Location Of Depocenters Research Articles

The Influence of Channel Planform and Slope Topography on Turbidity Current Overbank Processes: The Example of the Acquarone Fan (Southeastern Tyrrhenian Sea)

Overbank deposits provide a potentially valuable record of flows that have passed through a submarine channel. The architecture of overbank deposits has generally assumed to relate to autogenic processes related to channel construction. In previous models, which are largely based on passive margins, the distribution and geometry of these deposits is relatively simple, and hence generally predictable. Here, we show how the interaction of different flow types with the complex morphology on a highly-tectonically modified margin can profoundly affect overbank depositional processes, and hence also the resultant deposit geometry and architecture. Our case study is the Acquarone Fan, located in the intraslope Gioia Basin in the south-eastern Tyrrhenian Sea, whose topography is mainly controlled by the presence of the Acquarone structural ridge, which results in the confinement of the left south-west side of the channel-levee system. The research is carried out through analysis of multibeam bathymetric and high-resolution Chirp sub-bottom profiler data. Seven depositional units (Units I-VII) record the recent depositional history of the fan; their thickness has been mapped and their parent flow-types have been interpreted through their seismic response. According to unit thickness maps, two main patterns of deposition are recognized in the overbank area. Their depocenters coincide with different extensive sediment wave fields developed in specific tracts of the right levee and in the frontal splay area. We show that the location of the depocenters varies in time according to the prevalent flow-type and by its interaction with the surrounding seafloor topography and channel planform. We interpret that the lateral confinement of the channel by the structural high generates episodic rebound of the overspilling flow and the inversion of the channel asymmetry. The vertical stratification of the flow strongly influences the overbank deposition where the channel planform has a non-linear shape such as bends and knick-points. In particular, the vertical stratification influences the hydraulic jump size that conditions the amount of overspill and thus the location of overbank depocenters. This study highlights that variations in the sediment distribution and composition on the overbank can be related to the way different flows interact with tectonic setting.

Competition between 3D structural inheritance and kinematics during rifting: Insights from analogue models

AbstractThe competition between the impact of inherited weaknesses and plate kinematics determines the location and style of deformation during rifting, yet the relative impacts of these ‘internal’ and ‘external’ factors remain poorly understood, especially in 3D. In this study, we used brittle‐viscous analogue models to assess how multiphase rifting, that is changes in plate divergence rate or direction, and the presence and orientation of weaknesses in the competent mantle and crust, influences rift evolution. We find that the combined reactivation of mantle and crustal weaknesses without any kinematic changes already creates complex rift structures. Divergence rates affect the strength of the weak lower crustal layer and hence the degree of mantle‐crustal coupling; slow rifting decreases coupling, so that crustal weaknesses can dominate deformation localisation and surface structures, whereas fast rifting increases coupling and deformation related to mantle weaknesses can have a dominant surface expression. Through a change from slow to fast rifting mantle‐related deformation can overprint structures that previously formed along (differently oriented) crustal weaknesses. Conversely, a change from fast to slow rifting may shift deformation from mantle‐controlled towards crust‐controlled. When changing divergence directions, structures from the first rifting phase may control where subsequent deformation occurs, but only when they are sufficiently well developed. We furthermore place our results in a larger framework of brittle‐viscous rift modelling results from previous experimental studies, showing the importance of general lithospheric layering, divergence rate, the type of deformation in the mantle, and finally upper crustal structural inheritance. The interaction between these parameters can produce a variety of deformation styles that may, however, lead to comparable end products. Therefore, careful investigation of the distribution of strain localisation, and to an equal extent of basin depocenter locations over time is required to properly determine the evolution of complex rift systems, providing an incentive to revisit various natural examples.

The Teena Zn-Pb Deposit (McArthur Basin, Australia). Part I: Syndiagenetic Base Metal Sulfide Mineralization Related to Dynamic Subbasin Evolution

