Depositional Topography Research Articles

High-resolution X-ray fluorescence-based provenance mapping of Eocene fluvial distributary fans that fed ancient Gosiute Lake, Wyoming, USA

The Green River Formation of Wyoming, USA, is host to the world’s largest known lacustrine sodium carbonate deposits, which accumulated in a closed basin during the early Eocene greenhouse. Alkaline brines are hypothesized to have been delivered to ancient Gosiute Lake by the Aspen paleoriver that flowed from the Colorado Mineral Belt. To precisely trace fluvial provenance in the resulting deposits, we conducted X-ray fluorescence analyses and petrographic studies across a suite of well-dated sandstone marker beds of the Wilkins Peak Member of the Green River Formation. Principal component analysis reveals strong correlation among elemental abundances, grain composition, and sedimentary lithofacies. To isolate a detrital signal, elements least affected by authigenic minerals, weathering, and other processes were included in a principal component analysis, the results of which are consistent with petrographic sandstone modes and detrital zircon chronofacies of the basin. Sandstone marker beds formed during eccentricity-paced lacustrine lowstands and record the migration of fluvial distributary channel networks from multiple catchments around a migrating depocenter, including two major paleorivers. The depositional topography of these convergent fluvial fans would have inversely defined bathymetric lows during subsequent phases of lacustrine inundation, locations where trona could accumulate below a thermocline. Provenance mapping verifies fluvial connectivity to the Aspen paleoriver and to sources of alkalinity in the Colorado Mineral Belt across Wilkins Peak Member deposition, and shows that the greatest volumes of sediment were delivered from the Aspen paleoriver during deposition of marker beds A, B, D, and I, each of which were deposited coincident with prominent “hyperthermal” isotopic excursions documented in oceanic cores.

Simulating the postfailure behavior of the seismically- triggered Chiu-fen-erh-shan landslide using 3DEC

The dip-slope Chiu-fen-erh-shan landslide, estimated to have a volume of 36 × 106 m3, was triggered by the Mw 7.6 Chi-Chi earthquake. The debris covered an area of 1.95 km2 and claimed 36 lives. Although different discrete element methods have been applied to simulate the postfailure behavior of the Chiu-fen-erh-shan landslide in the last two decades, the lateral and dip-slope rock movements, the impact of the sliding debris against the hill in front of the slope toe and its final deposition have not been fully simulated. The major objective of this study was to investigate the possibility of applying three-dimensional (3D) numerical modeling code 3DEC to numerically model the debris movement and deposition of the Chiu-fen-erh-shan landslide under the impact of the Chi-Chi earthquake. By incorporating the spacing and orientation information of the joints and bedding planes, the slope was modeled as an assemblage of polyhedrons. Seismic ground motions were simultaneously applied in three dimensions. The shoveling effect was considered to erode the hill in front of the slope toe so as to extend the impact area and match more closely actual depositional topography. This study showed that the 3DEC simulations captured essential 3D characteristics of the landslide and resulted in a postfailure configuration similar to that observed in the field.

Entangled external and internal controls on submarine fan evolution: an experimental perspective

AbstractSubmarine fans are formed by sediment‐laden flows shed from continental margins into ocean basins. Their morphology represents the interplay of external controls such as tectonics, climate and sea level with internal processes including channel migration and lobe compensation. However, the nature of this interaction is poorly understood. Physical modelling was used to represent the evolution of a natural‐scale submarine fan deposited during an externally forced waxing‐to‐waning sediment supply cycle. This was achieved by running five successive experimental turbidity currents with incrementally increasing then decreasing sediment supply rates. Deposits built upon the deposits of earlier flows and the distribution of erosion and deposition after each flow was recorded using digital elevation models. Initially, increasing sediment supply rate (waxing phase) led to widening and deepening of the slope channel, with basin‐floor deposits compensationally stepping forwards into the basin, favouring topographic lows. When sediment supply rate was decreased (waning phase), the slope‐channel filled as the bulk of the deposit abruptly back‐stepped due to interaction with depositional topography. Therefore, despite flows in the waxing and waning phases of sediment supply having nominally identical input conditions (i.e. sediment concentration, supply rate, grain size, etc.), depositional relief led to development of markedly different deposits. This demonstrates how external controls can be preserved in the depositional record through the progradation of basin floor deposits but that internal processes such as compensational stacking progressively obscure this signal through time. This evolution serves as an additional potential mechanism to explain commonly observed coarsening and thickening‐upwards lobe deposits, with abrupt transition to thin fine‐grained deposits. Meanwhile within the slope channel, external forcing was more readily detectable through time, with less internally driven reorganization. This validates many existing conceptual models and outcrop observations that channels are more influenced by external forcing whilst internal processes dominate basin floor lobe deposits in submarine fans.

