construct a generalized model of the development of the Paleocene reefs, and a depositional model for the Paleocene carbonate shelf in Ajdabiya Trough that focuses on the initiation, growth, and demise of the reefs in this basin. The Intisar reefs began as algal-foraminiferal banks. Growth of the reefs was halted by a short-lived transgression that drowned reefs and led to shale deposition of Kheir shales. The inversion of 3-D seismic data to acoustic impedance has allowed for a better determination of the main facies in Intisar reefs. These facies reflect different depositional conditions and different ages. They are: 1) algal-foraminiferal packstone/grainstone; 2) bioclastic grainstone/packstone; 3) coralline floatstone/bindstone; 4) reefal boundstone and; 5) bio-lithoclastic talus. The Upper Sabil consists of a prograding carbonate shelf with ramp platform morphology, and its differentiated deposits indicate four main depositional facies tracts: 1) inner shelf margin; 2) barrier reefs; 3) pinnacle reefs and; 4) outer shelf margin. The proposed model for the growth of the Paleocene reefs in the Ajdabiya Trough represents consecutive stages of development: 1) the Lower Sabil Carbonate was deposited during the very early Upper Paleocene time; 2) the influx of the Sheterat Shale during the major marine transgression; 3) the sea-level began a very slow fall due to variable subsidence of the underlying strata (a deposition of micrite over the Sheterat Shale, the lower part of the Upper Sabil Carbonate); 4) the rise in sea level is evident from an increase in foraminifera and algae constituents over the high areas of the lower part of the Upper Sabil Carbonate; 5) the isolated Intisar reefs were born under the proper local circumstances. The rate of subsidence was very slow, and the coral reef had significant upward growth and; 6) a major transgression happened at the end of the Paleocene, and the consequent deepening of the water in the area drowned and killed the Intisar reefs and capped it with Kheir Shale.

Early Evolution of the Adelaide Superbasin

Continental rifts have a significant role in supercontinent breakup and the development of sedimentary basins. The Australian Adelaide Superbasin is one of the largest and best-preserved rift systems that initiated during the breakup of Rodinia, yet substantial challenges still hinder our understanding of its early evolution and place within the Rodinian supercontinent. In the past decade, our understanding of rift and passive margin development, mantle plumes and their role in tectonics, geodynamics of supercontinent breakup, and sequence stratigraphy in tectonic settings has advanced significantly. However, literature on the early evolution of the Adelaide Superbasin has not been updated to reflect these advancements. Using new detrital zircon age data for provenance, combined with existing literature, we examine the earliest tectonic evolution of the Adelaide Superbasin in the context of our modern understanding of rift system development. A new maximum depositional age of 893 ± 9 Ma from the lowermost stratigraphic unit provides a revised limit on the initiation of sedimentation and rifting within the basin. Our model suggests that the basin evolved through an initial pulse of extension exploiting pre-existing crustal weakness to form half-grabens. Tectonic quiescence and stable subsidence followed, with deposition of a sourceward-shifting facies tract. Emplacement and extrusion of the Willouran Large Igneous Province occurred at c. 830 Ma, initiating a new phase of rifting. This rift renewal led to widespread extension and subsidence with the deposition of the Curdimurka Subgroup, which constitutes the main cyclic rift sequence in the Adelaide Superbasin. Our model suggests that the Adelaide Superbasin formed through rift propagation to an apparent triple junction, rather than apical extension outward from this point. In addition, we provide evidence suggesting a late Mesoproterozoic zircon source to the east of the basin, and show that the lowermost stratigraphy of the Centralian Superbasin, which is thought to be deposited coevally, had different primary detrital sources.