Abstract Divergent genetic models have been proposed for clastic dominant (CD-type) massive sulfide Zn-Pb mineralization in the Proterozoic Carpentaria Zn Province. Due to varying degrees of tectonic overprint, it has been difficult to accurately constrain structural and paragenetic timing aspects of the CD-type genetic model, and the most basic timing constraints (syngenetic vs. epigenetic, synextension vs. syninversion) remain debated. The recently discovered Teena Zn-Pb deposit is hosted by an exceptionally well preserved subbasin that permits relative timing relationships to be well defined. Using a combination of geophysical, structural, sedimentological, and petrographic datasets, a new model for subbasin development and syndiagenetic hydrothermal replacement mineralization is developed for the Teena mineral system. At Teena, sulfide mineralization was deposited from lateral fluid flow beneath an impermeable seal several hundred meters below the paleosurface and maximum flood surface, after formation of fine-grained diagenetic pyrite (py1) and dolomite nodules. Sulfide mineralization resulted from syndiagenetic carbonate replacement and pore space cementation where thermochemical sulfate reduction took place. The sulfide mineralization is therefore partly cospatial but not cogenetic with its thick pyritic hanging wall, and its lateral alteration footprint is much smaller than predicted by sedimentary exhalative (SEDEX) models. An additional zone of low-grade Zn-Pb mineralization in the footwall W-Fold Shale Member represents a different style of mineralization not previously reported for Carpentaria CD-type Zn deposits: it is associated with strata-bound lenses of hydrothermal dolomite (HTD) that formed by both replacement and carbonate dissolution and infill, which yielded diverse cavity-infill textures that include coarse-bladed dolomite fans cemented by interstitial sphalerite, dolomite, and quartz. Volumetrically minor Zn mineralization is also present in a fault conduit hydrothermal breccia and in hanging-wall synorogenic vein sets derived by hydrothermal leaching and remobilization of Zn from the underlying mineralized zones. Whereas both the Teena and nearby McArthur River Zn-Pb deposits are located along the northern margin of the 3rd-order Hot Spring-Emu subbasin, they formed in separate 4th-order subbasins in association with local extensional growth faults. Growth fault movement in the Teena subbasin was initiated during deposition of the W-Fold Shale Member and persisted episodically until a weak structural disconformity associated with sedimentary facies regression developed in the Upper HYC unit. Shifting patterns in depocenter location, sedimentary facies development, mineralization, and alteration zonation are attributed to progressive growth and linkage of segments of a regionally anomalous ENE-trending, synsedimentary fault zone. Similar patterns of extensional subbasin development were repeated in other Zn-mineralized subbasins throughout the River supersequence across the northern Carpentaria Zn Province, and formed in response to a short-lived episode of north-northwest–south-southeast regional extension around ca 1640 ± 5 Ma, triggered by far-field subduction events.

How initial basin geometry influences gravity-driven salt tectonics: Insights from laboratory experiments

As a rifted margin starts to tilt due to thermal subsidence, evaporitic bodies can become unstable, initiating gravity-driven salt tectonics. Our understanding of such processes has greatly benefitted from tectonic modelling efforts, however a topic that has gotten limited attention so far is the influence of large-scale salt basin geometry on subsequent salt tectonics. The aim of this work is therefore to systematically test how salt basin geometry (initial salt basin depocenter location, i.e. where salt is thickest, as well as mean salt thickness) influence salt tectonic systems by means of analogue experiments. These experiments were analyzed qualitatively using top view photography, and quantitatively through Particle Image Velocimetry (PIV), and 3D photogrammetry (Structure-from-Motion, SfM) to obtain their surface displacement and topographic evolution. The model results show that the degree of (instantaneous) margin basin tilt, followed by the mean salt thickness are dominant factors controlling deformation, as enhancing basin tilt and/or mean salt thickness promotes deformation. Focusing on experiments with constant basin tilt and mean salt thickness to filter out these dominant factors, we find that the initial salt depocenter location has various effects on the distribution and expression of tectonic domains. Most importantly, a more upslope depocenter leads to increased downslope displacement of material, and more subsidence (localized accommodation space generation) in the upslope domain when compared to a setting involving a depocenter situated farther downslope. A significant factor in these differences is the basal drag associated with locally thinner salt layers. When comparing our results with natural examples, we find a fair correlation expressed in the links between salt depocenter location and post-salt depositional patterns: the subsidence distribution due to the specific salt depocenter location creates accommodation space for subsequent sedimentation. These correlations are applicable when interpreting the early stages of salt tectonics, when sedimentary loading has not become dominant yet.