Mass transport deposit (MTD) relief as a control on post-MTD sedimentation: Insights from the Taranaki Basin, offshore New Zealand

Mass transport deposits (MTDs) can affect the routing of turbidite systems and their resultant sedimentary architecture. With the aid of a high-quality 3D-seismic reflection dataset a buried Pleistocene MTD has been identified in the deep-water Taranaki Basin. Three MTD seismic facies have been identified and fall within three morphological domains – extensional, translational and contractional areas. Thickness relationships between the MTD and the overburden deposits were investigated quantitatively and matched using TWT-thickness maps. The results reveal that in proximal and distal areas with significant MTD relief, occupied by megaclasts and folded facies respectively, post-MTD accommodation is dominated by small ‘micro-basins’ with tortuous sediment pathways. In contrast the translational domain, containing homogeneous MTD facies, hosts larger low-relief basins. These characteristics can be used to define two main types of depositional topography: Type I, High-relief depositional topography, is observed prominently in the headwall and toe regions of MTDs, resulting in loosely-connected and isolated ponds of sediment; Type II, Low-relief depositional topography, is observed in the translational domain where the effects of sediment loading are clearly observed. Turbidite sandstone distribution affected by Type I topography, is characterised by irregular pinch-outs which present a significant risk for stratigraphic trapping whereas Type II topography may form overlying sedimentary bodies which are more extensive. Turbidite deposits in both settings are prone to syn-depositional remobilisation on top of the MTD, which may introduce considerable heterogeneity, particularly at reservoir margins.

Inherited Depositional Topography Control on Shelf-Margin Oversteepening, Readjustment, and Coarse-Grained Sediment Delivery to Deep Water, Magallanes Basin, Chile

A basin-margin depositional system is the stratigraphic product of terrigenous sediment delivery to the ocean, comprising a flat to low-gradient shelf, or topset, which transitions to a steeper deep-water slope, and, ultimately, a relatively flat basin floor. Erosional and depositional processes across these physiographic domains approximate a clinoform in the stratigraphic record. The shelf margin is a critical environment for terrigenous sediment dispersal because it links the shelf to deep water, and the coarse-grained deposits of associated strata are important subsurface reservoirs. Here, we characterize the shelf-margin and upper slope stratigraphy of the outcropping Upper Cretaceous Tres Pasos and Dorotea formations, Magallanes Basin, southern Chile. The Late Cretaceous Magallanes retroarc foreland basin was an elongate trough oriented parallel to the southern Andean arc and fold-and-thrust belt. The Tres Pasos and Dorotea formations record southward (basin axial) progradation of a high-relief shelf and slope system (>1000 m paleo-water depth) represented by a stratigraphic succession up to 3 km thick that is exposed for tens of kilometers along depositional dip. The character and distribution of deposits that define shelf margins contain evidence for a variety of processes related to deposition, erosion, sediment bypass, and mass wasting. The overall architecture of the Magallanes Basin strata is indicative of a graded basin-margin system interrupted by periods of slope oversteepening and development of out-of-grade conditions. These punctuated periods are recognized by sedimentological evidence for enhanced sediment bypass of the upper slope, and thick submarine fan successions in more distal segments. Development of oversteepened depositional topography is particularly significant as it instigated the only two major periods of coarse-grained sediment delivery to deep water over ~8 Myr during the Campanian. The controls on sediment dispersal beyond the shelf margin are commonly discussed in terms of allogenic forcings, such as tectonics, climate, and receiving-basin geometry, as well as autogenic behavior, such as delta-lobe switching. However, inherited depositional topography does not clearly fit within an allogenic/autogenic dichotomy. Depositional topography inherited from basin-margin evolution influences the position of subsequent shelf margins, which promote coarse-grained sediment delivery to deep water.

Axial and transverse deep‐water sediment supply to syn‐rift fault terraces: Insights from the West Xylokastro Fault Block, Gulf of Corinth, Greece

AbstractDeep‐water syn‐rift systems develop in partially‐ or transiently‐linked depocentres to form complicated depositional architectures, which are characterised by short transport distances, coarse grain sizes and a wide range of sedimentary processes. Exhumed systems that can help to constrain the tectono‐stratigraphic evolution of such systems are rare or complicated by inversion tectonics. Here, we document a mid‐Pleistocene deep‐water syn‐rift system fed by Gilbert‐type fan deltas in the hangingwall of a rift margin fault bounding the West Xylokastro Horst block, on the southern margin of the Gulf of Corinth, Greece. Structural and stratigraphic mapping combined with digital outcrop models permit observations along this syn‐rift depositional system from hinterland source to deep‐water sink. The West Xylokastro Fault hangingwall is filled by two distinct sediment systems; an axial system fed by coarse‐grained sediment gravity flows derived from fault‐tip Gilbert‐type fan deltas and a lateral system dominated by mass transport deposits fed from an evolving fault‐scarp apron. Abrupt changes in stratigraphic architecture across the axial system are interpreted to record changes in relative base level, sediment supply and tectonics. Locally, depositional topography and intra‐basinal structures controlled sediment dispersal patterns, from bed‐scale infilling of local rugose topography above mass transport complexes, to basin‐scale confinement from the fault scarp apron. These acted to generate a temporally and spatially variable, heterogeneous stratigraphic architecture throughout the basin‐fill. The transition of the locus of sedimentation from a rift margin to a fault terrace through the syn‐sedimentary growth of a basinward fault produced regressive surfaces updip, which manifest themselves as channels in the deep‐water realm and acted to prograde the system. We present a new conceptual model that recognises coeval axial and transverse systems based on the stratigraphic architecture around the West Xylokastro fault block that emphasizes the lateral and vertical heterogeneity of rift basin‐fills with multiple entry points.