Depositional elements and evolution of gravity-flow deposits on Lingshan Island (Eastern China): An integrated outcrop-subsurface study

Understanding how subaqueous sediment gravity flows (SSGF) evolve in time and space, and how their deposits vary spatially, is a key research focus for gravity flow sedimentology. This study investigates depositional facies, depositional elements, and sediment transport processes of supercritical flows and hybrid event beds (HEBs) of the Lingshandao Formation in Lingshan Island, Eastern China. Three kinds of depositional elements were recognized: mass transport deposits (MTDs), channel-lobe transition zone (CLTZs), and lobe complexes. MTDs can be sub-divided into proximal and distal deposits. CLTZs are characterized by facies changes from massive coarse-grained pebbly sandstone to backset bedding sandstone in a down-slope direction, which are the deposits of supercritical turbidity currents. HEBs are common in both proximal and distal lobe settings. Tripartite structure in HEBs, which may be caused by up-slope substrate erosion, implies a relatively proximal origin compared with bipartite HEBs, perhaps caused by fluid fractionation in the research area. The succession is formed by a prograding lobe unit, followed by MTDs which are themselves overlain by deposits from a CLTZ. This vertical stacking pattern implies the potential longitudinal facies tract from CLTZ; to lobe complex deposits, is accompanied by emplacement of MTDs at the slope break. The flow types accompanied by sediment transport processes imply that erosion by supercritical turbidity currents associated with a hydraulic jump in the channel-lobe transition zone may be the main reason for mud-clasts and matrix addition to the flow. The large-scale addition of mud-clasts and suspended mud may dampen turbulence in proximal to medial lobe settings, and result in medium-to thick-bedded HEBs with common erosional features and tripartite structures. The remaining suspension flow (with abundant mud) may further transport down-dip and form medium-to thin-bedded HEBs with bipartite structures and rare mud-clasts. These findings may be applicable to other SSGF systems with supercritical flow deposits and hybrid event beds, emphasizing the downdip and lateral variation in depositional elements associated with gravity flow evolution.

Bed-scale vertical and lateral distribution of massive sandstone in a topographically confined basin (Peïra Cava, SE France): Implications for flow processes

In high-density turbidite (HDT) systems, proximal deposits typically show thick beds of massive and/or stratified sandstone capped by a thin low-density turbidite (LDT) interval of finer-grained laminated and rippled sandstone. Distally, the massive/stratified HDT interval thins and is replaced by the LDT interval as the dominant facies. Whereas this empirical observation is common to many ancient systems worldwide, a very different relationship is found in the topographically confined Peïra Cava sub-basin in SE France. Eight prominaent beds can be correlated across much of the exposed basin (3 x 11 km). Three massive sandstone facies are recognised in these beds: (1) ungraded (2) graded, and (3) massive with patchy texture. In proximal and medial sections, these facies are capped by a relatively thick LDT interval comprising up to 30% of the bed thickness. Moving to distal sections, the massive facies come to dominate the bed, in some cases forming 100% of the bed thickness. Along a downslope transect, three facies tracts are defined that all differ considerably from previously described trends. Facies tract I and II comprises ungraded massive sandstone and massive sandstone with patchy texture in distal sections, respectively. Facies tract III comprises repetitive sequence of ungraded and graded massive sandstones in distal sections. Statistical analysis of the vertical internal character in the eight beds show non-random, non-cyclic facies trends, with each massive facies exhibiting some level of dependency on other facies, suggesting sediment deposition depended on temporally evolving flow conditions. The ungraded and graded massive facies are interpreted as deposits of high-concentration near bed layers beneath surge-type turbidity currents. In contrast, the massive sandstone with patchy texture is interpreted to result from liquefied debris flows. Bed thickness trends and the presence of distinctive grain size breaks indicate depositing flows were bipartite (debris flow with an overlying concentrated turbulent layer), depletive, unsteady and pulsatory in proximal locations, but evolved distally into a turbulent flows with a simple surge structure. The distal dominance of these massive facies is here interpreted as a consequence of flow interaction with confining topography. This interaction induced gravity and body transformations in flows of facies tract I and II, whereas generation and passage of internal waves in distal sections created the necessary conditions for massive sandstone deposition in facies tract III.

