Abrupt Facies Changes Research Articles

Early Pleistocene upper bathyal communities in fault-bounded paleovalleys of the island of Rhodes (Greece)

Abstract Two sediment sections are investigated at Cape Arkhangelos, island of Rhodes, where Pleistocene marine sediments crop out in horsts and grabens of a Mesozoic basement. There, hemipelagic sediments characterized by upper bathyal communities are atypically mixed with much shallower faunal components because they were deposited close to rugged coastal landforms. Biostratigraphic analyses show that the sections were deposited between 1.8 and 0.9 Ma, and between 1.8 and 1.6 Ma, respectively. By combining the planktonic/benthic foraminiferal ratio with 31 bathymetric indicators chosen among extant species of benthic foraminifera, mollusks, and bryozoans, we show that relative sea-level fluctuations can be reconstructed in these atypical settings despite the proximity of steep slopes that favored transportation of allochthonous fauna. The shallow-water components (including gravels and calcareous algae) were transported downslope by the combined action of gravity, currents, and tectonic disturbance that promoted drowning (with a maximum flooding recorded at ca. 1.7 Ma) and then uplift of fault-bounded paleovalleys that formed during the Early Pleistocene. Abrupt facies changes and age differences between sections have been triggered by the irregular paleotopography of the Mesozoic basement, which fostered differential depositional settings, with outer to middle neritic deposits above the horsts and upper bathyal deposits in paleovalleys.

Near - surface characterization of sediments of the Sokoto group exposed around Wamakko area, Northwestern Nigeria: an integrated approach

ABSTRACT An integrated approach involving sedimentological and geophysical studies was carried out to characterize the surface and subsurface geology of part of the southwestern Iullemmeden basin (Sokoto Basin, North-western Nigeria). The Kalambaina Formation (limestone) of the Sokoto Group is being investigated. The general objective of this study was to optimize lateral and vertical resolution amidst the subsurface lithological heterogeneity and that informed the choice of integrating sedimentology and geophysics (involving Vertical Electrical Sounding – VES and Electrical Resistivity Tomography – ERT). Surface geological mapping complemented by geophysical (4 VES and 2 ERT) surveys were carried out. Data integration involved geological (outcrop stratigraphic data), geophysics and data from down-hole core descriptions within the area of investigation. Results of 1D resistivity (VES) yielded QH and QHQ curve types with OH being predominant. Based on their contrasting resistivities, four-layer geoelectric sections was erected using VES data (ironstone topsoil, limestone, shale and clay/sandy clay layers). From the ERT modelling, four lithologic sections were also identified. These lithologic discriminations correlated well with few borehole data within the study area. With integration of geophysics with stratigraphic data , a subsurface facies distribution model (fence diagram) was developed having a high vertical and lateral resolution. This study revealed that during reconnaissance mapping, with integrated approach using resistivity and sedimentological data, accurate subsurface imaging could be achieved, at a reduced exploration cost, even in areas of abrupt facies changes (lithological heterogeneities).

Testing of Permian – Lower Triassic stratigraphic data in a half-graben/tilt-block system: evidence for the initial rifting phase in Antalya Nappes

Testing of Middle Permian – Lower Triassic stratigraphic data from the Antalya Nappes in a half-graben/tilt-block system has revealed the presence of episodic rifting events separated by periods of tectonic quiescence. Following a period of uplift during the Permian (Late Artinskian to Roadian), the basement rocks have been activated by displacement faults and several depocenters in half-graben-like asymmetrical basins began to be filled with Roadian to Wordian continental clastic deposits intercalated with coal and marine rocks. The Early Capitanian time was a period of tectonic quiescence. The second event occurred in Middle to Late Capitanian times and produced basaltic volcanic rocks intercalated in the shallow marine fossiliferous carbonate successions. Following the Lopingian (Wuchiapingian and Changhsingian) and Permian–Triassic boundary interval representing a long tectonic quiescence, the last rifting episode started with an abrupt facies change in the late Griesbachian. Variegated shales, limestones, volcanics, talus breccia, and debris flow deposits were laid down in a half-graben/tilt-block system. As normal faulting has become active, the deposition continued on the subsiding hanging wall side. The stratigraphic gap increased in magnitude as the erosional truncation has incised deeply the footwall side. This initial rifting phase in the Antalya Nappes is prior to the onset of a stronger and more continuous rifting event that occurred in the Anisian–Carnian interval including a variety of deepwater clastic and carbonate deposits, radiolarites containing sometimes blocks and clasts derived from the basin margins, and volcanic rocks carrying intraoceanic setting character.

