Basin Accommodation Research Articles

Watershed-scale provenance heterogeneity within Eocene nonmarine basin fill: Southern Greater Green River Basin, western USA

Weathering, erosion, and sediment transport in modern landscapes may be investigated via direct observation of attributes such as elevation, relief, bedrock lithology, climate, drainage organization, watershed extent, and others. Studies of ancient landscape evolution lack this synoptic perspective, however, and instead must rely more heavily on downstream records of fluvial deposits. Provenance analysis based on detrital grain ages has greatly enhanced the utility of such records but has often focused broadly on regional to continental scales. This approach may overlook important details of localized watersheds, which could lead to significant misinterpretation of past sediment dispersal patterns. The present study, therefore, explores the impact of geographic and stratigraphic sampling density on detrital zircon provenance, based on a high-density investigation of U-Pb ages (N = 23, n = 4905) obtained from a narrow chronostratigraphic range (∼2 m.y.) within a relatively small (∼25,000 km2) area of an Eocene nonmarine sedimentary basin. Based on multi-dimensional scaling and DZmix modeling, these strata comprise seven distinct, approximately isochronous detrital zircon (DZ) chronofacies, defined as “. . . a group of sedimentary rocks that contains a specified suite of detrital zircon age populations” (Lawton et al., 2010). Four of these DZ chronofacies reflect long-distance transport from extrabasinal source areas. DZ chronofacies CO-1 and CO-2 are interpreted to derive from a primary sediment source in central Colorado (USA), corroborating previously proposed long-distance sediment transport via the Aspen paleoriver. DZ chronofacies ID-1 and ID-2 are interpreted to have been delivered to the basin from central Idaho by the Idaho paleoriver. In contrast, DZ chronofacies UT-1 and UT-2 are interpreted to reflect local drainage from the Uinta Uplift south of the basin, and DZ chronofacies WY-1 is interpreted to have been sourced from the Rawlins, Granite, and Sierra Madre uplifts to the north and east via the Toya Puki paleoriver. Lateral transitions between different DZ chronofacies in some cases occur over distances as little as 5 km, implying that depositional systems carrying sand from disparate watersheds directly competed to fill available basin accommodation. The results of this study reveal a high degree of complexity of Eocene rivers that converged on the Greater Green River Basin, indicating that their deposits contain a rich record of fine-scale landscape evolution across much of the Laramide foreland and Cordilleran orogen. These results illustrate the need for adequate sample density when assessing basin-scale provenance and offer a cautionary consideration for researchers using sandstone (and incorporated authigenic cement) in other nonmarine basins as the basis for paleoaltimetry or detrital thermochronology studies.

Influence of a Rapidly Uplifting Orogen on the Preservation of Climate Oscillations

AbstractClimate oscillations preserved in sedimentary archives tend to decrease in resolution further back in Earth's history. High‐frequency climate cycles (e.g., ∼20‐Kyr precession cycles) are especially prone to poor preservation due to sediment reworking. Recent studies have shown, however, that given sufficient basin accommodation space and sedimentation rate, shallow‐marine paleoclimate archives record precession‐driven hydroclimate change in mid‐low latitude regions. Our study evaluates how the evolution of a rapidly uplifting orogen influences the recording of astronomical climate forcing in shallow‐marine sedimentary strata in the Taiwan Western Foreland Basin (WFB). Time‐series analysis of gamma‐ray records through the late Miocene–Pliocene Kueichulin Formation shows that during early stages of Taiwan orogenesis (before 5.4 Ma), preservation of precession‐driven East Asian Summer Monsoon variability is low despite increasing monsoon intensities between 8 and 3 Ma. The Taiwan Strait had not formed, and the southeast margin of Eurasia was open to the Pacific Ocean. Consequently, depositional environments in the WFB were susceptible to reworking by large waves, resulting in the obscuration of higher‐frequency precession cycles. From 5.4 to 4.92 Ma, during early stages of emergence of Taiwan, basin subsidence increased while sedimentation rates remained low, resulting in poor preservation of orbital oscillations. After 4.92 Ma and up to 3.15 Ma, Taiwan became a major sediment source to the WFB, and sheltered the WFB from erosive waves with the development of Taiwan Strait. The elevated sediment influx, increased basin accommodation as the WFB developed, and formation of a semi‐sheltered strait, resulted in enhanced preservation of precession‐driven East Asian Summer Monsoon variability.