Laterally accreted deposits in low efficiency turbidites associated with a structurally-induced topography (Oligocene Molare Group, Tertiary Piedmont Basin, NW Italy)

ABSTRACT The origin of laterally accreted deposits in ancient deep marine successions is often controversial. Indeed, not always do these features imply the occurrence of meanders or high-sinuosity turbidite channels, but they can be generated by other causes, such as sediment-gravity-flow dynamics controlled by the morphology of tectonically confined mini-basins. This work discusses laterally accreted deposits composed of sharp-based, normally graded beds in a very small tectonically controlled mini-basin. These beds, characterized by a well-defined asymmetrical cross-current facies tract, form well-developed lateral-accretion surfaces dipping in directions ranging between W and SW, and perpendicular to the paleocurrents directed towards the N. For this reason, these deposits have always been interpreted as point bars related to meandering channels. A new detailed stratigraphic framework and facies analysis have led to an alternative interpretation, namely that these deposits record lateral deflections of small volume, longitudinally segregated turbidite dense flows against a structurally controlled morphological high. This interpretation is also supported by a comparison to other tectonically controlled turbidite systems that are characterized by higher degrees of efficiency but show similar laterally accreted deposits and cross-current facies tracts.

Progradation of a mid‐Cretaceous distributive fluvial system: The upper member of the Bima Formation, Northern Benue Trough, Nigeria

AbstractBasin‐wide analysis of an ancient distributive fluvial system (the upper Bima Formation) was carried out using field‐based observation/measurement and ‘photo‐realistic’ virtual outcrop data from 19 locations with good exposure of the upper Bima Formation in the Northern Benue Trough. The goal of this study is to provide an assessment of the spatial distribution of both vertical and lateral trends within the upper Bima distributive fluvial system. Vertical and lateral variations of the different facies of the formation were assessed quantitatively; this includes the analysis of the vertical and lateral distribution of proximal, medial and distal facies, the thickness and widths of channel fill deposits, grain‐size variation and the proportion of floodplain to channel fill deposits across the 19 locations in the basin. Trends observed across the sections indicate a progradation of the mega‐fan. The facies tracts preserved within the different portions of the distributive fluvial system vary. In the proximal part, in Lamurde, Tashan Alaji, Ture and Tula, the sections show only medial passing up to proximal facies. The distal lacustrine facies at the base of the section are absent. In the medial part of the system at Hinna, the successions preserve distal passing upward into medial facies. The gradual lateral transition from proximal to medial to distal facies, upward decrease in floodplain to channel fill deposits, upward increase in channel fill thicknesses and widths, all suggest that the Bima system is a prograding distributive fluvial system.

Paleocene storm-related event beds in the Gaoyou Sag of the Subei Basin, eastern China: A new interpretation for these deep lacustrine sandstones

Lacustrine storm deposits in hydrocarbon-rich lake basins can be excellent reservoirs; therefore, the identification and interpretation of storm deposits have great significance for future petroleum exploration. Recent progress in facies analysis helps to discriminate the sedimentary effects of combined flow and oscillatory flow in the depositional processes. Through core and well log data and thin-section analysis, the sandy deposits of Paleocene E1F3 member in the middle part of the Gaoyou Sag, Subei Basin, are interpreted as storm-induced deposits. Petrologic analysis shows that the coarser sediments are mostly fine-grained to silt-sized feldspathic litharenite. Five lithofacies were identified based on core observation, including massive sandstone, cross-bedded sandstone, hummocky cross-stratification (HCS) sandstone, wave rippled sandstone and siltstone, and massive mudstone. Lithofacies associations revealed a storm “sequence” beginning with an erosional surface and then fining upward, indicating hydrodynamic conditions that ranged from gravity flow, unidirectional flow, combined flow to oscillatory flow, which are attributed to storm-related processes. According to the different lithofacies associations and depositional processes, a depositional model including four facies tracts of storm-related deposits are recognized in plane view: (1) storm-induced turbidity channel facies characterized by coarser particles and formed by storm-triggered turbidity current, (2) proximal facies dominated by gravity, unidirectional, and combined flows, (3) transitional facies influenced by all the flow regimes and exhibiting a relatively complete storm “sequence”, (4) distal facies formed by low-energy deep-lake turbidity flow. Because storm deposits have petroleum significance in an oil-bearing basin such as Subei Basin, this study provides practical examples and guidance for future hydrocarbon exploration in other ancient lacustrine basins.

