Fan-delta Deposits Research Articles

The controls of geomorphology and sediment supply on sequence stratigraphic architecture and sediment partitioning of the lacustrine rift basin in the Es3 of Liuzan area, Nanpu Sag, Bohai Bay Basin, China

ABSTRACTAn important hydrocarbon reservoir is hosted by the third member of the Shahejie Formation (Es3) in the Liuzan area, Nanpu Sag, Bohai Bay Basin, China. The Es3 can be divided into five third-order sequences (from base to top: Sq1, Sq2, Sq3, Sq4 and Sq5). Utilising well logs, cores and high-resolution 3-D seismic data, this study investigates the development characteristics and styles of sediment transport pathways in the steep-slope zone of the lacustrine rift basin. Two styles of sediment transport pathway are identified in the study area, including the faulted trough and fault slope-break zone. The faulted trough is divided into a single faulted trough, synthetic faulted trough and antithetic faulted trough. The fault slope-break zone is composed of synthetic fault slope-break zone and antithetic fault slope-break zone. The fan-delta plain and fan-delta front are recognised in the study area. The different styles of delivery conduits control depositional facies types and the spatial and temporal evolution of fan-delta depositional systems from Sq3 to Sq5. Based on paleo-geomorphological reconstruction, the dispersal pattern of the steep-slope zone system divided into a northern steep-slope system and an eastern steep-slope system is established. In Sq3, the northern steep-slope system develops large-scale mixed sandy–muddy fan-delta deposits derived from the NW. The eastern steep-slope system feeds the medium-scale gravel-rich fan-delta sediments. In Sq4, the large-scale mixed sandy–muddy fan-delta deposits develop from the NNE direction in the northern steep-slope system. The small-scale sand-rich fan-delta sediments are transported from the eastern steep-slope system. In Sq5, the large-scale mixed sand–mud fan-delta deposits are fed by both the northern steep-slope system and eastern steep-slope system. The strata stacking pattern exhibits a seesaw mode in the study area. Based on the analysis of the channel-belt thickness and the fan-scales, the paleogeomorphology of the drainage area in Sq3 and Sq4 and sediment supply in Sq5 alternatively control the sequence stratigraphic architecture and sediment partitioning in the sink area. The model proposed in this study may aid in the prediction of favourable reservoirs and good source–reservoir–seal development in lacustrine rift basin.

Lacustrine fan delta deposition alongside intrabasinal structural highs in rift basins: an example from the Early Cretaceous Jiuquan Basin, Northwestern China

Development of fan deltas alongside intrabasinal structural highs has been overlooked compared to those forming on basin margins. However, these fan deltas may provide important clues regarding the tectonic and climatic controls on deposition during rift development. This paper documents fan delta deposition alongside an intrabasinal structural high within the Early Cretaceous Xiagou Formation of the Jiuquan Basin, China, using subsurface geological and geophysical data. Deposits observed in drill core support fan delta deposition occurring almost exclusively through subaerial and subaqueous gravity flows. Subsurface mapping reveals a consistent decrease in the areal extent of fan deltas from lowstand to highstand system tracts, suggesting that deposition alongside the structural high is sensitive to lake-level changes. The temporal and spatial distribution of the fan deltas display retrogradational stacking patterns, where fan deltas exhibit a decreasing lateral extent up-sequence until fan delta deposition terminated and was replaced by deposition of fine-grained lacustrine deposits. The retrogradational stacking patterns observed alongside the intrabasinal structural high are not observed in fan deltas along the basin margin in the lower parts of the Xiagou Formation. Subsidence profiles also show differential subsidence across the basin during the earliest stages of this formation, likely resulting from border fault movements. These data suggest that non-uniform stacking patterns in the lower parts of the Xiagou Formation reflect basin-scale tectonic movements as the dominant control on synrift deposition patterns. However, later stages of Xiagou Formation deposition were characterized by uniform subsidence across the basin, and uniform retrogradational stacking patterns for fan deltas alongside the intrabasinal structural high and border fault. These observations suggest that basin-scale tectonic movements played a relatively limited role in controlling sediment deposition, and imply a potential change to regional-scale processes affecting fan delta deposition during later synrift stages. Climate change is favored here as the region-scale control on the uniform retrogradational fan delta stacking patterns. This assertion is supported by pollen assemblages, isotope signatures, and organic geochemical analyses, which collectively suggest a change from a humid to semi-arid environment during later synrift stages. We suggest that variations in stacking patterns between different fan delta systems can provide insights into the basin- and regional-scale processes that control rift basin deposition.