The Tectonically Controlled San Gabriel Channel–Lobe Transition Zone, Catalina Basin, Southern California Borderland

Abstract High-resolution geophysical data across the Catalina Basin, offshore southern California, USA, reveal a complex channel–lobe transition zone (CLTZ) and provide an opportunity to characterize an entire seafloor CLTZ in a tectonically active and confined-basin setting. The seafloor morphology, distribution of depositional and erosional features, and location of depocenters in the CLTZ are controlled by shifting confinement and seafloor gradient related to inherited basement structures, active faults, and basin margins. Below a Holocene hemipelagic drape, the Catalina Basin is dominated by CLTZ and lobe sedimentation from the San Gabriel Channel, with lesser accumulations from local sediment sources limited to basin margins. The San Gabriel Channel is structurally confined as it enters the Catalina Basin and appears unable to avulse; it continues into the basin as a channel that rapidly widens, decreases in relief, and becomes scoured at its margins. A CLTZ is imaged between the confined San Gabriel channel and its terminal lobes deposited > 50 km into the basin. Narrow, apparently disconnected channels with knickpoints occur throughout the proximal and mid-CLTZ and are concentrated near basement highs and basin-bounding Quaternary-active dextral strike-slip faults. A field of small-scale erosional crescent-shaped scours (∼ 100 m length, ∼ 200 m width, up to ∼ 10 m relief across ∼ 30 km2 region) occurs above a partially buried basement high that creates perturbations in seafloor gradient. Likewise, above a buried basement structure that locally increases seafloor gradient (up to 0.4°), the distal CLTZ may contain sediment waves (∼ 2–4 m wave height and ∼ 200–300 m wavelength) that are smaller than many other CLTZ examples. This study of the San Gabriel CLTZ in Catalina Basin provides high-resolution geophysical data coverage of a complete CLTZ and illustrates a tectonically controlled end-member CLTZ from the modern seafloor.

Effects of adjacent fault systems on drainage patterns and evolution of uplifted rift shoulders: The Lower Cretaceous in the Loppa High, southwestern Barents Sea

Abstract Sedimentological models for rift shoulders have poorly documented the effect of adjacent fault systems on drainage patterns. In this study we investigate the Loppa High, an ancient tilted rift shoulder located in the southwestern Barents Sea. We use seismic and well data, sedimentological log descriptions, and biostratigraphic information to understand the drainage patterns and the Early Cretaceous geological history of the Loppa High. This study provides an example of how the drainage systems in low-gradient flanks of a rift shoulder can be modified and confined by normal faults occurring almost orthogonal to the main fault system. These orthogonal faults might have acted as preferential sediment routes. Thus, potential deposits of the main drainage systems directed to the low gradient flank are found almost exclusively associated with grabens formed due to these orthogonal faults. The Early Cretaceous evolution of the Loppa High is summarized as follow: 1) during the Boreal Berriasian/Volgian to early Barremian, the Asterias and the Bjornoyrenna fault complexes were active. Diachronous shallow to eventually deep-marine fans and incised valleys were developed along the southern and western flanks of the Loppa High; 2) late Barremian–Aptian fault activity is interpreted along the Ringvassoy Loppa Fault Complex. A second generation of incised valleys and their related shallow-marine fans were formed in the western flank of the Loppa High; and 3) during late Aptian–early Albian the Loppa High and the Hammerfest Basin were tilted eastwards. The latter event triggered a switch in depocenter location and development of shelf-margin clinoforms downlapping in close proximity to the eastern flank of the high.

Deformation and sedimentary evolution of the Lake Albert Rift (Uganda, East African Rift System)

This study proposed a new reconstruction of the tectono-sedimentary evolution of the Lake Albert Rift based on a biostratigraphical, sedimentological and structural re-evaluation of the outcropping data and on an exceptional subsurface dataset. The infilling of the rift consists of lacustrine deposits wherein two major unconformities dated at 6.2 Ma and 2.7 Ma were characterized, coeval with major subsidence and climatic changes. Combined with the fault analysis, the evolution and distribution of the subsidence highlights a four-steps evolution of the rift after its initiation dated at 17.0 Ma. The first phase (17.0 – 6.2 Ma) consists of low and diffuse extension associated with low accommodation rates ranging from 150 to 200 m/Ma. Restricted in the southern part of the basin, the depocenter location is poorly controlled by faults, meaning that the basin extension was potentially larger at this time. The second time interval (6.2 – 2.7 Ma) shows an increase of accommodation rates with values reaching more than 800 m/Ma. These high rates combined with the location of the major depocenters down the bounding faults argue for a first true rifting phase. Between 2.7 Ma and 0.4 Ma, the accommodation rates decreases to reach less than 400 m/Ma and the individualization of major depocenters continue down the major fault in the southern and northwestern parts of the basin. Finally, between 0.4 Ma and present-day, a late uplift led the formation of the Ugandan scarp. Comparison of the Lake Albert Rift evolution with the data available in the rifts of both branches of the East African Rift System shows that most of the sedimentary basins experienced the same geometrical evolution from large basins with limited fault control during Late Miocene to narrow true rift in Late Pleistocene.