Stratigraphic evolution of the Neoproterozoic Callison Lake Formation: Linking the break-up of Rodinia to the Islay carbon isotope excursion

The ∼780 to 540 Ma Windermere Supergroup of western North America records the protracted development of the western Laurentian passive margin and provides insights into the nature, timing, and kinematics of Rodinia9s fragmentation. Here we present a refined tectono- and chemo-stratigraphic model for circa 780 to 720 Ma sedimentation in NW Canada through a study of the Callison Lake Formation (formalized herein) of the Mount Harper Group, spectacularly exposed in the Coal Creek and Hart River inliers of the Ogilvie Mountains of Yukon, Canada. Twenty-one stratigraphic sections are integrated with geological mapping, facies analysis, carbon and oxygen isotope chemostratigraphy, and Re-Os geochronology to provide a depositional reconstruction for the Callison Lake Formation. Mixed siliciclastic, carbonate, and evaporite sediments accumulated in marginal marine embayments formed in discrete hangingwall depocenters of a prominent Windermere extensional fault zone. Deposition of the Windermere Supergroup in NW Canada post dates the eruption of the circa 780 Ma Gunbarrel Large Igneous Province by ∼30 million years, is locally associated with compressional or transpressional tectonism, and predates the successful rift-drift transition by ∼200 million years. In order to accommodate evidence for coeval extensional and compressional tectonism, abrupt facies change, and Neoproterozoic fault geometries, we propose that NW Laurentia experienced strike-slip deformation during the ∼740 to 660 Ma early fragmentation of the supercontinent Rodinia. Sequence stratigraphic data from the Callison Lake Formation and other basal Windermere successions in the northern Canadian Cordillera delineate three distinct depositional sequences, or transgressive-regressive (T-R) cycles, that are coeval with similar stratigraphic packages in the ∼780 to 720 Ma Chuar-Uinta Mountain-Pahrump basins of the western United States. The global circa 735 Ma Islay carbon isotope excursion is consistently present in carbonate strata of the third T-R cycle and is interpreted to represent a primary perturbation to the global carbon cycle, possibly driven by the uplift and weathering of extensive shallow epicontinental seaways and evaporite basins.

Geochemistry of post-extinction microbialites as a powerful tool to assess the oxygenation of shallow marine water in the immediate aftermath of the end-Permian mass extinction

Rapid and profound changes in earth surface environments and biota across the Permian–Triassic boundary are well known and relate to the end-Permian mass extinction event. This major crisis is demonstrated by abrupt facies change and the development of microbialite carbonates on the shallow marine shelves around Palaeo-Tethys and western Panthalassa. Microbialites have been described from a range of sites in end-Permian and basal Triassic marine sedimentary rocks, immediately following the end-Permian mass extinction. Here, we present geochemical data primarily focused on microbialites. Our geochemical analysis shows that U, V, Mo and REE (Ce anomaly) may be used as robust redox proxies so that the microbialites record the chemistry of the ancient ambient sea water. Among the three trace metals reputed to be reliable redox proxies, one (V) is correlated here with terrigenous supply, the other two elements (U and Mo) do not show any significant authigenic enrichment, thereby indicating that oxic conditions prevailed during the growth of microbialites. REE profiles show a prominent negative Ce anomaly, also showing that the shallow marine waters were oxic. Our geochemical data are consistent with the presence of some benthic organisms (ostracods, scattered microgastropods, microbrachiopods and foraminifers) in shallow marine waters that survived the mass extinction event.