Isotope stratigraphy (87Sr/86Sr,13C) and depositional sequences of the Aruma Formation, Saudi Arabia: Implications to eustatic sea‐level changes

Lithofacies characteristics, depositional architecture (facies tracts and depositional sequences), stable isotopes (δ18O and δ13C), and87Sr/86Sr ratios were analysed from the Aruma Formation at the Khashm Buwaibiyat section (north‐east Riyadh, Saudi Arabia). The Aruma Formation consists of three formal members (base to top), Khanasir, Hajajah, and Lina members. Based on87Sr/86Sr and δ13C data, the lower age of the regionally distributed Aruma Formation (Khanasir Member) is now proposed as Late Campanian. This age assignment is supported by the identification of a distinct δ13C negative excursion (=Epsilon Event; Upper Campanian), and a globally conformable range of87Sr/86Sr values (0.707604–0.707703). Additionally, a review of previous fauna (ammonites and nannofossils) also favours this age assignment. The87Sr/86Sr values (0.707817–0.707848) from the upper part of the Hajajah Member reflects a Late Maastrichtian signature as also noted by the presence of a long‐term δ13C decrease. Additionally, based on lithofacies and depositional architecture for the Aruma Formation, nine facies types are identified. These are grouped into four facies associations (restricted peritidal, lagoon inner ramp, shoal transgressive lag, and coral reefs), reflecting four third‐order depositional sequences (transgression–regression cycles). These reflect strong influence of both eustatic sea‐level rise and regional tectonics (resulting in widespread unconformities and long‐time gaps). The shallowing and deepening of the sea level is inferred using sequence stratigraphy (system tracts) and lithofacies variations. This data suggests a gradual rise in sea level from the Upper Campanian to the Maastrichtian with an intervening fall around the Campanian‐Maastrichtian boundary.

Evaluating the depositional environment, lithofacies variation, and diagenetic processes of the Wolfcamp B and lower Spraberry intervals in the Midland Basin: Implications for reservoir quality and distribution

Detailed facies characterization in the Wolfcamp B and lower Spraberry intervals of two drill cores in the Midland Basin has yielded five main lithofacies with distinctive physical, chemical, and biologic attributes. The main attributes for facies identification include lithology and/or mineralogy, texture and/or fabric, porosity, hydrogen index, and total organic carbon content. The methodologies are focused on detailed core description, thin-section petrography, quantitative x-ray diffraction, and field emission scanning electron microscopy (SEM). Porosity data are primarily based on Gas Research Institute (Core Laboratories) measurements and field emission SEM assessment. Moreover, the depositional and diagenetic controls on facies development are addressed to assess the dominant geological processes that govern reservoir quality and distribution. A model of five end-member facies is proposed to characterize source and reservoir elements through delineating facies tracts. The five end-member facies are as follows: facies 1: silty mudstone, optimal source, and optimal to fair reservoir; facies 2: muddy siltstone (optimal source and optimal reservoir); facies 3: silty calcareous mudstone (good source and good reservoir); facies 4: bioclastic wackestone–floatstone (fair source and fair reservoir); and facies 5: packstone–grainstone (poor source and poor to excellent reservoir). The proposed facies scheme aims to provide a more comprehensive approach to capture the high vertical and lateral variability in this mixed carbonate and fine-grained clastic succession. Through this detailed textural, compositional, sedimentologic, and diagenetic approach, this facies model can be used to better understand reservoir quality and distribution throughout the Midland Basin.

Mixed carbonate–siliciclastic sedimentation in the Upper Cretaceous Nilkanth Formation, Garhwal Himalaya, India

The Upper Cretaceous Nilkanth Formation awaits a process-based depositional model despite being a topic of discussion between stratigraphers, palaeobiologists and structural geologists over the last few decades. Sedimentary facies analysis of a ~50 m thick section along a ~2.8 km long section along Rishikesh–Tal Bidhashini in Pauri Garhwal district of Uttarakhand allowed documentation of mixed carbonate–silicicalstic facies types, dominantly consisting of sand- and pebble-sized carbonate debris mixed with siliciclastics in a proximal to distal facies tract. Ten different facies types that include matrix-rich and matrix-poor shelly conglomerate, mixed clastic-carbonate wackestone, packstone, impure calcirudite and calcarenite, biomicrite and ferruginous sandstone are documented. Delineation of facies association and documentation of facies stacking pattern provide a post-Santonian mixed carbonate–siliciclastic sedimentation history of the Nilkanth Formation, deposited in the form of mass flows of varied rheology on a barred low- to moderate-gradient carbonate ramp, formed at the leading edge of the India plate before its collision with the Kohistan–Ladakh arc. Carbonate clasts comprising bivalves, crinoids, algae, bryozoan, etc., were produced in a narrow high-energy transgressive coastline and supplied across shelf along with reworked siliciclastics from clastic shoreface bar. It is argued that the reworked fossils, including the bryozoa Ceriocava Nilkanthi, present within massflows may not justify fixing of an absolute age for the formation but may definitely help in providing an age range.