The role of fault length, overlap and spacing in controlling extensional relay ramp fluvial system geometry

AbstractRelay ramps are integral components of normal fault systems that control sediment transport pathways in evolving rifts. We attribute differences in the geometry of fluvial systems that drain relay ramps to the scale of the ramp bounding fault segments, the spacing between segments and the amount of overlap between segments. Previous conceptual models for relay ramp geomorphological evolution have assumed that ramp fluvial catchments develop on the ramp surfaces and flow parallel to fault strike into the adjacent basin. Numerous examples exist in nature, however, that show that this is not ubiquitous. The fundamental question of what drives differences in fluvial geometry in these settings has, to date, not been fully addressed. We selected 27 relay ramps across the Basin and Range, western North America, and mapped, via GPS and remote sensing, the faults and ramp fluvial systems associated with each site. The sites represent a range of fault scales, which we define by the total outboard fault length, and a range of spacing and overlap values in order to better understand the structural controls on differences among ramp fluvial systems. Results show that the majority of a relay ramp surface drains parallel to fault strike when the outboard fault is less than about 15 km long. High overlap/spacing ratios are associated with relays along shorter (<15 km long) outboard faults, whereas lower overlap/spacing ratios are associated with relays along longer faults. Relays with lower overlap/spacing values may be more common along longer outboard faults because they survive for longer periods of time in the landscape. Our geomorphological observations can be used to predict synrift depocenter locations along segmented faults, but these observations only apply if the faults are short (<15 km long) and in early rifting stages. At longer fault lengths, ramp fluvial system geometry has no discernable relationship with any specific structural parameter.

The development of volcanic sequences at rifted margins: New insights from the structure and morphology of the Vøring Escarpment, mid‐Norwegian Margin

AbstractOn the Vøring Margin offshore mid‐Norway, Paleogene continental breakup was characterized by the extrusion of large volumes of flood basalts erupted in different depositional environments. The transition from subaerial to submarine emplacement environment is marked by the formation of the Vøring Escarpment which records the early encroachment of flood basalt into the basin and the buildup of a lava delta system. The increased availability of new and reprocessed high‐quality seismic data allows a more detailed characterization of the along‐strike and across‐strike continuity and variability of the different volcanic seismic facies units. Detailed seismic interpretation shows that the ~350 km long NE‐SW trending Vøring Escarpment is a prominent feature along the Vøring Margin with a height ranging between 200 and 1600 m. Structurally, the Vøring Escarpment is segmented along strike into five segments (E1–E5) with different controlling factors leading to variation in accommodation space. Relative sea level change and magma supply are the major controlling factors for segments E2 and E4 which are characterized by a well‐developed lava delta system and significant escarpment height. Tectonic movements along the Jan Mayen Fracture Zone resulted in second‐order segmentation of the E1 segment into pseudoescarpments with a very thin lava delta system and limited escarpment height. Segments E3 and E5, situated along the flanks of Cretaceous/Paleocene highs, are controlled by the structural highs, which were possibly reactivated during breakup time. Our mapping results provide crucial information about the paleogeography and yield important information regarding the paleo–water depth and depocenter locations prior to and during the breakup of the Vøring Margin.

Mass-Balance Effects In Depositional Systems

Abstract Net deposition is necessarily accompanied by overall loss of sediment mass from the transport system; on long time scales the deposition compensates for subsidence and creates the stratigraphic record. Although its spatial pattern changes and it can be locally stopped or reversed, depositional mass loss is a fundamental driving influence on the morphology and behavior of depositional systems. Mass extraction to deposition leads directly to facies changes as sediment flux declines; indirectly it usually leads to down-channel sediment fining as deposition preferentially removes coarser particles. Laboratory experiments illustrate the effects of systematic mass extraction on fluvial channel stacking and deposit grain size, and show how a mass-balance framework allows consistent comparison of facies despite major shifts in depocenter location. Mass-balance analysis of experimental fluvial and turbidite systems, and a turbidite mini-basin from the Gulf of Mexico, show a change from channel-dominated to lobe-dominated deposits at about 80% total mass extraction. Experimental and field studies also show a close connection between rate of mass loss and rate of downstream grain-size fining, as predicted by a mechanistic theory based on mass balance. These are first steps towards a framework for quantitative basin analysis based on mass extraction. A mass-balance framework allows for consistent, quantitative comparison across basins of varying scale and shape. Mass-balance frameworks can account for overall and/or generic mass-balance effects, and help separate them from local effects. One way to use such generalized models effectively is to view the results as reference cases of down-transport change resulting from the basic interplay of spatial mass-extraction pattern and sediment supply. Where such reference cases do not provide detailed predictions of specific field cases, they can still provide a baseline to separate local, case-specific features from generic system behavior. The approach is analogous to the way the geoid serves as a reference case against which to measure gravity anomalies.