Structural controls on the stratigraphic architecture of net‐transgressive shallow‐marine strata in a salt‐influenced rift basin: Middle‐to‐Upper Jurassic Egersund Basin, Norwegian North Sea

AbstractIn this study, we integrate 3D seismic reflection, wireline log, biostratigraphic and core data from the Egersund Basin, Norwegian North Sea to determine the impact of syn‐depositional salt movement and associated growth faulting on the sedimentology and stratigraphic architecture of the Middle‐to‐Upper Jurassic, net‐transgressive, syn‐rift succession. Borehole data indicate that Middle‐to‐Upper Jurassic strata consist of low‐energy, wave‐dominated offshore and shoreface deposits and coal‐bearing coastal‐plain deposits. These deposits are arranged in four parasequences that are aggradationally to retrogradationally stacked to form a net‐transgressive succession that is up to 150‐m thick, at least 20 km in depositional strike (SW‐NE) extent, and >70 km in depositional dip (NW‐SE) extent. In this rift‐margin location, changes in thickness but not facies are noted across active salt structures. Abrupt facies changes, from shoreface sandstones to offshore mudstones, only occur across large displacement, basement‐involved normal faults. Comparisons to other tectonically active salt‐influenced basins suggest that facies changes across syn‐depositional salt structures are observed only where expansion indices are >2. Subsidence between salt walls resulted in local preservation of coastal‐plain deposits that cap shoreface parasequences, which were locally removed by transgressive erosion in adjacent areas of lower subsidence. The depositional dip that characterizes the Egersund Basin is unusual and likely resulted from its marginal location within the evolving North Sea rift and an extra‐basinal sediment supply from the Norwegian mainland.

The full range of turbidite bed thickness patterns in submarine lobes: controls and implications

A widely misused criterion to interpret lobe deposits in submarine fan systems at outcrop, and in core and well logs, is a thickening and/or coarsening upward profile. Lobe deposits from the Laingsburg depocentre, SW Karoo Basin, demonstrate that a full range of bed thickness patterns exists within lobes. When lobes are defined by their laterally extensive bounding surfaces that are marked by abrupt facies changes, five types of bed stacking patterns are identified: thickening upward, thinning upward, thickening then thinning upward, thinning then thickening upward, and constant. The abrupt bounding surfaces are interpreted to record avulsion of feeder distributive channels. The stratigraphic bed thickness pattern preserved in a lobe reflects the internal organization of smaller-scale lobe elements, rather than lobe-wide initiation and progradation as implied by a thickening-upward only pattern. The full range of bed thickness patterns in lobes can be used to understand the stacking of lobe elements, the evolution of sediment deposition through time and space, and the relative movement of depocentres.

A Holocene sedimentary record of tectonically influenced reduced channel mobility, Skokomish River delta, Washington State, USA

At the Skokomish River delta in Washington State's Puget Lowland, coseismic uplift and tilting trapped the river against a valley wall, resulting in little to no channel migration for the last 1000years. The most recent earthquake occurred before AD 780–990, based on stratigraphic evidence such as sand blows and abrupt facies changes. Since the hypothesized tilting a 5-km-long section of the river has not migrated laterally or avulsed, resulting in reduced migration and a muddy intertidal flat that is 2km wider in the east than on the west side of Annas Bay. A ridge running perpendicular to the river may also have restricted channel mobility. The ridge may be either the surface expression of a blind thrust fault or a relict, uplifted and tilted shoreline. The uplift and tilting of the delta can be ascribed to any of three nearby active fault zones, of which the most likely, based on the orientation of deformation, is the Saddle Mountain fault zone, which produced a surface rupture 1000–1300years ago. The delta has experienced submergence since the earthquake. A forest that colonized an uplifted part of the delta about 800–1200years ago was later submerged by at least 1.6m and is now a brackish-water marsh.