The influence of variable progradation to aggradation ratio and facies partitioning on the development of syndepositional deformation in steep-walled carbonate platforms

Abstract One of the remaining questions in carbonate geology and reservoir studies is the origin of steep-walled carbonate platform margins and the role of gravity versus other structural processes in controlling the distribution of fracture networks and failure surfaces. This study is an attempt to document the role of gravity, interacting with stratigraphic architecture in controlling fracture patterns and margin collapse. We employ finite-discrete element models to illustrate that progradation/aggradation (P/A) ratio and facies tract partitioning in steep-walled carbonate platforms affects the development, distribution, and intensity of syndepositional deformation. All models are under the influence of only gravitational loading, where pore pressure is held constant. We utilize a modified Mohr-Coulomb constitutive law with a Rankine Rotating crack tensile corner to capture both tensile and shear brittle failure. Our results illustrate that P/A ratio affects the distribution and intensity of discrete fractures that form in steep-walled carbonate platforms. Our results suggest that deformation is more extensive in a progradational carbonate platform, where shelf edge angles are greatest and the clinoform is thickest. Alternatively, aggradational carbonate platforms experience localized deformation in front of the antecedent shelf edge where the clinoform is thickest and steepest. The introduction of mechanical heterogeneity associated with facies tract partitioning affects the intensity of fracturing in both progradational and aggradational models, with the greatest number of fractures developing in reef facies in both scenarios. Development of brittle deformation under the sole application of gravity and lack of seaward lithostatic confinement is consistent with the syndepositional nature of deformation in these settings. This work illustrates the interplay between carbonate platform geometry, facies distribution, and the resulting syndepositional deformation.

Revisiting the boundary between the Lower and Upper Vindhyan, Son valley, India

The placement of the boundary between the Lower and the Upper Vindhyan in the Son valley, an unconformity, has long been at the centre of a raging debate. At the Bundelkhand sector, it is placed between the Rohtas Limestone and the Sasaram Sandstone (Lower Quartzite). On the other hand, in the Son valley sector, it is placed between the Bhagwar Shale and the Kaimur Formation. The recent study reveals the existence of ca. 12 m thick sandstone between the Bhagwar Shale and Rohtas Limestone, traced over 150 km in the Son valley sector. Based on in-depth facies constituents and facies tracts, this sandstone is an exact equivalent of the Sasaram Sandstone in the Bundelkhand sector. Its base is strongly erosional and limestone and chert clasts derived from the underlying Rohtas Limestone are abundantly present at the basal part of the sandstone and the unconformity between the Upper and Lower Vindhyan are likely to be present in between.

Relative bathymetric position of opoka and chalk in the Late Cretaceous European Basin

Opoka is a distinctive, highly fossiliferous marine spiculitic limestone typical of the Upper Cretaceous epicontinental successions of Poland, western Ukraine and some other parts of Europe. Several recent works have suggested that opoka was deposited in pelagic offshore settings, more distal and deeper water than those of the chalk. We oppose this view based on facies characteristics and lateral relationships of selected Campanian and Maastrichtian successions of Poland and Ukraine, as well as on palaeoecology and taphonomy of fossil assemblages preserved in these deposits. Our conclusions are as follows: 1) when opoka and chalk occur in the same facies tract, opoka occupies a more proximal position than the chalk; 2) opoka could have been deposited in a variety of settings, ranging from relatively deep-water to shallow-water ones; 3) chalk was usually deposited at greater depth than opoka, but some intervals of chalk appear to have been laid down at the same depth as some opokas. Both opoka and chalk are important sources of data on Late Cretaceous marine communities of Europe. Therefore, a proper interpretation of the water depth relations between these facies is of key importance for our knowledge of these communities.