The evolution of a coastal wedge in response to Plio-Pleistocene climate change: The Northern Adriatic case

This work reconstructs the complex evolution of a coastal wedge in response to Pliocene tectonic activity and Pleistocene climate change recorded in the upper Neogene subsurface succession of the Friulian-Venetian Basin (FVB), since the latest Messinian sea-level drop event to the late Pleistocene. The FVB is the north-eastern portion of the wider Po Plain-Adriatic foreland basin (Northern Italy), which bounds to the east the whole Italian peninsula and is the place where most of the Italian hydrocarbon offshore fields presently occur. Also, the city of Venice and its surrounding lagoon is presently one of the sites most sensitive to land subsidence worldwide and even a few mm loss of ground elevations can significantly change the natural lagoon environments and threaten the city's survival. To this purpose, we have integrated seismics and well logs analysis with 1D-geohistory modelling on wells, using corrected paleobathymetric data, in order to extract decompacted sedimentation rates. The new age model is based on a complete chronostratigraphic revision of the continuously cored Venezia 1 well, correlated with other 12 hydrocarbon wells to form proper geological depth-calibrated profiles across the basin. Results suggest a major tectonic control on marine sedimentation and basin accommodation space changes during the entire Pliocene-early Pleistocene time, influenced by the Northern Apennines thrust fronts migration. On the contrary, the mid-upper Pleistocene deposition, characterized by a weak deformation, is mainly controlled by climate and basin geometry. During the early mid Pleistocene, an impressive progradational cycle (in the literature usually named “Sabbie di Asti group”) followed the deposition of deep-water turbidites; in this work, we subdivide the different clastic bodies the formation is made of. The uppermost part of the “Sabbie di Asti group” is dominated by shallow-water to continental deposits showing a prominent regressive-transgressive cyclicity, where unfortunately chronological ties are limited. Nevertheless, 6 main cycles driven by 100 kyr Milankovitch-type ciclicity were laterally correlated by means of the well electric logs and linked to marine ∂18O isotope and eustatic curves.

From marine bands to hybrid flows: Sedimentology of a Mississippian black shale

AbstractOrganic‐rich mudstones have long been of interest as conventional and unconventional source rocks and are an important organic carbon sink. Yet the processes that deposited organic‐rich muds in epicontinental seaways are poorly understood, partly because few modern analogues exist. This study investigates the processes that transported and deposited sediment and organic matter through part of the Bowland Shale Formation, from the Mississippian Rheic–Tethys seaway. Field to micron‐scale sedimentological analysis reveals a heterogeneous succession of carbonate‐rich, siliceous, and siliciclastic, argillaceous muds. Deposition of these facies at basinal and slope locations was moderated by progradation of the nearby Pendle delta system, fourth‐order eustatic sea‐level fluctuation and localized block and basin tectonism. Marine transgressions deposited bioclastic ‘marine band’ (hemi)pelagic packages. These include abundant euhaline macrofaunal tests, and phosphatic concretions of organic matter and radiolarian tests interpreted as faecal pellets sourced from a productive water column. Lens‐rich (lenticular) mudstones, hybrid, debrite and turbidite beds successively overlie marine band packages and suggest reducing basin accommodation promoted sediment deposition via laminar and hybrid flows sourced from the basin margins. Mud lenses in lenticular mudstones lack organic linings and bioclasts and are equant in early‐cemented lenses and in plan‐view, and are largest and most abundant in mudstones overlying marine band packages. Thus, lenses likely represent partially consolidated mud clasts that were scoured and transported in bedload from the shelf or proximal slope, as a ‘shelf to basin’ conveyor, during periods of reduced basin accommodation. Candidate in situ microbial mats in strongly lenticular mudstones, and as rip‐up fragments in the down‐dip hybrid beds, suggest that these were potentially key biostabilizers of mud. Deltaic mud export was fast, despite the intrabasinal complexity, likely an order of magnitude higher than similar successions deposited in North America. Epicontinental basins remotely linked to delta systems were therefore capable of rapidly accumulating both sediment and organic matter.

Using bar preservation to constrain reworking in channel-dominated fluvial stratigraphy