Concepts in halokinetic-sequence deformation and stratigraphy

Abstract Halokinetic sequences are unconformity-bound packages of thinned and folded strata adjacent to passive diapirs. Hook halokinetic sequences have narrow zones of deformation (50–200 m), >70° angular discordance, common mass-wasting deposits and abrupt facies changes. Wedge halokinetic sequences have broad zones of folding (300–1000 m), low-angle truncation and gradual facies changes. Halokinetic sequences have thicknesses and timescales equivalent to parasequence sets and stack into composite halokinetic sequences (CHS) scale-equivalent to third-order depositional cycles. Hook sequences stack into tabular CHS with sub-parallel boundaries, thin roofs and local deformation. Wedge sequences stack into tapered CHS with folded, convergent boundaries, thicker roofs and broad zones of deformation. The style is determined by the ratio of sediment-accumulation rate to diapir-rise rate: low ratios lead to tabular CHS and high ratios result in tapered CHS. Diapir-rise rate is controlled by the net differential load on deep salt and by shortening or extension. Similar styles of CHS are found in different depositional environments but the depositional response varies. CHS boundaries (unconformities) develop after prolonged periods of slow sediment accumulation and so typically fall within transgressive systems tracts in shelf settings and within highstand systems tracts in deepwater settings. Sub-aerial settings may lead to erosional unroofing of diapirs and consequent upward narrowing of halokinetic deformation zones.

Stable isotope record of the terminal Neoproterozoic Krol platform in the Lesser Himalayas of northern India

The terminal Neoproterozoic succession in the Lesser Himalaya of India, including the Infra Krol Formation and Krol Group, represent the thickest known accumulation of carbonate strata of this time period, and hence is an ideal target for chemostratigraphic and sequence stratigraphic investigation. High-resolution analyses reveal several negative δ 13C excursions. Only one of these is related to known Neoproterozoic ice ages, and many vary in amplitude and stratigraphic position in an oblique regional transect of the platform. A combination of stratigraphic and geochemical tests, and comparison with sections elsewhere, leads to an evaluation of the relative contributions of primary and secondary signals. Two excursions are associated with abrupt facies changes associated with shallowing and accumulation of carbonate in organic-rich restricted environments, or with karstification of a subaerially exposed platform; these are currently interpreted to be diagenetic artifacts. Other negative δ 13C excursions are recorded in open marine transgressive facies, and are interpreted to represent biogeochemical anomalies of global significance. The magnitude of these excursions, however, is complicated by their lateral inconsistency between adjacent sections. Two interpretations are proposed. One possibility is that the observed isotopic inconsistency is due to diagenesis not revealed by the application of existing geochemical criteria. A composite δ 13C curve constructed under this assumption is broadly consistent with that documented elsewhere, and may reflect modest biogeochemical changes of global scale. An alternative interpretation is that the inconsistency of δ 13C values relates to stratigraphic hiatus between correlated horizons, incomplete sampling, and/or lack of appropriate carbonates facies. The composite δ 13C curve constructed under this assumption shows large-magnitude (up to 15‰) negative δ 13C anomalies that are taken to imply remarkable perturbations of ocean geochemistry and the episodic input of 13C-depleted alkalinity during terminal Neoproterozoic time.

Synrift stratigraphic geometry in a transfer zone coarse‐grained delta complex, Miocene Pohang Basin, SE Korea

AbstractThe Gojusan fan delta is a coarse‐grained delta complex on the western margin of the Miocene Pohang Basin, SE Korea. The deposits consist of five facies associations (FA): alluvial‐fan conglomerates (FA I), shallow‐marine mouth‐bar sandstones (FA II), fluvial and mouth‐bar conglomerates (FA III), Gilbert‐type foreset conglomerates (FA IV) and hemipelagic mudstones (FA V). Different facies associations characterize the northern, central and southern parts of the delta complex. To the north, FA IV is laterally juxtaposed with FA I or the basement, with scarp‐derived breccias along the contact. Centrally, and to the south, FA I is laterally juxtaposed with FA II, with an abrupt facies change and local inliers of basement rocks along the contact. Early alluvial fan and mouth‐bar deposits are overlain by the topset (FA III) and truncated by the foreset (FA IV) of a Gilbert‐type delta in the central part of the fan delta complex, whereas FA II passes transitionally upward into FA III in the south, with the latter extending basinward into a gently inclined shoal water delta front. Gilbert‐type and shoal‐water geometries are therefore developed in the same delta complex. The composite delta geometry is interpreted as reflecting (1) its development near an extensional transfer zone where the hangingwall relief was variable over short distances along strike, and (2) the operation of intrabasinal faults. This interpretation contrasts with previous studies that viewed the delta complex as having formed along a pull‐apart basin margin.