Co-genetic, cohesive and non-cohesive delta front facies: A case study of flow transformation in a lacustrine setting, Camaquã Basin, southernmost Brazil

Sediment gravity flows comprise gravity-driven underflows with a large concentration of suspended load. Along the downslope transport, flow transformations from laminar to turbulent conditions or vice-versa can take place due to several factors, including the incorporation and segregation of clay into, or from the flow. These flow changes may produce hybrid behavior and resulting hybrid event beds. Flow transformation and hybrid events are widely discussed in marine settings, but studies on lacustrine environments are rare. The Ediacaran Western Santa Bárbara Rift represents one stage of the development of the Camaquã Basin and includes both cohesive (debrites) and non-cohesive (turbidites) gravity flow deposits associated with braidplain deltas deposited in shallow lake. A range of cohesive and non-cohesive density flow facies is here discussed in terms of triggering mechanism, genetic processes and related flow transformations. The analysis of aerial images combined with outcrop descriptions allowed the identification and mapping of key stratigraphic surfaces and therefore associated systems tracts. Sedimentological and gamma-ray logs, sampling and laboratory analyses (optical microscopy, X-ray diffraction –XRD - and Scanning Electron Microscopy – SEM) were performed to define facies and facies associations. Microscopy was useful to identify vermiculite as an important matrix component. The position of distinct facies within the genetic stratigraphic framework allowed the construction of a generalized facies tract model. The basinward shift of facies indicates three flow transformations, in addition to hybrid flows. Substrate erosion, and ensuing incorporation and fragmentation of mud clasts into turbulent flows caused changes from fluidal to plastic flow rheology. Generalized facies tracts suggest fluvial floods (and resulting hyperpycnal flows) as the triggering mechanism for the subaqueous density flows herein described.

Shifts in sediment provenance across a hierarchy of bounding surfaces: A sequence-stratigraphic perspective from bulk-sediment geochemistry

Geochemical differentiation of sediment packages framed by surfaces of chronostratigraphic significance represents an effective tool to unravel the stratigraphic architecture of multi-sourced sediment-supply systems on a variety of temporal scales and through a wide spectrum of lithofacies assemblages. Sediment provenance shifts were examined across three-orders of bounding surfaces, arranged in a hierarchy of 105 to 103years sediment packages, from late Quaternary alluvial, deltaic, coastal and shallow-marine strata of the Po Basin, where controlling factors of sedimentary evolution are firmly constrained by strong age control. A total of 150 samples were analyzed for bulk-sediment geochemistry by X-ray fluorescence spectrometry (XRF). Geochemically unique catchment lithologies, such as ultramafic rocks and dolostones, were used as end-member grain assemblages to assess clear geochemical signatures, and chemostratigraphic correlations were generated on the basis of key element abundances and ratios.Sequence boundaries originating in response to tectonic uplift and related to 105 years sediment packages are associated with marked, basin-wide changes in sediment composition that reflect phases of substantial basin rearrangement, with strong modification of the drainage patterns. On an intermediate temporal scale (104years depositional units), large magnitude sea-level fluctuations had a major impact on sediment pathways, producing sharp provenance shifts on a systems tract scale. The Holocene transgressive surface records a substantial change in clastic sediment flux, as the alluvial system turned into a wave-dominated estuary. On the other hand, no notable changes in composition were observed across the maximum flooding surface. Finally, identifiable provenance variations developed on a short-term, facies tract scale (103years sediment packages) and revealed by lower-rank bounding surfaces denote dramatic changes in sediment dispersal patterns in response to simple autogenic processes, such as channel avulsion and delta lobe switching. In the stratigraphic context provided by the ancient record, such provenance shifts could easily be misinterpreted as the sedimentary expression of higher-rank bounding surfaces.This study supports the robustness of a comprehensive geochemical approach to an improved interpretation of the subsurface geological record across a hierarchy of depositional elements. It shows, however, that provenance shifts at chemofacies boundaries alone cannot be used to infer allogenic or autogenic controls on sedimentation. High-resolution facies analysis within stratigraphically constrained time intervals needs to be considered before promoting predictive relationships between provenance shifts and basin evolution.