Ramp reef depositional facies model for the Mid-Pliocene Golden Gates Reef Member of the Tamiami Formation, South Florida

The depositional setting for northern most Atlantic coral reef development during the Mid-Pliocene Warm Period is a gentle sloping mixed carbonate–siliciclastic ramp. Five core transects document the distribution of the reef complex in an area approximately 65 by 12 km, and nine repetitive depositional facies are identified. Paleoecological and sedimentological evidence documents facies development along a water depth–energy gradient. The mid-Pliocene Tamiami Formation is characterized by coral boundstone developed over level skeletal rudstone depositional units dominated by mollusks. Hyotissa haitensis (Sowerby), one of the last Gryphaeid oysters, is the dominant fossil found in the most continuous skeletal facies and overlies deeper water green clay facies, the only facies with pelagic foraminifera. Ground penetrating radar documents reef depositional topography, onlapping stratigraphy and two episodes of reef growth. Two cycles of deposition are recognized, separated by subaerial exposure. The coral boundstone and the skeletal rudstone exhibit both high primary and secondary porosity and overlie the impermeable clay facies. The upper surface of the coral boundstone lies at ~ 4 m in elevation whereas contemporaneous estuarine deposits are found to the north at elevations of 20–25 m. High porosity bank reef complexes along a shallow dipping ramp provide an alternative to the standard model of reef and porosity development along the outer shelf margin. Understanding the differences in associated facies between these two depositional environments permits better interpretation of observed heterogeneities in subsurface geobodies associated with inner shelf and platform settings.

Turbidite, debrite, and hybrid event beds in submarine lobe deposits of the Palaeocene to middle Eocene Kapit and Pelagus members, Belaga Formation, Sarawak, Malaysia

Turbiditic flysch units of the Rajang Group form a large crescentic belt of deformed strata in central Borneo. Unfortunately, our understanding of its bed type characteristics and depositional setting is very poor. Here, we present a detailed bed type analysis of the Palaeocene to middle Eocene Kapit and Pelagus members of the Belaga Formation, based on detailed investigations of recent road‐cut exposures around Sibu, Sarawak, Malaysia. Five bed types are identified from the studied sections, representing deposition from turbidity currents, debris flows, and flows that show turbulent and laminar characteristics. The Belaga Formation deposits are divided into four bed type associations based on bed type assemblages, bed geometry, degree of bed amalgamation, vertical grain size, and bed thickness trends, for example, lobe axis, lobe off‐axis, lobe fringe, and slump. Hybrid event beds (HEBs) are not restricted only to the lobe fringe but are also a common element of the lobe axis sub‐environment in the Belaga Formation system. Their common occurrence in a proximal location in the lobes is probably due to enhanced seafloor erosion and rapid deceleration due to loss of confinement at the channel lobe transition further up‐dip. Evidence such as bidirectional ripple cross‐lamination in thin‐bedded turbidites and variable palaeocurrent orientations suggest a complex depositional topography, which also suppressed flow turbulence and promoted deposition of hybrid event beds in proximal locations through deflection and deceleration of incoming flows by confining counter slopes. Similar bed type assemblages within the slump and lobe deposits indicate a local origin for the slumps. Localized failures can form on low gradient slopes developed in tectonically active basins with complex topography.

A depositional model for deep‐lacustrine, partially confined, turbidite fans: Early Cretaceous, North Falkland Basin