The glacially overdeepened trough of the Salzach Valley, Austria: Bedrock geometry and sedimentary fill of a major Alpine subglacial basin

Overdeepened valleys are unambiguous features of glacially sculpted landscapes. They result from erosion at the bed of the glacier and their size and shape is determined by ice dynamics and the characteristics of the underlying bedrock. Major overdeepened valleys representing vertical bedrock erosion of several hundreds of meters are characteristic features of many trunk valleys in the formerly glaciated parts of the Alpine mountain belt. The thick sedimentary fill usually hinders attempts to unravel bedrock geometry, formation history and fill characteristics. Based on reflection seismic data and core-logs from multiple deep drillings we construct a detailed bedrock model of the Lower Salzach Valley trough, one of the largest overdeepened valleys in the European Alps. The analysed overdeepened structure characterized by a strongly undulating topography. Two reaches of enhanced erosion can be identified and are suggested to be related to variations in bedrock erodibility and a triple glacier confluence. The sedimentary fill shows clear characteristics of rapid infilling and subaqueous fan delta deposits indicate a strong influence of tributary streams. Associated surface lowering of the valley floor had a major impact on tributary stream incision but also on the available ice accumulation area at subsequent glaciations. The extent to which fills of earlier glaciations have been preserved from erosion during the last glacial maximum remains ambiguous and demands further exploration. To our knowledge the presented bedrock model is one of the best defined of any major overdeepened trunk valley.

The controlling factors of modern facies distributions in a half‐graben lacustrine rift basin: A case study from Hulun Lake, Northeastern China

Hulun Lake is located in Northeast China and is a well‐exposed modern lacustrine rift basin. In this study, recent fieldwork has demonstrated that it was mainly filled by alluvial fan and fan delta deposits along the western escarpment, which were controlled by the Eerguna Fault Zones. However, it was filled by river‐deltaic and eolian deposits along the eastern slope. The distributions of the depositional systems were found to be controlled by their positions within the basin. The western escarpment was determined to have influenced the distributions of the alluvial fan and fan delta deposits. In contrast, the river‐deltaic and eolian deposits were controlled by the sediment supply, paleogeomorphology, and wind forces. The asymmetric geometry of the basin was determined to have resulted in different facies associations. First, a region of maximum sediment thickness and high‐gradient escarpment was found to be located near a normal fault foot (Eerguna Fault). Second, a region of a shallower and low‐gradient slope was located near the margin of a river‐lake transition. The depositional characteristics obtained from the prospecting trench indicated that the coarse deposits on the western side were mainly derived from unexpected flooding, gravity, and episodic flooding in an ascending order. In contrast, the fine deposits on the eastern side were mainly sourced from a meandering river. A clear understanding of controlling factors on facies associations can potentially reveal the sedimentary process and evolution of the depositional systems, as well as provide an important methodology for the prediction of facies in other sedimentary basins with similar characteristics.

Sequence stratigraphy, sedimentary systems and implications for hydrocarbon exploration in the northern Xujiaweizi Fault Depression, Songliao Basin, NE China

Recent drilling data from the northern Songliao Basin reveal considerable hydrocarbon exploration potential in deep-seated small-scale faulted basins. In this paper, the Shahezi Formation of the northern Xujiaweizi Fault Depression within the Songliao Basin was investigated as a case study. Sequence stratigraphy, sedimentary systems and the hydrocarbon exploration potential of the small-scale faulted basin were investigated through observations and descriptions of cores; analyses of conventional log curves, imaging data and logging data; and interpretations of 3-D seismic profiles. Four third-order sequences, mainly controlled by sediment supply and fault activity, were identified based on unconformities at the basin margins and in sedimentary successions. System tracts in the third-order sequences were also recognized through lithological associations and geophysical response characteristics. The distributions of sedimentary systems, including fan deltas, braided deltas, lacustrine deposits, subaqueous fans, volcaniclastic deposits and incised-valley deposits, in the sequence framework are associated with basin evolution. During the early stage of the Shahezi Formation (SQ1 and SQ2), the sedimentary area was mainly covered by fan deltas, and lacustrine deposits were not very developed. Additionally, incised-valley systems formed on basin margins and subaqueous fans formed along basin-controlled faults. During the middle stage (SQ3), fan delta deposits on gentle slopes were gradually replaced by braided delta deposits, and the sizes of lacustrine facies increased with basin expansion. In the late stage (SQ4), the basin and lacustrine deposit areas reached their maximum sizes, and braided deltas became the dominant sedimentary systems on gentle slopes. With continuous sediment supply, fan and braided deltas migrated forward, and the lacustrine system was locally distributed. In this small-scale faulted basin, the lacustrine mudstones and coal seams widely developed in SQ4, especially in SQ4-HST, and they also exhibited good source rock properties for gas generation. Sandbodies of fan deltas and braided deltas in the upper Shahezi Formation, exhibiting good contact relationships with underlying source rocks, can be favourable reservoirs for hydrocarbon accumulation, as confirmed by the newly drilled Well Songs9H (with a daily gas production of 20×104 m3). However, due to low porosity and permeability (averages of 4.7% and 0.28×10−3µm2, respectively), these reservoirs may need fracturing for commercial hydrocarbon production.