Lithostratigraphy and sedimentology of the latest Cretaceous-early Burdigalian Tamezzakht succession (Northern Rif, Morocco): consequences for its sequence stratigraphic interpretation

The Tamezzakht succession (Maastrichtian–middle Burdigalian), situated at the fringe between the Internal and the External zones, displays contrasting lithologies with abrupt facies changes, discontinuities, and/or coarse-grained calciturbidite in between. These criteria allow the definition of seven main lithostratigraphic formations. Depositional environments (oxygenation levels, trophic conditions, omission histories, among others) and/or transgressive/regressive trends are inferred from integrated sedimentologic data including facies change, cyclicity pattern and the textural composition of the turbidite facies tracts. Special emphasis is given to the ichnological features. Taking into account the extended time-range, the position between the internal zones and the external ones, as well as the clear differentiation into several contrasting sedimentary formations, the Tamezzakht succession is expected to provide useful stratigraphic data for the regional correlations.

Cuyania, an Exotic Block to Gondwana: Review of a Historical Success and the Present Problems

A review of the early history of the Cuyania terrane and the numerous pioneering works of the past century provides the present robust framework of evidence supporting a derivation from Laurentia, travel towards Gondwana as an isolated microcontinent, and final amalgamation to the protomargin of western Gondwana in Middle to Late Ordovician times. The major remaining uncertainties and inconsistencies, such as the time of deformation and collision with Gondwana, the lack of evidence of Famatinian-derived zircons, the effects of strike-slip displacements proposed along the suture, as well as the potential sutures defined by ophiolite assemblages, are discussed. The precise boundary along the northern and southern limits is not yet well defined. The most suitable hypothesis based on present data is that Cuyania originated as a conjugate margin of the Ouachita embayment, south of the Appalachian platform during Early Cambrian times. The subsequent travel toward the Gondwana protomargin is clearly depicted by the changing faunal assemblages in the carbonate platform. New geochemical and age data on K-bentonites presented by several authors reinforce the strong connection between Cuyania ash-fall tuffs and Famatina volcanics by 468–470 Ma, indicating Cuyania and Gondwana were in close proximity at that time. Extension related to flexural subsidence, preceded by the drowning of the carbonate platform in early Llanvirnian times, is recorded by abrupt facies changes in the sedimentary cover during late Llanvirnian and early Caradocian times. This episode marked the beginning of contact between Cuyania and Gondwana. The subsequent evolution of the foreland basin indicates that deformation lasted until latest Silurian-Early Devonian times. The time of collision is tracked by the cessation of arc-related magmatic activity in the upper plate (Gondwana protomargin), at about 465 Ma in western Sierras Pampeanas, and ages around 454 Ma corresponding to syncollisional and postcollisional magmatism. The age of the collision is also preserved in the lower plate (Cuyania), where both angular unconformities in the sedimentary cover and the ages of peak of regional metamorphism in the basement rocks point to 460 Ma as the most probable age for the beginning of the collision. Evidence from the upper plate is essentially identical with an age of 463 Ma. Thermal gradients along this suture vary from 13°C/km in the lower plate, to 18°C/km in the fore arc upper plate, reaching more than 30°C/km along the Famatinian arc. Based on these data, a Llandelian-Caradocian age for the collision can be postulated on firm grounds. Deformation continued through most of the early Paleozoic until amalgamation of the Chilenia terrane by the Late Devonian.

Depositional cycles: an approach to the sequence stratigraphy of the Dakhla Formation, west Dakhla-Farafra stretch, Western Desert, Egypt