Discontinuity surfaces and microfacies in a storm‐dominated shallow Epeiric Sea, Devonian Cedar Valley Group, Iowa

AbstractDiscontinuity surfaces develop in carbonate successions in response to a range of environmental changes and represent an integral part of the stratigraphic record. In Palaeozoic shallow epeiric basins that are typified by extremely slow subsidence and intermittent sedimentation, discontinuity surfaces may represent the majority of the time‐rock record. A depositional and sequence‐stratigraphic model was developed through microfacies analysis and discontinuity surface characterization using three cores in a proximal to distal transect across the Middle to Upper Devonian Iowa Basin. Twelve microfacies are recognized, spanning supratidal to deep subtidal facies tracts. A total of 105 discontinuity surfaces were documented and classified as either submarine omission surfaces, subaerial exposure surfaces or submarine erosional surfaces. Omission surfaces increase in frequency basinward, indicating increased sediment starvation in the offshore direction. Exposure surfaces increase in frequency shoreward, indicating more frequent subaerial exposure in a shallower setting. Erosional surfaces are dominant in the inner and middle ramp and interpreted as the base of storm beds (tempestites); these surfaces are rare in the outer ramp due to its generally deeper setting below storm wave base. Moreover, discontinuity surfaces exhibit systematic groupings stratigraphically (vertically) across the three localities spanning the Devonian carbonate ramp. Zones of either exposure‐dominated, erosion‐dominated or omission‐dominated surfaces were recognized and correlated with their landward or basinward equivalents (along with shifts in major facies belts) and interpreted in a sequence‐stratigraphic context. This study highlights the importance of including a detailed characterization of both depositional facies and non‐depositional discontinuity surfaces in order to better understand the stratigraphic history of a basin. The framework of analysis provided here is particularly useful for marine carbonate strata deposited in epeiric basins, which are especially common in the Palaeozoic and where non‐deposition and erosion occur frequently, but can also be applied to other geological time periods and settings.

Hybrid event bed character and distribution linked to turbidite system sub‐environments: The North Apennine Gottero Sandstone (north‐west Italy)

AbstractThis study documents the character and occurrence of hybrid event beds (HEBs) deposited across a range of deep‐water sub‐environments in the Cretaceous–Palaeocene Gottero system, north‐west Italy. Detailed fieldwork (>5200 m of sedimentary logs) has shown that hybrid event beds are most abundant in the distal confined basin‐plain domain (>31% of total thickness). In more proximal sectors, hybrid event beds occur within outer‐fan and mid‐fan lobes (up to 15% of total thickness), whereas they are not observed in the inner‐fan channelized area. Six hybrid event bed types (HEB‐1 to HEB‐6) were differentiated mainly on basis of the texture of their muddier and chaotic central division (H3). The confined basin‐plain sector is dominated by thick (maximum 9·57 m; average 2·15 m) and tabular hybrid event beds (HEB‐1 to HEB‐4). Their H3 division can include very large substrate slabs, evidence of extensive auto‐injection and clast break‐up, and abundant mudstone clasts set in a sandy matrix (dispersed clay ca 20%). These beds are thought to have been generated by highly energetic flows capable of delaminating the sea floor locally, and carrying large rip‐up clasts for relatively short distances before arresting. The unconfined lobes of the mid‐fan sector are dominated by thinner (average 0·38 m) hybrid event beds (HEB‐5 and HEB‐6). Their H3 divisions are characterized by floating mudstone clasts and clay‐enriched matrices (dispersed clay >25%) with hydraulically fractionated components (mica, organic matter and clay flocs). These hybrid event beds are thought to have been deposited by less energetic flows that underwent early turbulence damping following incorporation of mud at proximal locations and by segregation during transport. Although there is a tendency to look to external factors to account for hybrid event bed development, systems like the Gottero imply that intrabasinal factors can also be important; specifically, the type of substrate available (muddy or sandy) and where and how erosion is achieved across the system producing specific hybrid event bed expressions and facies tracts.