Facies controls on the distribution of diagenetic alterations in fan delta deposits: a case study from the Lower Cretaceous sandstone, Chagan Sag, Inner Mongolia, China

AbstractBy integrating diagenesis and sedimentary facies, the distribution of diagenetic alterations and their impacts on reservoir quality were investigated within a lacustrine fan delta depositional environment in the Lower Cretaceous of the Western sub‐sag in the Chagan Sag, Yin‐E Basin, northern China. Core observation and analyses of petrography and geochemistry of the sandstones revealed that the eogenetic alterations display spatial and temporal distribution patterns associated with sedimentary facies, including distributary channels of a fan delta front (DC), distributary bays (DB) and sheet sands (SS) and mouth bar (MB) (SS and MB are collectively referred to as S). Percolation of meteoric waters occurring in high permeability DC sandstones resulted in leaching of feldspar, the formation of kaolinite, as well as mechanical infiltrated clays around detrital grains. Conversely, the DB deposits containing abundant ductile lithic fragments were subjected to mechanical compaction and thus the development of a pseudomatrix. During a long burial residence time (the Early Cretaceous period) in a lacustrine environment, abundant early carbonate cements were precipitated, especially in the distal S sandstones. The sandstones lacking eogenetic cements were subjected to stronger mechanical compaction. Additionally, eogenetic alterations have an important impact on the distribution of the mesogenetic alterations. Sandstones containing few eogenetic cements or thin or discontinuous infiltrated clay rims around the detrital grains were subjected to quartz cementation. However, during the hydrocarbon generation stage, the most efficient percolation of meteoric waters and organic acid dissolved feldspar minerals and carbonate cements, resulted in DC sandstones having more intragranular and intergranular porosity. Owing to high matrix contents and early calcite cement contents, the DB and S sandstones had lower permeability and were rarely dissolved. During the late mesogenetic stage, late carbonate cements occurred in all sedimentary facies and iron and magnesium ions were released from the transformation from kaolinite to illite or to chlorite. The results from this study show possible diagenetic evolutionary pathways in the reservoir sandstones within the fan delta depositional environment, which in turn provides some insights into the controls on reservoir potential. Copyright © 2016 John Wiley & Sons, Ltd.

The characteristics of dolomite reservoir in saline lacustrine Basin, Qaidam, China

Qaidam is a plateau saline lacustrine basin in China. The Eocene in western Qaidam mainly consists of lacustrine carbonate, and the sedimentary rocks mainly include micrite, bioclastic limestone, algal limestone, dolomicrite, algal dolomite and so on. Among these rocks, dolomite is the most important petroleum reservoir rock, which mainly includes algal dolomite and dolomicrite. The pores of carbonate are mainly the intercrystalline pores, and there are dissolution pores locally. The analysis based on electronic probe energy spectrum scanning instrument shows that the enrichment region of element Mg in the algal dolomite is highly consistent with the distribution of algal laminas, which means the algal dolomite is a biogenic primary dolomite, because the microbial activities can overcome the kinetic obstacles and create a favorable microenvironment for the formation of dolomite. The analysis based on X-ray diffraction instrument shows that most dolomicrite includes clastic particles and argillaceous components because of the combined sedimentation of clastic materials brought from the peripheral fan delta deposit. The rock composition analysis based on the electronic probe shows that the average content of FeO in the dolomicrite is 0.942 %, and that of MnO is 0.052 %, which are obviously lower than the average contents of Fe and Mn in the hydrothermal dolomite. Meanwhile, due to the high contents of SiO2 and Al2O3, it is considered that the dolomicrite has a penecontemporaneous metasomatic formation mechanism. This type of dolomicrite generated a lot of disseminated intercrystalline pores during the metasomatism. The pore radius is small and the pore number is enormous, which are the main cause of relatively high porosity and relatively low permeability. The type of dolomicrite is a good petroleum reservoir in the study area, so adequate attention shall be given to the dolomicrite during the oil and gas exploration in the carbonate rock reservoir of Eocene in Qaidam Basin.