Fluvial deposits comprising more than 80% channel facies are often thought to have accumulated during intervals of relatively slow subsidence in sedimentary basins. This interpretation stems from the conceptual model that migrating and avulsing rivers rework their own deposits during times of limited accommodation creation, preferentially removing and bypassing fine floodplain deposits. Alternatively, channel-dominated stratigraphy may reflect avulsion patterns that favor channel preservation over floodplain preservation, or channel networks fed by sand-rich sediment sources that never deposited significant floodplain sediments. These potential origins of channel-dominated stratigraphy cannot be differentiated without a way of independently assessing how ancient rivers eroded and reworked their own alluvium. Here we propose a new method that uses fluvial-bar preservation as a proxy for reworking in channel-dominated stratigraphy. We apply this approach to the lower Castlegate Sandstone (Upper Cretaceous, Utah, USA) and use geometric modeling to investigate the degree to which sediment supply and avulsion dynamics influence fluvial deposit preservation. Castlegate exposures in central Utah show up to 80% bar preservation in some localities, suggesting that, in contrast to previously published interpretations, Castlegate channel deposits are not heavily reworked and accommodation-creation rates during Castlegate deposition might have been relatively high. Model comparisons indicate that well-preserved Castlegate deposits could have resulted from random avulsions in a rapidly aggrading basin with a sand-dominated sediment supply, or under lower aggradation conditions if river avulsions avoided previously occupied locations. This approach represents a new advance for interpreting the relationships between basin accommodation, sediment supply, and avulsion dynamics on ancient landscapes and provides a new method for estimating sediment reworking and bypass from ancient river deposits.

Controls on amorphous organic matter type and sulphurization in a Mississippian black shale

Paleoredox proxies (Fe speciation, trace element and δ34Spy) integrated with sedimentological and palynological observations link the distribution and type of particulate organic matter (OM) preserved to hydrocarbon source rock potential. In the Mississippian Bowland Shale Formation (Lancashire, UK), particulate OM is dominated by “heterogeneous” amorphous OM (AOM), primarily “sharp-edged, pellet-like” (AOMpel) and “heterogeneous, granular” (AOMgr) types. AOMpel is abundant in muds deposited under anoxic and moderately to highly sulphidic conditions and most likely represents the fecal minipellets of zooplankton and/or pellets of macro-zooplankters. We recognize two intervals, “A” and “B,” which exhibit Sorg/TOC > 0.04, suggesting a bulk Type II-S kerogen composition. The Interval A palynofacies is typified by pyritized AOMpel (AOMpyr) particles that contain high-relief organic spheres surrounding individual pyrite framboids, within each AOMpyr particle. These textures are interpreted as sulphurized OM local to pyrite framboids (Sorg-PF). Sorg-PF is rarely observed in Interval B, and absent in all other samples. Redox oscillation between ferruginous and euxinic conditions during early diagenesis of Interval A likely promoted S cycling in microenvironments surrounding pyrite framboids, which generated reactive S species and reactive OM required for sulphurization. Early diagenetic redox oscillation processes were apparently triggered by relative sea level fall, associated with an increased supply of FeHR from adjacent shelves into the basin. Interval B represents deposition during the late stages of basin infill and transition from anoxic to (sub)oxic bottom waters, where AOMpel is replaced by AOMgr as the dominant type of AOM. A large particle diameter at the limit of the mesh size (500 μm), sheet-like, fragmented character, and presence of candidate organic sheaths suggests AOMgr at least partially represent fragments of benthic microbial mats, probably as sulphide-oxidizers. A ternary plot of AOMpel + AOMpyr versus AOMgr versus spores + phytoclasts links the observed palynofacies to bottom and pore water redox conditions, water column productivity and proximity to fluvial (deltaic) supply of spores and phytoclasts. These variables were moderated by changing basin accommodation, driven primarily by eustatic sea level fluctuation. A sequence-stratigraphic control on AOM type and sulphurization is important for understanding the link between source rock heterogeneity and the timing of hydrocarbon generation and expulsion from this source rock.

Interconnected Accommodation Space Controls Between Sand-Charged Shallow Tidal Channels and Wind-Wave Truncated Tidal Flats During Latest-Holocene Sea Level Rise (~3.0 m) in a Large Mesotidal Wave-Dominated Estuary, Grays Harbor, Washington, USA