AbstractThis paper presents a model of facies distribution within a set of early Cretaceous, deep‐lacustrine, partially confined turbidite fans (Sea Lion Fan, Sea Lion North Fan and Otter Fan) in the North Falkland Basin, South Atlantic. As a whole, ancient deep‐lacustrine turbidite systems are under‐represented in the literature when compared with those documented in marine basins. Lacustrine turbidite systems can form extensive, good quality hydrocarbon reservoirs, making the understanding of such systems crucial to exploration within lacustrine basins. An integrated analysis of seismic cross‐sections, seismic amplitude extraction maps and 455 m of core has enabled the identification of a series of turbidite fans. The deposits of these fans have been separated into lobe axis, lobe fringe and lobe distal fringe settings. Seismic architectures, observed in the seismic amplitude extraction maps, are interpreted to represent geologically associated heterogeneities, including: feeder systems, terminal mouth lobes, flow deflection, sinuous lobe axis deposits, flow constriction and stranded lobe fringe areas. When found in combination, these architectures suggest ‘partial confinement’ of a system, something that appears to be a key feature in the lacustrine turbidite setting of the North Falkland Basin. Partial confinement of a system occurs when depositionally generated topography controls the flow‐pathway and deposition of subsequent turbidite fan deposits. The term ‘partial confinement’ provides an expression for categorising a system whose depositional boundaries are unconfined by the margins of the basin, yet exhibit evidence of internal confinement, primarily controlled by depositional topography. Understanding the controls that dictate partial confinement; and the resultant distribution of sand‐prone facies within deep‐lacustrine turbidite fans, is important, particularly considering their recent rise as hydrocarbon reservoirs in rift and failed‐rift settings.

Statistical pattern analysis of surficial karst in the Pleistocene Miami oolite of South Florida

A robust airborne light detection and ranging digital terrain model (LiDAR DTM) and select outcrops are used to examine the extent and characteristics of the surficial karst overprint of the late Pleistocene Miami oolite in South Florida. Subaerial exposure of the Miami oolite barrier bar and shoals to a meteoric diagenetic environment, lasting ca. 120 kyr from the end of the last interglacial highstand MIS 5e until today, has resulted in diagenetic alteration including surface and shallow subsurface dissolution producing extensive dolines and a few small stratiform caves.Analysis of the LiDAR DTM suggests that >50% of the dolines in the Miami oolite have been obscured/lost to urbanization, though a large number of depressions remain apparent and can be examined for trends and spatial patterns. The verified dolines are analyzed for their size and depth, their lateral distribution and relation to depositional topography, and the separation distance between them. Statistical pattern analysis shows that the average separation distance and average density of dolines on the strike-oriented barrier bar versus dip-oriented shoals is statistically inseparable. Doline distribution on the barrier bar is clustered because of the control exerted on dissolution by the depositional topography of the shoal system, whereas patterning of dolines in the more platform-ward lower-relief shoals is statistically indistinguishable from random. The areal extent and depth of dissolution of the dolines are well described by simple mathematical functions, and the depth of the dolines increases as a function of their size. The separation and density results from the Miami oolite are compared to results from other carbonate terrains. Near-surface, stratiform caves in the Miami oolite occur in sites where the largest and deepest dolines are present, and sit at, or near, the top of the present water table.

Unsung Pioneers of Sequence Stratigraphy: Eliot Blackwelder, Joseph Barrell, Amadeus Grabau, John Rich and Harry Wheeler

Few facies analysts or stratigraphers would argue against the contention that the transition to facies-driven sequence strati- graphic correlations represents one of the most important paradigm shifts in modern stratigraphy. What is less commonly appreciated is that most of the fundamental underpinnings of sequence stratigraphy were derived from a relatively small number of pioneers in the early to mid-1900s; most of them largely unknown or underappreciated by the current generation of sequence stratigraphers. Chief among these are Eliot Blackwelder, Amadeus Grabau, Joseph Barrell, John Rich, and Harry Wheeler. Blackwelder was perhaps the first to point out the presence and significance of regional unconformities in the packaging of strata in North America, which would later come to form the basis of the well-known cratonic megasequences of Larry Sloss. However, Blackwelder is best appreciated as the mentor and colleague of not only these other pioneers, but also of key workers who themselves ultimately mentored some of the most renowned sequence stratigraphers of the 1970s and 1980s. Amadeus Grabau focused his attention on expounding the law of the correlation of facies of Johannes Walther and bringing facies-driven correlations into stratigraphy; an approach at odds with the then prevailing view of lithostratigraphy as the principal physical stratigraphic framework. Grabau recognized that the rock record contains numerous temporal gaps that partition facies succes- sions, for which he defined the term “hiatus”. His Pulsation theory was used to explain the cycles of deposition and hiatal erosion/bypass, and although the mechanism is different, the net outcome of such “oscillations” is broadly similar to the effects of eustacy and tectonism that we assign to such changes today. Joseph Barrell developed the concept of base level and explored its role in controlling erosion versus deposition. Like Grabau, Barrell insisted that sedimentation was not continuous, leading to a stratigraphy that is replete with breaks of varying durations. For these he coined the term “diastems” for the small but more numerous breaks, which could be contrasted with those reflecting longer breaks which he referred to as “discontinuities”. John Rich evaluated depositional topography, for which he coined the terms clinoform, undaform and fondoform. He recognized that such depositional bodies necessarily link genetically related associations of sediment deposited from the shoreline to the basin center; what today we regard as the depositional system. The resulting architectures bounded by these surfaces form the underpinning of all systems tracts in modern sequence stratigraphy. Harry Wheeler formalized the concept of time-stratigraphy, demonstrating that temporal gaps in stratigraphy are as important in understanding the rock record as the rocks themselves. His novel approach of developing stratigraphic cross-sections with time on the vertical axis pioneered the concept of chronostratigraphy through the development of what are now referred to as Wheeler Diagrams. His resurrection of the base-level concept expounded by earlier workers was instrumental in defining sequences as unconformity-bounded units, re-establishing the concepts pio- neered by Blackwelder himself. In concert, these stratigraphic visionaries erected a stratigraphic framework focused on the understanding of the stratigraphic record as opposed to its simple lithostratigraphic mapping or biostratigraphic dating. These unsung pioneers put into place virtually all components of modern sequence stratigraphy more than two decades before its popularization in the scientific commubity.