Paleocurrents and Paleohydraulics Studies of the Proterozoic Kolhan Siliciclastic Unit (India): A Case Study from the Chaibasa-Noamundi Basin, Jharkhand, India

Detailed palaeocurrent and palaeohydraulic analyses of the fluvio-deltaic Proterozoic Kolhan siliciclastic unit has provided new insights into the sedimentological knowledge of the fan delta deposition, allowing for the reconstruction of paleogeographic and paleoclimatic conditions prevalent in the Chaibasa-Noamundi intracratonic basin of India. The azimuth reading of cross-bed stratifications of fluvial deposits and the preferential grain orientation of different lithofacies have indicated at least two major paleo-flow systems, NW and NE trending. The local occurrence of sedimentary structures of high energy depositional environments has indicated the superimposition of retrogradational strata pertaining to an older progradational fan complex. The fluvio-deltaic Proterozoic sedimentation has followed the emplacement of volcanic dykes, basic in composition, indicating a thermal rejuvenation of the continental crust. Finally, the partial development of limestones and dolomites in the Kolhan succession has pointed out the peneplanation of the cratonic areas and the development of carbonate sedimentation on stable carbonate platform.

Mineralogical characterization of Martian Jezero crater from MRO CRISM hyperspectral images

Abstract. Martian mineral detection and mapping can provide important information and constraints on Martian aqueous history, which can be used to assess the potential habitability of Mars. The key parameters to Martian aqueous alteration are the depth and extent of the Martian hydrous mineral. Therefore, it is important to know detailed minerals and chemical induction of the existence of water on the Martian surface at past or present. The Jezero crater located in the Nili Fossae region of Mars is the once-flooded crater, which has rich fan-delta deposit clays. It is a good case to study the clays and mineral components at Jezero crater, so as to know the geogloical processes and evolution on Mars. The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard the Mars Reconnaissance Orbiter (MRO) is a visible and near infrared spectrometer with enhanced spectral resolution, which provides an opportunity to map detailed and large-area mineralogy on Mars. In this paper, CRISM nearinfrared spectral data are analyzed using the mixture tuned filtering (MTMF) along with spectral angle mapper (SAM), and mineral components at Martian Jezero region are recognized, including the phyllosilicate, carbonate, nitrates and tectosil. Some detailed characteristics and implications of minerals at Martian Jezero crater are further studied and discussed, including implications on Martian climate change and geological evolution.

Depositional environment of a fan delta in a Vistulian proglacial lake (Skaliska Basin, north-eastern Poland)

ABSTRACT The study reconstructed the environment of a fan delta filling the vast end depression of the Skaliska Basin, and its overlying aeolian deposits. The formation of the large fan delta is associated with the presence of an ice-dammed lake functioning during the retreat of the Vistulian Glaciation (MIS 2). The examined material was collected from five boreholes. Sediments were analysed for their granulometric composition and subjected to analyses of frosting and rounding of quartz grains. Grain size analysis showed that the fan delta deposits are built of sand sediments of very low lateral and vertical variability. The fan delta was supplied with fluvioglacial sediments. Accumulation of sediments occurred in shallow water with a very low-gradient slope. The exposed fan delta became a site conducive to aeolian processes after the lake waters fell and the Skaliska Basin depression dried. Dune deposits overlying the fan were affected by short-distance transport so they did not acquire features typical for aeolian deposits.