The late-Holocene (5–0 ka) record of accommodation space controls of tidal channel and tidal flat deposition in the shallow mesotidal wave-dominated Grays Harbor estuary (236 km2 surface area) was investigated in previously reported drill cores (n=15) and new vibracores (n=20), reaching 3–10 m depth subsurface. Continuous vibracore facies sequences (3–4 m depth subsurface) discriminate between tidal channel and tidal flat deposition and demonstrate responses of both depositional settings to interseismic uplift and coseismic subsidence (1±0.5 m vertical) from cyclic neotectonic forcing (200–800 yr recurrence intervals) in the Cascadia subduction zone. Vibracore channel samples, at 0.5 m or 1.0 m depth intervals, were analyzed for sediment grain size (sample n=124) and sand source mineralogy (sample n=67). The mean and standard deviation of sand size in the sand fraction is 175±x34 1σ µm. Sediment 14C dates (n=29) range from 376 to 6,579 median calyrBP and establish long-term sedimentation rates in subtidal channel accretionary banks (average 4.2 m ka-1), intertidal channel accretionary banks (average 3.7 m ka-1), and tidal flats (average 1.1 m ka-1). Tidal channel accretionary bank deposition largely reflects reworking of pre-existing estuary deposits. Long-term total basin sediment accumulation rates (232x106 m3 ka-1) are tied to rates of net sea level rise (1.0 m ka-1) or increasing basin accommodation space. In latest Holocene time (3–0 ka) littoral sand import (117x106 m3 ka-1) was about twice as large as the retention of river sand and mud in the estuary. The selective export of winnowed mud from the estuary provided the necessary accommodation space for the import of littoral sand in latest-Holocene time. Shallow intertidal settings in Grays Harbor (60% by surface area) are maintained by self-regulating conditions of channelized sediment import, wind-wave erosion of tidal flats, and tidal prism forcing of tidal channel discharge. Hind-casted wind-wave bottom orbital velocities (>20 cm sec-1) are sufficient to truncate tidal flat elevations to lower-intertidal levels, which maintain substantial tidal prism volumes (modern MLLW-MHHW ~6.1 x 108 m3) and associated tidal channel discharge in the shallow estuary. Net sediment deposition in the estuary is controlled by the interaction of limiting accommodation space controls in the tidal flats and tidal channels. The balance between sediment supply, energy of sediment transport/resuspension, and sediment export has survived small changes in relative sea level (1±0.5 m) from cyclic neotectonic forcing. However, the prehistoric (natural) balance could be altered by future anthropogenic impacts from sustained global sea level rise (> 1.5 m during the next century) or diminished wind-wave fetch distances, which could result from tidal flat diking/filling or uncontrolled spread of non-native invasive stabilizing sea grass (Spartina). In this regard, the susceptibilities of prehistorically-balanced sediment dynamics in Grays Harbor serve as warning for other similar mesotidal wave-dominated estuaries that could be impacted by future global sea level rise, changing sediment inputs, and/or tidal flat diking/filling, which could reduce intertidal habitat and associated ecosystem functions.

High-resolution compositional analysis of a fluvial-fan succession: The Miocene infill of the Cacheuta Basin (central Argentinian foreland)

The Miocene infill of the Cacheuta Basin (central Argentinian foreland) comprises the Mariño and La Pilona formations, which record continental sedimentation related to the major phases of uplift of the Andean chain around 33° S over 1500m of stratigraphy. Sedimentological and stratigraphic evidence suggest the succession to represent progradation of a fluvial-fan sourced from the western, orogenic margin of the foreland basin. The integration of compositional data and sedimentological observations consent to disentangle the relative roles of allogenic factors over long time scales, and to separate them from system-scale autogenic dynamics. The geochemical, mineralogical and radiogenic-isotope composition of sandstones through the succession show a progressive change in the composition of magmatic source rocks from more primitive (basaltic-andesitic) to relatively more evolved (dacitic), testifying the uplift of the western part of the Principal Cordillera, followed by the sequential eastward advance of the thrust and volcanic fronts toward the foreland basin, in agreement with established chronologies of Andean structural development at these latitudes. Sandstones from the La Pilona Formation record compositional signatures suggesting the first stages of uplift of the Frontal Cordillera. The gradual changes in the compositional signal are attributed to changing weathering conditions related to climate and to source-rock changes, related to the Miocene tectonic evolution of the Andean range. However, results suggest that the influence of large-scale allogenic factors is partly blurred by the effects of autogenic fluctuating depositional processes, especially during deposition of the La Pilona Formation. The gradual transitions in provenance and weathering signals suggest that vertical architectural and facies changes through the succession are likely related to the long-term, progradational evolution of the fluvial-system, rather than to allogenic changes in basin accommodation or climate-related sediment supply.

Tectonic significance of Cenozoic exhumation and foreland basin evolution in the Western Alps

The Alps are the archetypical collisional orogenic system on Earth and yet our understanding of processes controlling topographic growth in the Cenozoic remains incomplete. Whereas ideas and models on the Alps are abundant, data from the foreland basin record able to constrain the timing of erosion and sedimentation, mechanisms of basin accommodation and basin deformation are sparse. We combine seismic stratigraphy, micropaleontology, white mica 40Ar/39Ar, detrital zircon (U-Th)/He and apatite fission track thermochronology to Miocene-Pliocene samples from the retro-wedge foreland basin (Saluzzo Basin in Italy) and to Oligocene-Miocene sedimentary rocks from the pro-wedge foreland basin (Bârreme Basin in France) of the Western Alps. Our new data show that exhumation in the Oligocene-Miocene was non uniform across the Western Alps. Topographic growth was underway since the Oligocene and exhumation was concentrated on the pro-side of the orogenic system. Rapid and episodic early Miocene exhumation of the Western Alps was concentrated instead on the retro-side of the orogen and correlates with a major unconformity in the proximal retro-foreland basin. A phase of orogenic construction is recorded by exhumation of the proximal pro-foreland in both the Central and Western Alps at ca. 16 Ma. This is associated with high sedimentation rates, and by inference erosion rates, and suggests that an increase in accretionary flux associated with the dynamics of subduction of Europe under Adria controlled orogenic expansion in the Miocene.