The latest Campanian-Middle Palæocene Dakhla Formation has been the subject of few sedimentological studies, despite its great areal distribution. This shale/mudstone unit exhibits marked facies variations in a northwest-southeast direction. The facies distribution patterns, stratal geometries and type, as well as frequency of the associated microfauna, allow a possible subdivision of the Dakhla Formation into ten depositional cycles. These cycles are delineated from each other by a bounding surface, which is defined by an abrupt facies change and/or intensively bioturbated hardground. The sediments of these cycles have been accumulated in a shallow subtidal-upper intertidal-flat environment during Late Cretaceous and in a middle/outer shelf-lower intertidal suite during Palæocene times. Deposition took place in a topographically irregular basin under a constantly high clastic input derived largely from the south and southwest. The cyclic pattern of deposition that characterises the Dakhla Formation, points to repeated relative sea level fluctuations. The submarine palæorelief also plays a significant role on the facies pattern. It is found that the succession was deposited along the marginal part of a palæostructural low. Its deeper area is located around present-day Edmonstone. The study of sedimentary facies characteristics and cyclicity of the Dakhla Formation leads to the recognition of four depositional sequences bounded by five type 2 sequence boundaries. The first and second type 2 depositional sequences correspond to the Late Maastrichtian Mawhoob Shale and lower-middle part of the Beris Mudstone Members, respectively. The third sequence comprises the Beris Mudstone Member ‘upper part’ and the Latest Maastrichtian partition of the Kharga Shale Member. The well-documented Cretaceous-Tertiary boundary marks the type 2 sequence boundary at the top of this sequence. The last depositional sequence coincides with the late Early-Middle Palæocene subdivision of the Kharga Shale Member. These sequences comprise nine systems tracts, mainly of transgressive and highstand systems tract deposits.

Syngenetic Au on the Carlin trend: Implications for Carlin-type deposits

A new type of gold occurrence recently discovered in the Carlin trend, north-central Nevada, is clearly distinct from classic Carlin-type gold ore. These occurrences are interpreted to be of sedimentary exhalative (sedex) origin because they are stratiform and predate compaction and lithification of their unaltered Devonian host rocks. They contain barite that exhibits δ34S and δ180 values identical to sulfate in Late Devonian seawater and sedex-type barite deposits. Abrupt facies changes in the host rocks strongly suggest synsedimentary faulting and foundering of the carbonate shelf during mineralization, as is characteristic of sedex deposits. Gold occurs both as native inclusions in synsedimentary base-metal sulfides and barite, and as chemical enrichments in sulfide minerals. The absence of alteration and lack of δ13C and δ180 isotopic shift of primary carbonates in these rocks is strong evidence that this gold was not introduced with classic Carlin-type mineralization. Collectively, these features show that the Devonian strata were significantly enriched in gold some 300 m.y. prior to generation of the mid-Tertiary Carlin-type deposits. These strata may have been an important, perhaps even vital, source of gold for the latter. Although gold is typically low in most Zn-Pb-rich sedex deposits, our evidence suggests that transport of gold in basinal fluids, and its subsequent deposition in the sedex environment, can be significant.

RECENT ADVANCES IN THE SEQUENCE STRATIGRAPHY OF THE TRIASSIC TO LOWER CRETACEOUS SUCCESSION IN THE NORTHERN CARNARVON BASIN, AUSTRALIA

Application of sequence stratigraphy to well and seismic data has resulted in major revisions to the stratigraphic subdivision and understanding of the hydrocarbon system in the Northern Carnarvon Basin of Australia.A sequence stratigraphic nomenclature which integrates the biostratigraphic control and sequence terminology has been developed. This has proven to be an invaluable tool in enabling effective communication between geologists, biostratigraphers, and seismic interpreters. This nomenclature reduces the need to refer to absolute time scales and Epoch or Stage names, all of which are subject to change. The revised subdivision and new terminology can be used to reliably correlate across wide areas on the North West Shelf and possibly beyond.Six first and second order megasequences are recognised within the Triassic to Lower Cretaceous succession. These megasequences are mostly bounded by transgressive surfaces that are expressed on well logs as abrupt facies changes and on seismic as major downlap surfaces, indicating significant acceleration in subsidence rates and an increase in accommodation space in the basin. Each megasequence consists of an initial transgressive section of mainly fine-grained clastics overlain by a regressive and usually extensive succession of coarse-grained deposits. Recognition of the significance of these megasequence boundaries, and the stages of extension, has resulted in a vastly improved understanding of surfaces, some of which have previously been mis-interpreted as 'break-up unconformities'. Two distinct stratigraphic events, a Callovian transgression and an Oxfordian Iowstand, have been recognised as separate megasequence boundaries, which has a significant impact on the prospectivity of the Northern Carnarvon Basin.