Highly Variable Precambrian Fluvial Style Recorded In the Nelson Head Formation of Brock Inlier (Northwest Territories, Canada)

Abstract Spectacular canyon exposures of the ∼ 1 Ga Nelson Head Formation along the modern Brock River, Northwest Territories, provide a rare opportunity to assess the deposits of pre-vegetation, braided to sinuous-channelized fluvial systems. We analyze the sedimentology, architecture, and depositional evolution of 16 stacked fluvial-channel belts at this site, demonstrating greater variability in sedimentary style and morphodynamics than is typically interpreted for Precambrian rivers. Inferences on fluvial planview based on integrated analysis of depositional architecture and paleoflow reveal complex patterns of channel-planform evolution, and four depositional stages are recognized: (1) floodbasin-splay progradation with bounding episodes of eolian reworking; (2) deposition in wandering-channel belts characterized by deep anabranches and prominent lateral accretion; (3) progressive shift towards shallow, braided-channel belts confined within alluvial valleys and dominated by downstream accretion; and (4) deposition by marine-influenced braided-channel belts with mixed downstream, lateral, and upstream accretion. Overall, the studied fluvial-channel belts point to significant morphodynamic complexity and geomorphic variability, challenging the assumption that all pre-Silurian rivers shared poor channelization and low sinuosity. Observed channel bodies also have width:thickness ratios strongly overlapping with those of rivers postdating the rise of vegetation. In both wandering- and braided-channel belts, complex patterns of channel migration, bar accretion, and dissection were the result of co-acting processes such as development of transverse-velocity zones within the channels, local paleoflow disturbance induced by depositional topography, and avulsion. In concert with the regional extent and paleoflow, and increasingly distal character of correlative stratigraphic units towards the northwest, the magnitude of the observed trunk-fluvial channels is consistent with a mature drainage capable of transecting the Laurentian craton over thousands of kilometers.

Along-strike sequence stratigraphy across the Cretaceous shallow marine to coastal-plain transition, Wasatch Plateau, Utah, U.S.A.

Current sequence stratigraphic models deal with stratal packages almost exclusively along depositional-dip. A high-resolution outcrop example detailing sequence stratigraphic architecture along depositional-strike is largely lacking. This outcrop study provides a detailed sequence stratigraphic correlation along depositional-strike for a length of ~50km. Moreover, this study focuses on the facies and stratigraphic relationships particularly at the very transition between coeval marine and nonmarine strata, which is relatively underdocumented.Within the Mesaverde Group of central and eastern Utah, the Upper Cretaceous shallow-marine Star Point Sandstone and paralic Blackhawk Formation are relatively well studied along depositional-dip in the Book Cliffs, but the nature of their spatio-temporal transition particularly along depositional-strike in the NNE-SSW trending Wasatch Plateau remains poorly documented. Facies-scale to stratigraphic-scale data were gathered from ten outcrop “windows” along the eastern margin of the Wasatch Plateau. The vertical and lateral transition between marine and nonmarine strata varies in complexity within the study area. In the southern part of the study area near the central Wasatch Plateau, the shallow-marine Star Point Sandstone passes stratigraphically upward into the nonmarine Blackhawk Formation in one simple transition without any intercalation of marine and nonmarine strata. In the northern part of the study area, however, where the Star Point Sandstone to Blackhawk Formation transition is complex, aggradationally stacked shallow-marine sandbodies (i.e., parasequences) taper and completely pinch-out within coastal-plain mudstones in a paleo-landward direction over short distances (<1km), representing potential stratigraphic traps. These marine sandbodies are intercalated with coeval nonmarine strata within the Blackhawk Formation. Fourth-order sequence boundaries and flooding surfaces were correlated along the lower Campanian depositional-strike.The results reveal along-strike undulations in the early Campanian depositional topography, which was characterized by alternating incised valleys, interfluvial topographic highs with mature soil development, and shoreline embayments. In particular, the variable along-strike transition-complexity from shallow-marine to coastal-plain strata is the result of differential tectonic subsidence, where the transition-complexity increases and the stacking pattern is more aggradational towards an early Campanian depocenter in the north of the study area. This study reveals that differential subsidence is the key in controlling along-strike variability of sequence stratigraphy and that correlation of sequence stratigraphic surfaces are far more challenging in nonmarine strata than in adjacent marine strata.