Tectonic and dynamic controls on the topography and subsidence of the Argentine Pampas: The role of the flat slab

We analyze the Pampean foreland (the Pampas) along the modern flat-slab segment of the south-central Andes between 31° and 33° South latitude and to the east of the Argentine “flat-slab” province, using flexural and gravity studies and computations of dynamic topography. Bouguer anomalies and flexural analysis predict a foredeep of ∼250km width and a peripheral bulge amplitude of ∼25m, which match the regional morphologies of the modern Argentine Pampean Plain. However, these studies do not account for the subsurface Miocene–Quaternary basin preservation, represented by sedimentary thicknesses >400m and with depocenters >400km eastward with respect to flexural models. The discrepancy suggests that two mechanisms, acting at different wavelengths, influence the Argentine Pampas. The basin preservation is likely the result of a large-scale geodynamic forcing. Models of mantle flow, driven by realistic flat-slab subduction geometry and density contrasts, reproduce the depocenter location and the wavelength of subsidence as well as most of the remaining amplitude. Nonetheless, more sophisticated studies (e.g. considering lateral viscosity variations in the mantle wedge) might help reduce the dynamic amplitudes and better reproduce the observed geological record.

Ozona sandstone, Val Verde Basin, Texas: Synorogenic stratigraphy and depositional history in a Permian foredeep basin

The Ozona sandstone is a record of Permian synorogenic sedimentation in a foreland basin. The Ozona comprises terrigenous clastic slope and basin systems overlain by mixed clastic-carbonate shelf-ramp systems. Ozona sandstones form low-permeability gas reservoirs in Crockett County, Texas. I used wire-line logs and cores to map Ozona genetic stratigraphy and to reconstruct the depositional and tectonic history during the final phase of the Ouachita orogeny. Ozona depositional systems are composed of sandy turbidite channel and lobe genetic facies enclosed in laterally extensive muddy turbidite sheets and hemipelagic drapes. Channel and lobe complexes and turbidite sheets together form basin-floor apron systems. Coeval slope systems are mud-dominated products of mass transport processes. Sediment dispersal systems evolved from point sourced to line sourced.Ozona sequence development was primarily controlled by tectonic uplift and subsidence. Stratal geometries, facies associations, sediment input patterns, and depocenter locations are stable within sequences, but they change across sequence boundaries in response to tectonically driven changes in basin geomorphology. Onlapping stratal geometries in the lower sequence record excess accommodation space in the study area as plate convergence progressed from south to north. Offlapping strata in the middle and upper sequences formed in response to reduced accommodation space and intraforeland uplift in the north. At the base of each sequence, sandstone depocenters step toward the thrust belt in response to thrust-sheet loading and foredeep subsidence, but within each sequence, depocenters migrate away from the thrust belt. Foredeep migration through time provides a predictive tool for locating Ozona reservoir analogs farther south in Val Verde Basin.

Inner-Forearc Sequence Architecture in Response to Climatic and Tectonic Forcing Since 150 ka: Hawke's Bay, New Zealand

The influence of eustasy, tectonic deformation, and sediment flux as controlling parameters on basin stratigraphy and depositional sequence development are largely accepted. Eustasy is usually considered as the dominant mechanism of sequence generation, especially for Pleistocene successions. In active subduction-margin settings, the high rates of tectonic deformation are expected to have a stronger influence on basin-fill architecture, while sediment flux is generally less well constrained, and therefore less frequently considered. The active Hikurangi subduction margin in New Zealand offers the opportunity to quantitatively assess the relative roles of tectonic, climatic, and eustatic drivers. We present a quantitative source-to-sink-like study of the late Pleistocene succession from the Hawke's Bay sector of the inner forearc domain (c. 150 ka to present). The interpretation of a grid of high-resolution marine seismic data, onland and offshore core and well descriptions, and the integration of geomorphic studies enabled identification of system tracts. In turn these comprise two sea-level-cycle depositional sequences (LPS1 and LPS2), including one complete 100 ka sequence (LPS1). Isopach maps of both sequences reveal changes in sediment distribution and preservation that reflect the relative roles of tectonic deformation and eustasy. Eustasy dominates development of sequence architecture at relatively short time scales (i.e., 100 kyrs). Four long-lasting depocenters are identified over the inner forearc domain and located in four subsiding basins (Kidnappers, Mahia, Lachlan, and Motu-o-Kura basins). Significant shifts of the depocenter location in the basins are correlated with eustatic sea-level changes. Estimates of sediment volumes and masses from isopach maps indicate higher mass accumulation rates during climato-eustatic extremes, which we correlated to the onland erosional response. Sediment distribution and landscape evolution are strongly influenced by the interaction of the structural deformation and sediment flux. We present paleogeographic reconstructions for the inner forearc domain coincident with two paleoclimatic extremes (Last Glacial Maximum and Holocene Optimum). These illustrate the importance of eustatic changes, structural deformation, and sediment flux on the pattern of sediment distribution, accumulation, and sequence architecture.