Bioestratigrafía del parche arrecifal de la Quebrada Brazo Seco, Plio-Pleistoceno, Limón, Costa Rica

The Plo-Pleistocene stratigraphic sequence of Brazo Seco Creek-Limón is composed by fossiliferous and non-fossiliferous sandstones, and limestones (blaflestones) constituted by coral fragments into a sandy matrix. The depositional environment was shallow marine and they belong to the Río Banano Fomtation. Overlying this sequence, are conglomerates of the Suretka Formation, genetically related to the uplifting of the Talamanca Range, and they correspond with fan delta deposits. In this paper, we include the description of the fossil content: mollusca, corals, bryozoa, arthrophoda and ichnofossils.Based on the fossils and outcrops the depositional environmet of this section of the Río Banano Formation was litoral to sub-litoral, with patch reefs, whose growth was interrupted by fast sedimentation and/or uplifting.

Research on Main Control Factors of Coalbed Methane Reservoir Formation in Turpan-Hami Basin

Turpan-Hami Basin is a typical low coal rank coal bearing basin in China, with coal resources accounting for 50% of the total low coal rank resources. Structural evolution of the basin plays important controlling roles to generation；Fan delta and braided delta deposit systems are thick and stable sedimentary systems which are favorable for coal beds development；Hydrogeological condition is a major force for the adjustment and construction of coal-bed gas reservoirs.

Depositional age and provenance of the Itajaí Basin, Santa Catarina State, Brazil: Implications for SW Gondwana correlation

The Itajaí Basin is a fault-bounded late- to post-orogenic basin deposited adjacent to the Dom Feliciano Belt in the state of Santa Catarina, Brazil. The basin is composed of alluvial-deltaic and platformal successions followed by a turbiditic complex and fan-delta deposits. U-Pb zircon dating of a tuff and five sandstone samples indicates a maximum depositional age of 563 ± 3 Ma for the intermediate and upper sections of the Itajaí Basin, and zircon dating of a rhyolite stock indicates a minimum depositional age of 549 ± 4 Ma. These dates are compatible with the previous assignment of an Ediacaran age to the sediments based on the discovery of Ediacara-type fossils such as Parvancorina, Charniodiscus, Cyclomedusa, and Aspidella. The U-Pb detrital zircon ages show a change in the sedimentary input from the base to the top. At the base, zircon grains from the Transamazonian and Brasiliano/Pan-African orogenic cycles occur as major sources with some Archean contribution. The upper section is dominated by Brasiliano/Pan-African zircon grains, most of which range in age from 824 to 770 Ma and 656 to 563 Ma. Pb and Sm-Nd whole rock isotope data suggest that the main source area for the basin infill was a reworked crust such as the Dom Feliciano Belt. We also present the regional correlation of the Itajaí Basin with the basins formed during the late stages of the SW Gondwana.

Magnetochronology of synorogenic Miocene foreland sediments in the Fars arc of the Zagros Folded Belt (SW Iran)

ABSTRACTThe timing of deformation in the northern Zagros Folded Belt is poorly constrained because of the lack of an accurate absolute chronology of the syntectonic sedimentary sequences. The foreland basin infill in the northern part of the Fars arc is composed of supratidal sabkha deposits (Razak Fm), medium‐grained deltaic deposits (Agha Jari Fm) and coarse conglomerates of nearshore fan delta deposits at the base (Bakhtyari Fm, Bk1) and continental alluvial deposits at the top of the section (Bakhtyari Fm, Bk2). A magnetostratigraphic study was carried out in a composite section spanning about 1300 m on the northern flank of the Chahar–Makan syncline. Magnetostratigraphic correlation of the Razak Fm with chron C6n yields an age of 19.7 Ma at the base of the composite section. The transition to Agha Jari Fm is correlated with chron C5Cn, yielding an age of 16.6 Ma. The transition to the conglomerates of the Bakhtyari Fm (Bk1) correlates with the chron C5AD at approximately 14.8 Ma, which is considerably older than previously thought. The base of the Bakhtyari Fm growth strata, and thus the beginning of the deformation in northern Fars, is dated at 14–15 Ma. The topmost preserved Bakhtyari Fm (Bk1) is folded and unconformably overlain by Bakhtyari Fm (Bk2) conglomerates. This indicates that tectonic deformation in northern Zagros was already underway in the Middle Miocene.