Allostratigraphy and paleogeography of the Harmon and Cadotte members of the Peace River Formation (Middle Albian), west-central Alberta and adjacent British Columbia

Abstract The mudstone-dominated Harmon and sandstone-dominated Cadotte members of the Peace River Formation were deposited during the middle Albian in the southern end of a Boreal marine embayment that occupied the foredeep of the Western Canada Foreland Basin. Sandstone of the Cadotte Member represents a shoreface environment and has traditionally been interpreted as the updip equivalent of offshore Harmon mudstone. Allostratigraphic correlation, based on marine flooding surfaces and unconformities shows that the Harmon alloformation can be divided into five regionally-mappable allomembers. Similar erosion surfaces allow the Cadotte alloformation to be divided into three allomembers. The new allostratigraphy shows that the informal Harmon and Cadotte alloformations do not have an interfingering relationship and hence are not genetically related. Harmon and Cadotte allomembers have been mapped over about 100 000 km2, but show no evidence of stratal lap-out or depositional topography; it is concluded that deposition took place on a shallow marine ramp of very low relief. The vertical stacking of Harmon and Cadotte allomembers across the foredeep indicates that rates of accommodation and sediment supply remained in approximate equilibrium throughout deposition of these units. In the far southern and eastern parts of the study area, changing gamma ray log response suggests that marine deposits of both the Harmon and Cadotte alloformations grade laterally into coeval coastal plain deposits that are mapped in outcrop as Gates Formation whereas in subsurface wireline log picks, these rocks are usually included in the Upper Mannville Group. The lateral boundary between offshore mudstone and coeval nearshore muddy sandstone can be mapped within individual Harmon allomembers; the mudstone to sandstone boundary has a lobate to cuspate pattern in plan view, suggesting that, at the southern extremity of the ‘Harmon Sea’ rivers built elongate deltas in a very shallow-water, low wave-energy setting. Lenticular, sharp-based sandstone bodies 17–42 m thick hang from allomember bounding surfaces in both the Harmon and Cadotte alloformations; these sandbodies probably represent paleovalley fills. Valley incision may have been linked to relative sea-level changes that also gave rise to the marine transgressive surfaces that are traceable over the entire study area. In the east, both the Cadotte and Harmon alloformations are completely truncated by a bevelling unconformity at the base of the Paddy and the laterally-contiguous Joli Fou alloformation.

불투과성 퇴적지형에 의한 다열 수중구조물 성능 평가

오랜 시간동안의 인접한 두 수중구조물 사이 퇴적 작용은 수중구조물의 성능을 감소시켜 해안 도시 안전에 위협을 가할 수 있다. 본 연구에서는 다열 수중구조물 사이 불투과성 퇴적지형에 의한 파랑의 반사율을 고유함수전개법을 이용하여 해석하였다. 경사를 표현하는데 필요한 구간의 수와 반사율 계산에 필요한 소멸파 성분의 수를 제안하였으며 기존 결과와 비교하였다. 다열 수중구조물 사이 퇴적양상, 퇴적 양에 따른 성능 변화와 퇴적에 따른 Bragg 반사 특성 변화를 살펴보았다. 퇴적지형이 다열 수중구조물 성능에 영향을 주는 것을 확인하였다. 그러므로 다열 수중구조물 설계 시, 퇴적에 의한 성능 변화를 고려해야하다. The deposition between the two adjacent submerged structures over a long time can decrease the roles of structures and also threaten the safety of the coastal communities. In this study, the reflection of the incident water waves caused by deposition among multi-arrayed submerged structures are investigated by using the eigenfunction expansion method. The proper number of the steps representing a slope and the evanescent modes for calculating the reflection coefficient was suggested, and the resulting reflection coefficients were compared with the previous results. Not only the performance change due to difference styles of deposition and the amount of deposition between multi-arrayed submerged structures, but also the characteristic changes in the Bragg reflection caused by the deposition were studied. We found that the depositional topography affects the performance of multi-arrayed submerged structures. Thus, when designing multi-arrayed submerged structures, we have to consider the performance change by deposition.