Growth patterns of a proximal terrigenous margin offshore the Guadalfeo River, northern Alboran Sea (SW Mediterranean Sea): glacio-eustatic control and disturbing tectonic factors

The shelf-upper slope stratigraphy offshore and around the Guadalfeo River on the northern continental margin of the Alboran Sea, Western Mediterranean Basin, has been defined through the interpretation of a grid of Sparker seismic profiles. We tried to identify evolutionary trends in shelf growth, as well as to determine the regional/local factors that may modify the influence of glacio-eustatic fluctuations. Four major depositional sequences are identified in the sedimentary record by a detailed seismic interpretation, which defines three significant intervals of shelf-upper slope progradation, dominated by deposition of shelf-margin wedges, which resulted in uniform patterns of shelf-margin growth in response to significant sea-level falls. In contrast, the record of transgressive intervals is more variable, mainly as the result of distinct patterns of regressive-to-transgressive transitions. Major progradational wedges are internally composed of seaward-prograding, landward-thinning wedges, interpreted to represent shelf-margin deltaic deposits. In contrast, the last aggradational interval is composed of shelf-prograding wedges that show distinct characteristics, in terms of seismic facies, morphology and distribution when compared with previous shelf-margin wedges. These shelf wedges are thought to represent the particular case of Regressive Systems or Shelf Margin Systems Tracts, and their development seems to be controlled by a drastic change in main depocenter location, which moved from the upper slope to the shelf during the Pleistocene. The stacking pattern of seismic units, the shallowness of the acoustic basement and the migration of the shelf break are used to infer spatial and temporal changes in tectonic subsidence-uplift rates, which interact with low-order glacio-eustatic changes. For much of the Pliocene-Quaternary, uplifted sectors alternated laterally with sectors experiencing more subsidence. Subsequently, a significant change from lateral outgrowth to vertical accretion is recognised. This stratigraphic change could be related to the combined influence of increased subsidence rates on the shelf and the onset of higher-frequency glacio-eustatic cyclicity after the Mid Pleistocene Revolution that occurred around 1 Ma.

Landscape evolution, alluvial architecture, environmental history, and the archaeological record of the Upper Mississippi River Valley

The alluvial fill in the Upper Mississippi River Valley (UMV) is a palimpsest of past landscapes, environments, and physical evidence of human life ways. The valley has undergone significant changes in fluvial style during the time humans have occupied its landscapes, including changes in channel pattern, location of depocenters, and sediment lithology. Remnants of late-glacial braidplains that predate human presence in the Upper Midwest occur as sandy terraces and terrace fills. A subsequent major change in alluvial architecture resulted from fundamental changes in seasonal water and sediment input that marked the end of glacial meltwater input into the valley about 12.4 ka (10,500 14C yr B.P.). A shift to net transport of sandy bedload and storage of fine-grained overbank sediment on the floodplain accompanied the change to an island-braided channel pattern at that time. The Holocene channel belt has been significantly narrower, and the zone of sediment storage is reduced relative to that of the late-glacial river. Major Holocene depocenters include alluvial fans and colluvial slopes, floodbasins, natural levees, and fluvial fans at the junction of large tributaries. Climate models, and paleobotanical, and isotopic studies indicate that shifts in large-scale patterns of atmospheric circulation and moisture transport into the mid-continent of North America induced hydrologic and vegetation changes that strongly influenced flood frequency and magnitude, the delivery of sediment from tributary basins, and the evolution of the UMV landscape. Understanding the alluvial architecture of the valley, and the temporal/spatial distribution of biotic environments and processes that have buried, mixed, altered, or destroyed archaeological deposits is essential to develop strategies for sampling the valley for evidence of past human activity and for properly interpreting the archaeological record.