Accommodation space controls on incised-valley sediment accumulation rates during the Holocene marine transgression (0–11 ka) in Grays Harbor, a large meso-tidal estuary, Washington, USA

Holocene deposits (0–11ka) that were drill cored (275 samples from 17 sites) in the meso-tidal Grays Harbor estuary were analyzed for 1) sediment texture and source components and 2) calibrated radiocarbon age. These data were used with basin 10m-contour interval volumes to establish relations between rates of increasing basin accommodation space and sediment volume accumulation. Rates of increasing accommodation space (0.3–2.1×106m3yr−1) are based on interval volumes and a sea level curve (0–11.5ka) for the Grays Harbor basin. Total sediment accumulation rates (0.41–1.4×106m3yr−1) are computed for the same 10m-depth intervals (0–60m depth MSL) and corresponding 10m-interval sedimentation durations (0.7–5.0×103ka) from a sediment level curve. The rates of basin increasing accommodation space and total sediment accumulation are positively correlated (R2=0.81), but sediment accumulation lagged behind increasing accommodation space (23–37%) during early- and middle-Holocene time (7–10ka). Slowing rates of sea level rise after 8ka, permitted combined river and littoral inputs of sediment to reduce basin subtidal water depths (~10m) to intertidal water depths (~1m). The ratios between sand and mud components in the analyzed Holocene deposits are negatively correlated to basin water depth. Shallowing tidal flats promoted the export of river mud out of the estuary and the import of littoral sand into the central estuary areas. Variable rates of increasing basin accommodation space 1) directly controlled the total sediment accumulation rate in the evolving basin and 2) indirectly controlled the relative proportions of mud and littoral sand accumulation, due to changing water depths in the basin. The late-Holocene accumulation of littoral sand (52% of total sediment volume) is used to compute the net littoral sand sink (~100×106m3) in the Grays Harbor basin from a potential 1.0m rise of future sea level following predicted global warming. The contribution of littoral sand from adjacent barrier sand spits (20km in length) to fill the increasing accommodation space in the Grays Harbor estuary could result in 0.5–1.0km of shoreline retreat following a 1.0m rise in sea level.

To what extent can intracrater layered deposits that lack clear sedimentary textures be used to infer depositional environments?

Craters within Arabia Terra, Mars, contain hundreds of meters of layered strata showing systematic alternation between slope- and cliff-forming units, suggesting either rhythmic deposition of distinct lithologies or similar lithologies that experienced differential cementation. On Earth, rhythmically deposited strata can be examined in terms of stratal packaging, wherein the interplay of tectonics, sediment deposition, and base level (i.e., the position above which sediment accumulation is expected to be temporary) result in changes in the amount of space available for sediment accumulation. These predictable patterns of sediment deposition can be used to infer changes in basin accommodation regardless of the mechanism of deposition (e.g. fluvial, lacustrine, or aeolian). Here, we analyze sedimentary deposits from three craters (Becquerel Crater, Danielson Crater, Crater A) in Arabia Terra. Each crater contains layered deposits that are clearly observed in orbital images. Although orbital images are insufficient to specifically determine the origin of sedimentary deposits, depositional couplets can be interpreted in terms of potential accommodation space available for deposition, and changes in the distribution of couplet thickness through stratigraphy can be interpreted in terms of changing base level and the production of new accommodation space. Differences in stratal packaging in these three craters suggest varying relationships between sedimentary influx, sedimentary base level, and concomitant changes in accommodation space. Previous groundwater upwelling models hypothesize that layered sedimentary deposits were deposited under warm climate conditions of early Mars. Here, we use observed stacking patterns to propose a model for deposition under cold climate conditions, wherein episodic melting of ground ice could raise local base level, stabilize sediment deposition, and result in differential cementation of accumulated strata. Such analysis demonstrates that a first-order understanding of sedimentary deposition and accumulation—despite a lack of textural information that inhibits interpretation of depositional mechanism—can provide insight into potentially changeable depositional conditions of early Mars.