The Influence of A Lateral Slope On Turbidite Lobe Development On A Modern Deep-Sea Slope Fan (Villafranca Deep-Sea Fan, Tyrrhenian Sea)

Abstract In deep-sea fans, fan lobes form from the stacking of the deposits of turbidity currents and other sediment gravity flows beyond the channel mouth. Slopes lateral or oblique to the main direction of turbidity currents exiting channel confinement can profoundly affect flow behavior and the distribution of facies and lithology in fan lobes. In this paper, we report on the analysis of multibeam and CHIRP subbottom data aimed at the study of the effects of a lateral slope on turbidity-current behavior and resultant geomorphology and stratigraphy of a modern fan lobe. The Villafranca fan lobe, the subject of this study, develops on the 0.5° dipping seafloor of the Gioia intraslope basin in the southeastern Tyrrhenian Sea. The Villafranca is a transient fan lobe that has its downslope limit coinciding with a 200-m-high bathymetric step formed by the erosional flank of the Stromboli slope valley, perpendicular to the fan-lobe trend. A “lobe complex” hierarchical level is assigned to the Villafranca fan lobe. The lobe complex develops beyond the mouth of the Villafranca leveed channel, which trends N and is oblique to the regional NW-dipping slope. The depositional topography created by an adjacent channel levee wedge to the west is the cause of the observed obliquity. The western and the eastern “lobes” constitute the Villafranca lobe complex. Both lobes have mainly NW-trending channels, and therefore they do not conform to the classical divergent pattern of channels observed in deep-sea fan lobes. The channel trend is a result of the inability of flows to freely spread sideways, due to the presence of the lateral slope and the dip of the seafloor at the channel mouth. The western lobe has an up-dip apex with respect to the eastern one and is characterized by channels that nucleate on the downslope side of a channel-mouth bar and that die out distally before reaching the flank of the Stromboli slope valley. The eastern lobe, in contrast, has channels that are depositional upslope but on approaching the flank of Stromboli slope valley become mainly erosional or bypass features. The two lobes develop concomitantly, therefore an evolution due to compensational stacking with the two lobes being in different stages of development is here disregarded. It is therefore feasible that turbidity currents, which due to different initiating mechanisms have distinct magnitude and efficiency, deposit their load either in the eastern or in the western lobe. Another possibility is that single turbidity currents exiting the Villafranca channel mouth form two separate flow portions with different properties, which independently but simultaneously feed the eastern and the western lobe. In both cases, the effect of the lateral slope and the obliquity between the dip of the slope and the trend of the channel mouth are here considered as the primary factors controlling the differences in depositional style of the two lobes.

Karstification of reef-bank facies carbonate and its control on pore distribution: A case study of Upper Ordovician Lianglitage Formation in eastern Tazhong area, Tarim Basin, NW China

Abstract The karstification type and characteristics and its control on pore distribution in reef-bank carbonate rock in the Upper Ordovician Lianglitage Formation (O 3 l) in eastern Tazhong area were studied using cores, thin sections and well logging data. The area experienced two types of karstification: syndepositional karstification and early diagenetic karstification near surface. During the depositional stage of reef-bank complex, meteoric diagenetic lens with lots of selective dissolution pores were formed by the syndepositional karstification, they were developed in the middle and upper parts of reef-bank sedimentary cycles, and a single porous/cavity layer has a thickness of 10–30 m, increasing the absolute porosity by 3%–4%. After depositional stage, sediments were uplifted and exposed without experiencing burial diagenesis, the depositional topography was influenced by early diagenetic karstification near surface and turned into karst paleotopography and formed four karstification zones within 100 m below the top of the Upper Lianglitage Formation. The surface karstification zone and radial flow karsitifiction zone have favorable porous connectivity, increasing the absolute porosity by 4%-8%. These two types of karsitifiction are continuous in diagenesis stage and the formed porosity is successive in distribution. The favorable space-time coupling is the key reason for effective pores being largely developed in the O 3 l 3 –O 3 l 1 members.

The uncertainties involved in extracting amplitude and frequency of orbitally driven sea‐level fluctuations from shallow‐water carbonate cycles

AbstractIn outcrops, shallow‐water carbonates often form thick, vertically stacked, metre‐scale, exposure‐bounded depositional packages, here called carbonate cycles. Because the facies within carbonate cycles can be tied precisely to water depths, they are considered ideal for reconstructing past sea‐level changes. There is, however, increasing evidence that these depositional cycles are unreliable recorders of both the frequency and the amplitude of orbitally driven sea‐level fluctuations. Carbonate cycles record only a fraction of the amplitude of a sea‐level cycle: a portion of the rise and nothing of the fall. Cores through the Pleistocene cycles on Great Bahama Bank illustrate this shortcoming. Although the amplitudes of the last nine sea‐level changes are each a hundred metres or more, the thickness of the cycles varies from a few metres to ca 15 m. The lack of correlation between sea‐level amplitude and cycle thickness is not eliminated for cycles deposited during times of lower sea‐level amplitudes, for example, the Cretaceous. Upper Cretaceous cycles on the Maiella platform margin document the irregularly filled accommodation space and the resultant variability in cycle thickness and frequency. Uncertainties in assessing the frequencies of sea‐level changes from shallow‐water carbonate cycles are caused by ‘missed beats’ and metre‐scale oscillations of sea‐level within highstands that potentially produce cycles of very short duration. The random amplitude variability during the last 57 glacio‐eustatic sea‐level changes illustrates the difficulty of assessing ‘missed beats’, where a sea‐level fluctuation is not recorded because the sea‐level rise does not reach the platform top. ‘Missed beats’ are also produced by the depositional topography that is created by irregularly filled accommodation space. As a result, variable numbers of cycles are deposited across the platform. Further complicating orbital frequency analyses are decametre‐scale oscillations of the sea‐level during highstands. The amplitude of these sub‐orbital sea‐level oscillations (up to 17 m within the last interglacial, Marine Isotope Stage 5e) are sufficient to expose shallow platforms like Great Bahama Bank and subsequently produce an additional depositional cycle with similar facies successions. The combined effects of missed beats and oscillations within highstands are likely to produce cycles and hiatuses of variable duration that are difficult to extract from the rock record. Consequently, estimates of the orbital forcing mechanisms and frequencies from ancient shallow‐water carbonate cycles carry large uncertainties.