History and evolution of the North-Betic Strait (Prebetic Zone, Betic Cordillera): A narrow, early Tortonian, tidal-dominated, Atlantic–Mediterranean marine passage

The North-Betic Strait, located in the Prebetic Zone (outermost part of the Betic Cordillera), was a narrow seaway connecting the Atlantic Ocean and the Mediterranean Sea through the Betic Cordillera during the Early Late Miocene. Strait facies consist of heterozoan bioclastic carbonates and mixed siliciclastics–carbonates. Local fan-delta deposits (conglomerates) also occur. The North-Betic Strait deposits exhibit huge trough cross-bedded structures. Single troughs are up to 5 m high and point to the east in easternmost outcrops. In central outcrops, they reach up to 15 m in height and some tens of metres in length and point both to the east and to the west. The westernmost outcrops show the largest cross-bedded structures, up to 20 m in height and some hundred of metres in length, all pointing to the west. The large-scale cross-bedding was generated by the migration of very large dunes moved by tides. The largest structures are inferred to have formed at depths of 90 m. Depth estimations indicate that the North-Betic Strait shallowed to the east and deepened to the west. The Miocene evolution towards the formation of the North-Betic Strait can be divided into a series of steps. The palaeogeography evolved from a south-facing platform, marginal to a northeastern relief, to a wide-open marine passage limited by a southern platform and, finally, to a tidal-dominated strait. The closing of the North-Betic Strait is recorded by the presence of lagoonal, silty sediments covered by a stromatolite layer and crowned by a red soil in the westernmost outcrops.

Geotechnical mapping for alluvial fan deposits controlled by active faults: a case study in the Erzurum, NE Turkey

Erzurum, the biggest city of Eastern Anatolia Region in the Turkey, is located in Karasu Plain. Karasu Plain, located on the central segment of the Erzurum Fault Zone, is an intermountain sedimentary basin with a Miocene-Quaternary volcanic basement, andesitic-basaltic lava flows and fissure eruptions of basaltic lava. It was filled in the early Quaternary by lacustrine fan-delta deposits. The basin is characterized by NNE-SSW trending sinistral wrench faults on its eastern margin and ENE-WSW trending reverse faults on its southern margin. Both systems of active faults intersect very near to Erzurum, which is considered to be the most likely site for the epicenter of a probable future large earthquake. Historical records of destructive earthquakes, morphotectonic features formed by paleo-seismic events and instrument seismic data of region indicate to a very high regional seismicity. The residential areas of Erzurum are located on thick alluvial fan deposits forming under the control of faults on the central segment of the Erzurum Fault Zone, which is one of the most active fault belts of the East Anatolian Region. Over time, the housing estates of city such as Yenisehir and Yildizkent have been expanded toward to the west and southwest part of Erzurum as a consequence of rapid and massive construction during the last 30 years. Geotechnical investigation has therefore been undertaken the residential areas of city in order to characterize geotechnical properties over the varied lithologies examine the potential for geotechnical mapping and assess the foundation conditions of the present and future settlement areas. The geological field observations and operations have been performed to make the soil sampling and characterize the lateral and vertical changes in thickness of the alluvial deposits in trenches, excavations and deep holes with 6–12 m sections. The soil samples have been subjected to a series of tests under laboratory conditions to obtain physical and mechanical properties. Furthermore, the standard penetration tests have been applied to the soils under field conditions. The geological field observations, geotechnical data and distribution of bearing capacity have been considered for the geotechnical mapping. Based on the geotechnical map, there are five geotechnical zones distinguished in the study area.

Dynamics of a Transgressive Prodeltaic System: Implications for Geography and Climate Within a Pennsylvanian Intracratonic Basin, Colorado, U.S.A.

Detailed sedimentological analysis of a prodeltaic turbidite unit within the Pennsylvanian Minturn Formation provides insight into the nature of fan-delta deposition, and allows reconstruction of paleogeographic and paleoclimatic conditions within a tectonically active cratonic basin. The formation, a thick succession of fan-delta deposits, exhibits a complex stratigraphic architecture of alluvial, fluvial, and deltaic fithofacies that were deposited in the Central Colorado Basin (CCB), U.S.A., a fault-bounded depression within the Ancestral Rockies. An ~ 20 to 35-m-thick, regionally extensive, unconformity-bounded prodelta unit was deposited during a large eustatic sea-level rise that temporarily interrupted coarsegrained fluvial-deltaic deposition. The unit consists of dark-green shale and sandstone event beds with tool marks produced by abundant plant debris. The sandstone event beds were previously interpreted as turbidites because they contain current ripples, parallel lamination, and sole marks consistent with deposition from turbidity currents in the absence of storm-generated surface gravity waves. We have found, however, that proximal prodeltaic deposits contain evidence for combined oscillatory and unidirectional flow (i.e., asymmetric hummocky cross stratification (HCS), quasi-planar lamination, and combined-flow ripple stratification), indicating deposition under large waves in relatively shallow water. Paleohydraulic calculations indicate that formation of the HCS beds requires a fetch of at least 75 km to generate the necessary wave periods. Given the position of the CCB in the lee of the ancestral Front Range relative to the prevailing trade winds, and the narrow seaway to the south, storms either tracked across the Cordilleran seaway from the west or formed in situ within the CCB. The former hypothesis is consistent with paleoclimate models that purport the development of summer monsoonal circulation patterns over western equatorial Pangea during the late Paleozoic, and regional observations of southeastward eolian bedform migration directions.