Clinoform Progradation by Turbidity Currents: Modeling and Experiments

Abstract Clinoform morphologies and growth patterns are typically viewed as the product of a particular mode of sediment transport, but process-specific models for their generation from turbidity currents are few, despite observations of turbidity currents on modern clinoforms and turbidites in ancient clinoform deposits. We present a simple morphodynamic model, supported by laboratory experiments, which shows how net-depositional turbidity currents can build sedimentary clinoforms. Most conceptual models for clinoform evolution assume that under constant (sea level, sediment supply) forcing progradation occurs via continuous basinward migration of a depocenter localized near the clinoform rollover. Abrupt basinward shifts in depocenter location are therefore an indication of allogenic variability in forcing. In contrast, our results document a unique style of progradation driven by autogenic cycles of slope steepening, sediment bypass, and depositional backstepping on the foreset slope. In experiments designed to investigate this slope–flow feedback, deposition from a continuous turbidity current transporting fine sand and silt repeatedly steepened a clinoform foreset to a graded slope which bypassed sediment to the slope base, depositing a sediment lobe. Continued deposition then caused the lobe to backstep up the slope, building a lower-slope foreset and eventually reinitiating the cycle. Our model shows how this cyclic depositional pattern arises from a morphodynamic feedback between the foreset gradient and the rate of sediment resuspension by the overriding turbidity current. Using our model to scale experimental results to the field, we predict gradual foreset steepening in prograding turbidity-current- dominated clinoform strata, with slope grading and cyclic deposition favored where these clinoforms build over steep basin slopes under conditions of high sediment supply and/or lateral confinement. We demonstrate how our results can add new insight to process-based interpretation of clinoform strata by application to a modern and an ancient field example.

Mesozoic sedimentary-basin development on the allochthonous Wrangellia composite terrane, Wrangell Mountains basin, Alaska: A long-term record of terrane migration and arc construction

The ∼7000 m of Upper Triassic–Upper Cretaceous strata of the Wrangell Mountains basin depositionally overlie the allochthonous Wrangellia composite terrane in south-central Alaska. New sedimentologic, compositional, and geochronologic data from these strata provide a record of the migration of the terrane from an intraoceanic setting at low paleolatitudes (∼12°N) to its present position along the continental margin of southern Alaska (∼61°N). We recognize several distinct stages of basin development: (1) Upper Triassic–Lower Jurassic carbonate strata represent an intraoceanic carbonate platform built on a remnant volcanic arc at low paleolatitudes (∼12°N). (2) During the Middle to Late Jurassic, a subduction zone formed along the southern margin of the Wrangellia composite terrane, prompting development of an intraoceanic arc and backarc basin on top of the terrane. (3) A narrow thrust belt and retroarc foreland basin formed along the inboard margin of the arc during the latest Jurassic. The foreland basin and arc were subsequently folded, uplifted, and eroded during the latest Jurassic–Early Cretaceous as recorded by an angular unconformity and isotopic ages from clasts in conglomerate. Regional deformation of the foreland-basin strata, shortening and uplift of the Wrangellia composite terrane, and uplift and erosion of the Middle to Late Jurassic arc are interpreted to represent the initial collision between the terrane and the continental margin of western North America. (4) Following regional deformation, a new volcanic arc was constructed inboard (northward) of the Wrangell Mountains basin. Upper Lower to Upper Cretaceous siliciclastic strata were deposited by shallow- to deep-marine deposystems in a continental-margin forearc basin. Distributions of lithofacies types and formation-thickness changes across intrabasinal normal faults document synextensional deposition. (5) The final stage of basin development was characterized by shortening and coarse-grained sedimentation along a fault system that separated the trenchward (southern) margin of the forearc basin from the subduction complex. (6) The Wrangell Mountains basin arrived at its current position by northward translation along orogen-parallel strike-slip fault systems. Comparison of the sedimentary record of the Wrangell Mountains basin, located on the outboard margin of the Wrangellia composite terrane, with the sedimentary record of the Nutzotin basin, located along the inboard margin of the terrane, demonstrates distinct changes in the locations of depocenters, the timing of deformation, and the composition of sediment. Similar stratigraphic and structural variations characterize outboard and inboard segments of the Wrangellia composite terrane in southeastern Alaska and coastal British Columbia.