Controls on the deposition and preservation of the Cretaceous Mowry Shale and Frontier Formation and equivalents, Rocky Mountain region, Colorado, Utah, and Wyoming

Regional variations in thickness and facies of clastic sediments are controlled by geographic location within a foreland basin. Preservation of facies is dependent on the original accommodation space available during deposition and ultimately by tectonic modification of the foreland in its postthrusting stages. The preservation of facies within the foreland basin and during the modification stage affects the kinds of hydrocarbon reservoirs that are present. This is the case for the Cretaceous Mowry Shale and Frontier Formation and equivalent strata in the Rocky Mountain region of Colorado, Utah, and Wyoming. Biostratigraphically constrained isopach maps of three intervals within these formations provide a control on eustatic variations in sea level, which allow depositional patterns across dip and along strike to be interpreted in terms of relationship to thrust progression and depositional topography. The most highly subsiding parts of the Rocky Mountain foreland basin, near the fold and thrust belt to the west, typically contain a low number of coarse-grained sandstone channels but limited sandstone reservoirs. However, where subsidence is greater than sediment supply, the foredeep contains stacked deltaic sandstones, coal, and preserved transgressive marine shales in mainly conformable successions. The main exploration play in this area is currently coalbed gas, but the enhanced coal thickness combined with a Mowry marine shale source rock indicates that a low-permeability, basin-centered play may exist somewhere along strike in a deep part of the basin. In the slower subsiding parts of the foreland basin, marginal marine and fluvial sandstones are amalgamated and compartmentalized by unconformities, providing conditions for the development of stratigraphic and combination traps, especially in areas of repeated reactivation. Areas of medium accommodation in the most distal parts of the foreland contain isolated marginal marine shoreface and deltaic sandstones that were deposited at or near sea level lowstand and were reworked landward by ravinement and longshore currents by storms creating stratigraphic or combination traps enclosed with marine shale seals. Paleogeographic reconstructions are used to show exploration fairways of the different play types present in the Laramide-modified, Cretaceous foreland basin. Existing oil and gas fields from these plays show a relatively consistent volume of hydrocarbons, which results from the partitioning of facies within the different parts of the foreland basin.

Symmetrical Cone-Shaped Hills, Abaco Island, Bahamas: Karst or Pseudokarst?

2 Abstract: Abaco Island, Bahamas, contains a large number of conical hills that strongly resemble cone karst, a dissolutional landform associated with karst processes in tropical carbonate localities around the world. Field investigation demonstrated that the conical hills are roughly symmetrical in shape and consist of mid to late Pleistocene eolian calcarenites (carbonate sand dunes) partly mantled by talus. The original hummocky depositional topography of the eolian ridges has been dissected by a variety of processes including dissolution pit formation that enhances slope failure, vegetative disruption of the rock surface, and fire-induced exfoliation of the bedrock. The original asymmetrical shape of continuous dune ridges has thus been modified into a series of hills with a symmetrical cone shape, in which the steeper leeward depositional slopes of the dunes have been masked by talus to create a lesser slope that approximates the dip of the exposed bedrock slope of the more gentle windward depositional faces of the dunes. The conical hills are primarily constructional in nature, modified by mass-wasting slope processes only partly influenced by dissolution; therefore these conical hills are not true cone karst, but pseudokarst, despite their cone shape and their development in soluble carbonate rock. Abaco is the only island in the Bahamian Archipelago with both high eolian relief and a significant positive water budget; that water budget supports the degree of dissolution and forest growth necessary to create the root wedging and fire- induced exfoliation that most modify the steep leeward slopes of the dunes. As the eolian calcarenites were deposited only during the brief glacioeustatic sea-level highstands of the last few hundred thousand years, the development of cone-shaped hills in these eogenetic rocks has been geologically very rapid.