Accommodation Space Controls on the Latest Pleistocene and Holocene (16–0 ka) Sediment Size and Bypassing in the Lower Columbia River Valley: A Large Fluvial–Tidal System in Oregon and Washington, USA

Peterson, C.D.; Gates, E.B.; Minor, R., and Baker, D.L., 2013. Accommodation space controls on the latest Pleistocene and Holocene (16–0 ka) sediment size and bypassing in the Lower Columbia River Valley: a large fluvial–tidal system in Oregon and Washington, USA.In this study, we establish the roles that increasing basin accommodation space have on sediment size and bypassing in the transgressive fill (16–0 ka) in the submerged Lower Columbia River Valley (LCRV). The antecedent forearc valley (225 km in length, 4–8 km in width, and 60–115 m in axial valley depth) is characterized by high sediment supply rates (10–15 million t y−1) but no delta at its mouth to the Pacific Ocean. Core sample sediment textures (N = 1600) are analyzed from 3000 m of borehole sections in 58 representative boreholes to characterize the ancestral valley fill: 57% sand, 17% muddy sand, 12% sandy mud, and 14% mud (total fill volume = 77 km3). Decreasing mud endmember texture from between 30 and 60% (early Holocene) to 18% (late Holocene) is directly related to (1) declining rates of increasing basin accommodation space and (2) increasing sediment bypassing through the LCRV. Both conditions result from decreasing rates of sea level rise (15.0–1.5 mm y−1) in Holocene time. The overall coarsening of sediments, both upvalley and upsection, is controlled by declining rates of increasing basin accommodation space (5.66 million–0.57 million m3 y−1) during middle to late Holocene time. Differences between middle and late Holocene sediment accumulation rates yield bedload bypassing rates of 5– 6 million m3 y−1, under assumed conditions of constant sediment supply rates in middle to late Holocene time.

Deriving a long paleoseismic record from a shallow-water Holocene basin next to the Alpine fault, New Zealand

A sedimentary sequence that was highly sensitive to fault rupture–driven changes in water level and sediment supply has been used to extract a continuous record of 22 large earthquakes on the Alpine fault, the fastest-slipping fault in New Zealand. At Hokuri Creek, in South Westland, an 18 m thickness of Holocene sediments accumulated against the Alpine fault scarp from ca. A.D. 800 to 6000 B.C. We used geomorphological mapping, sedimentology, and paleoenvironmental reconstruction to investigate the relationship between these sediments and Alpine fault rupture. We found that repeated fault rupture is the most convincing mechanism for explaining all the features of the alternating peat and silt sedimentary sequence. Climate has contributed to sedimentation but is unlikely to be the driver of these cyclical changes in sediment type and paleoenvironment. Other nontectonic causes for the sedimentary alternations do not produce the incremental increase in basin accommodation space necessary to maintain the shallow-water environment for 6800 yr. Our detailed documentation of this near-fault sedimentary basin sequence highlights the advantages of extracting paleoearthquake records from such sites—the continuity of sedimentation, abundance of dateable material, and pristine preservation of older events.

Mechanisms Controlling the Clustering of Fluvial Channels and the Compensational Stacking of Cluster Belts

Abstract Analysis of sand-body distribution reveals that fluvial channel sands in the Upper Cretaceous lower Williams Fork Formation in the central Piceance Basin, Colorado, USA, are not randomly distributed but are predictable in their spatial and stratigraphic position. Sand bodies are organized in channel-belt clusters that are compensationally stacked on different temporal scales. While channel clustering occurred over a shorter period (channel and channel-belt scale 10s ky to 100s ky), compensational stacking occurred over a much longer time scale (channel cluster belts, ~ 400 ky). The lower Williams Fork Formation consists of three distinct intervals of clustered channel belts, each ~ 400 ft (~ 122 m) thick, topped by a thin coal layer. The primary control on the cluster formation is autogenic channel avulsion during an overall aggradational phase. Short-lived changes in in basin accommodation caused by either changes in tectonic subsidence or high-frequency eustatic changes due to Milankovitch-band orbital forcing at the end of each cluster interval result in a distinct increase in channel thickness and coal formation towards the end of each cycle. Coals at the end of each sub-cluster span the entire study area, but their overall thickness is greater in the inter-cluster, floodplain-prone areas. The resulting differential early coal compaction is greatest in the inter-cluster areas and has a significant control on the subsequent cluster-belt position overlying the coal. Channel sands overlying the coals are concentrated in the previously low net/gross floodplain-prone inter-cluster areas resulting in compensationally stacked cluster belts. On the longest time scale (~ 1 to 1.5 My), which encompasses the entire lower Williams Fork, changes in channel geometries and sand-body thickness seem to be controlled mainly by long-term changes in eustasy, and autogenic processes are of lesser importance.