Deposits from Wave-Influenced Turbidity Currents: Pennsylvanian Minturn Formation, Colorado, U.S.A.

Turbidity currents generated nearshore have been suggested to be the source of some sandy marine event beds, but in most cases the evidence is circumstantial. Such flows must commonly travel through a field of oscillatory flow caused by wind-generated waves; little is known, however, about the interactions between waves and turbidity currents. We explore these interactions through detailed process-oriented sedimentological analysis of sandstone event beds from the Pennsylvanian Minturn Formation in north-central Colorado, U.S.A. The Minturn Formation exhibits a complex stratigraphic architecture of fan-delta deposits that developed in association with high topographic relief in a tectonically active setting. An ~20–35-m-thick, unconformity-bounded unit of prodelta deposits consists of dark green shale and turbidite-like sandstone beds with tool marks produced by abundant plant debris. Some of the sandstone event beds, most abundant at distal localities, contain reverse-to-normal grading and sequences of sedimentary structures that indicate deposition from waxing to waning flows. In contrast, proximal deposits, in some cases less than a kilometer away, contain abundant beds with evidence for deposition by wave-dominated combined flows, including large-scale hummocky cross-stratification. We interpret the majority of these event beds as a record of deposition from hyperpycnal flows, i.e., turbidity currents generated directly from highly concentrated river plumes, which accelerated and decelerated in response to a rising and falling flood discharge. Additional support for this interpretation includes the following: (1) a variety of sole marks including flute and gutter casts, as well as tool marks made by relatively large (up to tens of centimeters across) woody debris (i.e., groove, prod, and chevron marks); (2) consistent unimodal orientation of sole marks and abundant ripple cross-stratification, which indicate strong downslope-directed flow; (3) a well documented sedimentological framework for the formation of fan-delta deposits adjacent to nearby highlands; and (4) plant fossils typical of middle- to high-elevation habitats that are abundant in the turbidite beds but absent in underlying and overlying shoreline and marginal marine deposits, which have a separate floral assemblage. Differences in grain sizes, vertical stratification sequences, and bed thicknesses between outcrops are interpreted to result from the spatial distribution of wave effects, the time history of hyperpycnal flows, and the interaction of these processes. The latter varied both spatially and temporally and produced a wide range of bed types, which are incorporated into a new conceptual model for storm-influenced hyperpycnal flows.

The Middle Cambrian Mount Roosevelt Formation (new) of northeastern British Columbia: evidence for rifting and development of the Kechika Graben System

The eastern margin of the Kechika Graben in the vicinity of Muncho and Moose lakes, northeastern British Columbia, contains Middle Cambrian rift-related strata herein named the Mount Roosevelt Formation. The formation conformably overlies a quartzite package equivalent to the uppermost clastic unit of the informal late Early Cambrian Gataga group ( Bonnia – Olenellus zone) and is conformably overlain by a thick, unnamed carbonate sequence of Middle Cambrian age ( Plagiura – Poliella zone). The Mount Roosevelt Formation is subdivided into three members. The lowermost member is characterized by oöid-bearing siltstone and sandstone, interbedded with dolostone, limestone, and hematitic conglomerate. Conformably above this, the middle member is a thick sequence of polymict cobbly pebble conglomerate. The upper member includes karstified dolostone, calcareous-cemented conglomerate and sandstone, and limestone. Collectively the Mount Roosevelt Formation reflects alluvial fan delta progradation into a transgressive marine environment. Deposition occurred in an active fault-controlled basin, located on the eastern margin of the Kechika Graben adjacent to the Muskwa High. Basin initiation in the late Early Cambrian coincided with the reactivation of pre-existing regional faults. High rates of subsidence during the initial phase of extension were accommodated on these faults, which provided a locus for fan delta deposition. Continuing high rates of subsidence limited basinward fan delta development. Deposition of the formation ended with base-level transgression in the early Middle Cambrian that drowned the fan deltas and the adjacent Muskwa High and allowed development of the Kechika Trough above the older graben system.