Facies Associations and Depositional Environment of the Proterozoic Carbonate-hosted Microbanded Manganese Oxide Ore Deposit, Penganga Group, Godavari Rift Basin, India

The late Proterozoic Penganga Group, Pranhita-Godavari valley, South India, is a sequence of siliciclastic-carbonate sediments that was deposited in an intracratonic rift basin. The nonvolcanogenic sedimentary succession containing listric basin-marginal faults shows abrupt facies changes from craton to trough with rich resources of stratabound microbanded manganese oxide ore hosted within the trough facies. The cratonic sediments of the basin are mature siliciclastics with clear imprints of tide-dominated, shallow-subtidal deposition. The siliciclastics show rapid lateral transition into a thick sequence of nonstromatolitic and micritic ribbon limestone containing sediment-gravity-flow deposits in the basal part and a persistent horizon of bedded manganese ore deposit in the upper part. The limestone is overlain by a thick sequence of basinal shale facies through an interbedded sequence of limestone and shale. Uniformly thick, laterally persistent, parallel-sided and thin-bedded limestone-shale rhythmites and parted limestone rhythmites characterize the ribbon limestones. Except for widespread thin parallel lamination, structures formed by waves, tidal currents, and storm-induced currents are totally absent. The limestone is brown and argillaceous in the basal part, and changes upwards to siliceous and pyritiferous light gray to dark gray varieties and finally to carbonaceous black limestone. The physical attributes of the limestone suggest complete drowning of the carbonate interval in response to higher sea level caused by tectonic subsidence and a deep-water slope and basinal environment of carbonate deposition under continued transgression. The bedded manganese oxide ore interstratified with chert, simulating iron formation, typically shows micron-scale to millimeter-scale thin parallel laminae of almost uniform thickness, texture, and composition. The manganese ore and chert laminae chiefly have a cryptocrystalline to microcrystalline fabric consisting of grains of clay size to fine silt size. Todorokite and birnessite are the most abundant sedimentary manganese oxides; they were locally transformed to braunite and bixbyite during diagenesis. The centimeter-thick laminated chert beds are analogous to microbanded chert mesobands of the iron formations and are composed of microcrystalline quartz. The sedimentary attributes of the ore-chert sequence suggest deposition from suspension fallout in a low-energy environment. Penecontemporaneous deformation of the ore deposit indicates slow downslope movement of the semiconsolidated sediments over the basin floor. The sedimentology and stratigraphy of the Penganga Group suggest a sediment-gravity-flow-influenced deep-water, base-of-slope/basinal environment of manganese accumulation during the thermal subsidence stage of basin development within an extensional rift setting. On the basis of sedimentological evidence and parallel studies done on iron formations, a possible hydrothermal source of manganese is also considered. This represents a significant departure from the prevalent views of high-energy, shallow-water shelf environments of manganese deposition and terrigenous sources of metal in cratonic basins hosting several important manganese deposits, and thus it has important implications for genesis of manganese ores.

Sequence Stratigraphy of Pleistocene Sediments, Northwestern Green Canyon, Gulf of Mexico: ABSTRACT

ABSTRACT A sequence stratigraphic study of Pleistocene sediments in the northwestern portion of the Green Canyon Area has identified five depositional sequences: 1.1-0.8, 0.8-0.7, 0.7-0.5, 0.5-0.4, and 0.4-0.3 Ma. Detailed seismic and geologic facies maps were constructed for each of these sequences. The data base for this study comprises 1420 km (882 miles) of multifold seismic, petrophysical information from 20 wells and 23 biostratigraphic reports. The area is characterized by several large mini-basins, separated by complex salt bodies and minor faults. The depositional settings are structurally controlled by these salt bodies and normal faults. Fields/discoveries in the area are in Green Canyon 18, and in the Mahogany sub-salt discovery (Ship Shoal 349). Paleoecology indicates that the sequences were deposited in water depths ranging from lower slope to middle shelf. Bathyal turbidite systems represent most of sediments. Prominent basin-floor fan deposits are present only in sequences 1.1-0.8 Ma and 0.8-0.7 Ma in the southeastern portion of the area. All of the sequences consist primarily of channel-levee systems and overbank deposits (slope fan facies). Abrupt facies changes within these deposits are common. Well logs indicate that most of the area is mud-dominated, with sands primarily occurring in channel-fill facies. Extensive slides are also present in these slope fan facies. In the northern part of the area, the younger sequences (0.7 Ma and younger) also contain sediments deposited in prodelta/delta environments (prograding complex/HST). Relatively thick TST are present and stacked along the downthrown sides of faults.