Magnetostratigraphy of the Neogene Chaka basin and its implications for mountain building processes in the north‐eastern Tibetan Plateau

ABSTRACTMagnetostratigraphy of sedimentary rock deposited in the Chaka basin (north‐eastern Tibetan Plateau) indicates a late Miocene onset of basin formation and subsequent development of the adjacent Qinghai Nan Shan. Sedimentation in the basin initiated at ∼11 Ma. In the lower part of the basin fill, a coarsening‐upward sequence starting at ∼9 Ma, as well as rapid sedimentation rates, and northward paleocurrents, are consistent with continued growth of the Ela Shan to the south. In the upper section, several lines of evidence suggest that thrust faulting and topographic development of the Qinghai Nan Shan began at ∼6.1 Ma. Paleocurrent indicators, preserved in the basin in the proximal footwall of the Qinghai Nan Shan, show a change from northward to southward flow between 6.5 and 3.8 Ma. At the same location, sediment derived from the Qinghai Nan Shan appears at 6.1 Ma. Finally, the initiation of progressively shallowing dips observed in deformed basin strata and a change to pebbly, fluvial deposits at 6.1 Ma provide a minimum age for the onset of slip on the thrust fault that dips north‐east beneath the Qinghai Nan Shan. We interpret a decrease in sediment accumulation rates since ∼6 Ma to indicate a reduction in Chaka basin accommodation space due to active faulting and folding along the Qinghai Nan Shan and incorporation of the basin into the wedge‐top depozone. Declination anomalies indicate the beginning of counter‐clockwise rotation since 6.1 Ma, which we associate with local deformation, not regional block rotation. The emergence of the Qinghai Nan Shan near the end of the Miocene Epoch partitioned the once contiguous Chaka‐Gonghe and Qinghai basin complex. In a regional framework, our study adds to a growing body of evidence that points to widespread initiation and/or reactivation of fault networks during the late Miocene across the north‐eastern Tibetan Plateau.

Tectonic and eustatic controls on Pliocene accommodation space along the front of the southern Apennine thrust-belt (Basilicata, southern Italy)

The stratigraphical organization of the Pliocene thrust-top deposits cropping out at the front of the Southern Apennine thrust-belt has been debated for a long time taking a great importance in the context of the geodynamics of the Central Mediterranean area. During this time, spreading episodes in the Apennine backarc zone alternate with important phases of overthrusting in the thrust-belt. As a consequence, the Pliocene succession appears to be arranged in a series of stacked units, recording the poliphase tectonic history that leads to the building of the front of the southern Apennine thrust-belt. Although there is not yet an accordance on the nature and position of the main unconformities bounding the thrust-top units, all authors agree that the creation of new accommodation space is mainly ruled by contractional tectonics consequent to the eastward nappe propagation according to the Apennine vergence polarity. A detailed geological survey, carried out along a large portion of southern Apennine thrust-belt front, running south of the Vulture volcano, allowed the collecting of new data concerning the basinal-formation mechanisms acting during the sedimentation of Pliocene deposits. From this analysis, it is clear that even if contractional tectonics is the predominant factor controlling the creation or destruction of accommodation space, other mechanisms, as well as wedge uplift-related extensional tectonics and eustasy, could have also played a significant role in the basin accommodation. In order the considered sector of southern Apennines can provide an useful example about the complex phenomena occurring at mountain belt front where the accommodation space results from a concomitance of eustatic and tectonic factors mainly linked to the accretionary wedge activity.

Synorogenic evolution of large-scale drainage patterns: Isotope paleohydrology of sequential Laramide basins

In the past decade, we and others have compiled an extensive dataset of O, C and Sr isotope stratigraphies from sedimentary basins throughout the Paleogene North American Cordillera. In this study, we present new results from the Piceance Creek Basin of northwest Colorado, which record the evolving hydrology of the Eocene Green River Lake system. We then place the new data in the context of the broader Cordilleran dataset and summarize implications for understanding the synorogenic evolution of large-scale drainage patterns. The combined data reflect (1) a period of throughgoing foreland rivers heading in the Sevier fold-and-thrust belt and flowing east, (2) ponding of freshwater lakes in the foredeep as Laramide uplifts blocked drainage, (3) hydrologic closure that led to both intensive evaporation in the terminal sink of the Piceance Creek Basin and integration of catchments over length-scales >1000 km, (4) infilling of basin accommodation by southward migrating magmatism in distal catchments, leading to the freshening and demise of intraforeland lakes that also stepped south over time.