Late Cretaceous to middle Tertiary basin evolution in the central Tibetan Plateau: Changing environments in response to tectonic partitioning, aridification, and regional elevation gain

Research Article| May 01, 2007 Late Cretaceous to middle Tertiary basin evolution in the central Tibetan Plateau: Changing environments in response to tectonic partitioning, aridification, and regional elevation gain P.G. DeCelles; P.G. DeCelles 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar P. Kapp; P. Kapp 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar L. Ding; L. Ding 2Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100029, People's Republic of China Search for other works by this author on: GSW Google Scholar G.E. Gehrels G.E. Gehrels 3Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar Author and Article Information P.G. DeCelles 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA P. Kapp 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA L. Ding 2Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100029, People's Republic of China G.E. Gehrels 3Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Publisher: Geological Society of America Received: 13 Jul 2006 Revision Received: 09 Nov 2006 Accepted: 27 Nov 2006 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 GEOLOGICAL SOCIETY OF AMERICA GSA Bulletin (2007) 119 (5-6): 654–680. https://doi.org/10.1130/B26074.1 Article history Received: 13 Jul 2006 Revision Received: 09 Nov 2006 Accepted: 27 Nov 2006 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation P.G. DeCelles, P. Kapp, L. Ding, G.E. Gehrels; Late Cretaceous to middle Tertiary basin evolution in the central Tibetan Plateau: Changing environments in response to tectonic partitioning, aridification, and regional elevation gain. GSA Bulletin 2007;; 119 (5-6): 654–680. doi: https://doi.org/10.1130/B26074.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Located in the Bangong suture zone between the Lhasa and Qiangtang terranes of central Tibet, the Nima basin records Cretaceous–late Miocene sediment accumulation during a period of drastically changing paleogeography and paleoelevation. The Bangong suture zone originally formed during Late Jurassic–Early Cretaceous time as the Lhasa terrane collided with the Qiang-tang terrane. During Early to middle Cretaceous time, the region lay at northern near-equatorial paleolatitudes, near sea level. By Aptian time the Nima basin was above sea level and was strongly influenced by nearby volcanic activity and crustal shortening in the reactivated Bangong suture zone. In the southern Nima basin, an ∼50 m.y. (Late Cretaceous–Eocene) depositional hiatus correlates with major crustal shortening and ensuing voluminous ignimbrite eruptions in the Lhasa terrane. In the northern Nima basin, deposition continued during latest Cretaceous time, recording arid paleoclimate in evaporitic lacustrine and eolian dunefield deposits. By Oligocene time the Nima basin comprised two independent depocenters that accumulated coarse-grained alluvial, fluvial, lacustrine (evaporitic), and fan-delta deposits in close association with reactivated thrusts in the Bangong suture zone.Carbon and oxygen isotope data from Oligocene paleosol carbonate, reported elsewhere, indicate that regional paleoelevation during the late Oligocene was >4.6 km, as it is today. Overall, the depositional record of the Nima basin, combined with ongoing structural and geochronologic studies, demonstrates that the Bangong suture zone was reactivated during middle Cretaceous and middle Tertiary time, that the intervening ∼50 m.y. interval was a time of regional upper crustal shortening in the Lhasa terrane followed by regional ignimbrite eruptions, and that arid paleoclimate and high paleoelevation were established by the Late Cretaceous and late Oligocene, respectively.Within the context of other data sets from the Lhasa terrane, the record of deposition in the Nima basin is consistent with low-angle subduction of Neotethyan oceanic lithosphere and reactivation of the Bangong suture zone during the Early Cretaceous, followed by shortening within the Gangdese retroarc and northern Lhasa terrane thrust belts during the Late Cretaceous, lithospheric delamination-dripping and regional magmatic flare-up during latest Cretaceous through early Tertiary time, and underthrusting of Indian lower crust and lithosphere as far north as the Bangong suture zone during late Oligocene time. 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