Shannon Sandstone in Hartzog Draw-Heldt Draw Fields (Cretaceous, Wyoming, USA) Reinterpreted as Lowstand Shoreface Deposits

ABSTRACT The lower Campanian Shannon Sandstone at Hartzog Draw-Heldt Draw (Powder River Basin, Wyoming) is interpreted in the literature as a shelf-ridge complex deposited 160 km from the contemporaneous shoreline. This interpretation poses difficult problems, and does not discuss the significance of bounding discontinuities preserved in the sandbodies. Cross sections show that the Shannon Sandstone is present at several stratigraphic horizons, and is bounded by regionally extensive discontinuities. The presence of these bounding discontinuities suggests that fluctuations in relative sea level were important in controlling geometry of sandbodies on the Shannon shelf. In the Hartzog Draw-Heldt Draw area of Wyoming the base of the Shannon is a planar surface with respect to a lower bentonite marker and is characterized by the abrupt appearance of bioturbated sandstone on top of black basinal mudstone. Siderite pebbles < 1 cm in diameter are common at this contact. In Hartzog Draw the bioturbated sandstone interval has an average thickness of 10 m and is separated from an overlying unit of cross-bedded fine to medium sandstone (up to 20 m thick) by a ubiquitous (50 cm thick) horizon of mud clasts in a cross-bedded, highly glauconitic, medium sandstone. These three units are laterally continuous in Hartzog Draw except where removed by erosion. The top of the Shannon is marked by an abrupt change from the cross-bedded sandstone or bioturbated medium sandstone to laminated black mudstone. Mud clasts, siderite pebbles, chert grains, and glauconite mark this abrupt contact, suggesting erosional transgression. This surface can be traced westward, where in many areas it is a mud-on-mud contact marked by a lag of shell debris. This transgressive surface, which terminates Shannon deposition, is characterized by at least one regionally extensive onlapping marker in the Hartzog Draw-Heldt Draw area. The presence of depositional discontinuities in the Shannon Sandstone is suggested by (1) the sharp basal contact, (2) the lateral continuity of individual facies, (3) the abrupt vertical facies changes and the association of mud clasts, siderite pebbles, and glauconite accompanying these abrupt juxtapositions of facies, (4) the stratigraphic variability, and (5) the termination of Shannon deposition by erosive transgression, marked by onlapping markers. These depositional discontinuities are the bounding discontinuities. These bounding discontinuities have asymmetrical one-sided scour morphologies, with steeper landward margins that flatten basinward. These geometries and the linearity of the overlying sandstone and the troughs defined by these scoured surfaces suggest incised shoref ce profiles truncated by erosive transgression. Thus, the Shannon Sandstone in Hartzog Draw-Heldt Draw is reinterpreted as a lowstand shoreface deposit.

Interpretation of the Reagan Fault, Garvin, Johnston, Murray, and Stephens Counties, Oklahoma

The Reagan fault, which lies between the Mill Creek syncline and the Tishomingo anticline, is one of the major faults in the Arbuckle Mountains. The fault's surface expression extends for more than 24 mi, and it can be traced in the subsurface at least an additional 26 mi west. The relative upthrown side of the fault changes at least four times along its length and it is manifest in different segments as both an apparent reverse fault and an apparent normal fault. Subsurface cross sections show abrupt facies changes within formations across the Reagan fault and isochore maps of individual units indicate a large-scale component of left-lateral movement along the fault. The geometry of the fault, as well as its displacement, also is consistent with a wrench-fault interpretation of the Reagan fault. Synorogenic conglomerates indicate that in at least one locality the Reagan fault had ceased movement, whereas the Washita Valley fault was still active.