Tectonostratigraphic development and hydrocarbon reservoir quality on a convergent margin: East Coast, North Island, New Zealand

Hydrocarbon exploration on the East Coast of the North Island has not yet yielded significant commercial reserves, even though the elements of a working petroleum system are all present (Field et al, 1997). Exploration has focussed on the shallow, Neogene part of the succession, built up during plate margin convergence over the last ∼24 million years. Convergent margins are generally characterised by low-total organic carbon (TOC) source rocks and poor clastic reservoir quality due to poor sorting and labile grains. However, the obliquely-convergent Hikurangi subduction margin of the East Coast has high TOC source rocks that pre-date the subduction phase, and its reservoir potential, though variable, has several aspects in its favour, namely: deep-water rocks of high porosity and permeability; preservation of pore space by overpressure; the presence of fractured reservoirs and hybrid reservoirs, where low clastic permeability is enhanced by fractures. The East Coast North Island is a Neogene oblique subduction margin, with Neogene shelf and slope basins that developed on Late Cretaceous-Paleogene passive margin marine successions. The main hydrocarbon source rocks are Late Cretaceous and Paleocene and the main reservoir potential is in the Neogene (Field et al, 2005). Miocene mudstones with good seal potential are common, as is significant over-pressuring. Neogene deformation controlled basin development and accommodation space and strongly-influenced lateral facies development and fractured reservoirs. Early to Middle Miocene thrusting was followed by later Neogene extension (e.g. Barnes et al 2002), with a return to thrusting in the Pliocene. Local wells have flow-tested gas shows.

Climate Change Across Continental Sequence Boundaries: Paleopedology and Lithofacies of Iglesia Basin, Northwestern Argentina

Subdivision of sedimentary rocks into genetically related sequences reflects the interaction between the extrinsic and intrinsic processes that affect a basin's accommodation potential as well as its environment. In nonmarine settings, sequence-bounding surfaces are often difficult to locate, and several causal mechanisms for sequences (e.g. tectonism, climate change, and autogenic variability) combine in complex and still poorly understood ways. Here we draw upon paleosols as sources of evidence for causes of sequence boundaries in the upper Miocene–lower Pliocene continental Iglesia wedgetop basin, San Juan Province, Argentina. Stable-isotope compositions (δ13C and δ18O), clay mineralogy, inorganic carbon weight percentages, and micromorphologic features of paleosols that span 9.0–4.6 Ma reveal environmental conditions concurrent with the development of ten continental stratigraphic sequences. Charaetcristics of compound pedofacies and regionally recognizable, abrupt vertical lithofacies changes suggest fluctuations in both accumulation rate and climate across sequence boundaries and between structural subbasins. We interpret that the Iglesia Basin strata accumulated under conditions that were generally semiarid with seasonal precipitation. Isotopic ratios and soil inorganic carbon weight percentages indicate increased aridity at 6.9 and 5.2 Ma, each a time of sedimentary hiatus. Notable enrichment of the δ13C signal at 6.9 Ma also suggests the onset of dominance of C4 plant biomass in northwestern Argentina. In combination with seismic data, we interpret that four of the ten sequence boundaries formed during intervals of marginal surface uplift concurrent with greater aridity, circumstances that led to erosion of proximal strata and starved the central basin of sediment input, resulting in hiatuses in sediment accumulation.

Development and response of a coupled catchment fan system under changing tectonic and climatic forcing

Sediment fans are a potentially useful and underexploited recorder of Earth's climatic and tectonic history, but historical observations have led to conflicting views on the importance of tectonic, climatic, and lithologic variables in controlling fan morphology and deposition. A one‐dimensional model of a sediment fan and its associated catchment is used to explore the sensitivity of such simple sediment routing systems to perturbations in fault slip and precipitation rates. A transport‐limited catchment is coupled to a fan whose surface slope is set by the balance between catchment sediment efflux and the available tectonically generated basin accommodation. Rock uplift rate is spatially variable across the model space. Increasing the fault slip rate, or decreasing the precipitation rate, leads to an increase in fan slope, temporary back‐stepping of the fan toe, and a pronounced angular unconformity. Conversely, a decrease in slip rate, or an increase in precipitation rate, results in a decrease in fan slope, and progradation and eventual stabilization of the fan toe. Once perturbed, the system evolves toward a new equilibrium state with time constants of ∼0.5 to 2 Myr; these response times are insensitive to slip rate but are strongly dependent on precipitation rate. Variations in fan slope are well described by a dimensionless parameter that expresses equilibrium slope as a function of slip rate, precipitation rate, system size, and catchment lithology. This parameter holds promise as a predictive tool in inverting the morphology of natural fans for environmental variables.