Sedimentary record of late Paleozoic to Recent tectonism in central Asia — analysis of subsurface data from the Turan and south Kazak domains

The Turan and south Kazak domains (TSK) are in central Asia, between the Caspian Sea and the Tien Shan. The area is covered by sediments, deposited since the Late Permian during a series of tectonic events closely related to the history of two oceanic domains, Paleotethys and Neotethys. Sedimentary basins on the TSK therefore provide constraints on the tectonic development of the southern margin of Eurasia since the Late Permian. Our study is based on structure-contour maps and isopach maps of five key stratigraphic markers, of Late Permian to Tertiary age. Isopach maps help locate major faults and delimit sedimentary basins, providing information on vertical motions and, in some instances, horizontal motions. Subsidence associated with extension appears to have dominated the TSK, from the Late Permian to the Eocene. The extension may have been of back-arc type in southern Eurasia, next to the active margin, where the Paleotethys and Neotethys successively subducted toward the north. Here, sedimentary basins are both wide and deep (up to 15 km). During the Mesozoic, two compressional events of regional significance occurred in association with accretion of continental blocks at the southern margin of Eurasia. The first one, at the end of the Triassic, led to strong selective inversion of basins over the Turan domain. The second one, during the Late Jurassic–Early Cretaceous, had weaker effects. Since the Oligocene, following collision of both India and Arabia with Eurasia, inversion has become more generalized and compressional basins have formed on the TSK. Throughout the entire history of development of the TSK, from the Late Permian to the Tertiary, structures of Paleozoic and early Mesozoic age have exerted a strong control on sedimentation and especially on the location of depocenters. The south Kazak domain has registered little subsidence, in comparison with the Turan domain, where some basins have become very deep.

Shelf-Slope Break Sedimentation and Erosion During Deposition of the Claiborne Group in Colorado and Wharton Counties, Texas: ABSTRACT

ABSTRACT Geological and geophysical data over a 6,500 square kilometer area across Wharton and Colorado Counties, Texas, are utilized to understand sedimentary processes that occurred along the shelf-slope break of the Eocene Claiborne Group. Sedimentation models based on seismic profile data and well log data are critical to the oil and gas industry. It can be demonstrated that Claiborne deposition generally took place in middle neritic water depths on the shelf, however, some sediment did bypass the shelf and was deposited in deeper waters downdip. Sands of the Yegua Fm. were transported basinward from the deltaic and fluvial depositional environments in the updip area of Colorado County, bypassing shelf deposition. In general, there was little sedimentation taking place along the shelf except during short periods of transgression. Downdip at the shelf-edge, subsidence rates were higher and sedimentation became more aggradational. Possible drops in sea-level and/or changes in depocenter locations brought about downdip deposition of sediment beyond the shelf edge during most periods of Yegua-age deposition. It appears that even during low accumulation times on the shelf, sediment was being deposited downdip beyond the shelf-edge. Slumping along the over-steepened shelf margin was common during Claiborne deposition. The style of Claiborne age slumping was controlled by many factors including the placement of the Wilcox shelf. During high sedimentation times, erosion of the shelf margin from slump related carving appears to have taken place.

Rift flank uplifts and Hinterland Basins: Comparison of the Transantarctic Mountains with the Great Escarpment of southern Africa

Uplifted rift margins are a common feature of continents and oceans. Two variants of rift flank morphologies have been recognized: One in which the topography warps down from an inland high toward the continental margin, and one where the tropographic peak lies close to the continental margin. The Great Escarpment of southern Africa and the Transantarctic Mountains are examples of the first and the second variants of rift flanks, respectively. Both rift flanks are bordered on their landward side by broad continental basins: the Kalahari and the Wilkes hinterland basins. If these basins are interpreted as flexural “outer lows” that deepen in unison with the uplift of the rift flanks, the lithosphere on the uplifted side is very rigid in both cases (elastic thickness Te of 100 ± 20 km for southern Africa and 110 ± 20 km for East Antarctica). We suggest that the variation in rift flank morphology is caused by the isostatic response to uplift forces of elastic plates sharing different boundary conditions. We model the uplift of the Transantarctic Mountains as an upward deflection of an elastic plate which is broken at the front of the Transantarctic Mountains, and we model the uplift of the Great Escarpment as an upward deflection of a continuous elastic plate that is modified by the downward load of sediments on the continental margin. Although the Transantarctic Mountain uplift is young (60–0 Ma) and the southern African uplift is old (<100 Ma), the different isostatic responses of the two margins are not a function of age, because most loading (sedimentation) and unloading (erosion) took place shortly after rifting. Detailed modeling of topography, gravity, geological markers, and the locations of depocenters suggests that lithospheric rigidity decreases under the Transantarctic Mountains, whereas in southern Africa the decrease occurs not under the Great Escarpment but far seaward under the continental shelf and slope. If the distribution of lithospheric rigidity is indicative of the thermal regime of the lithosphere, then uplifted rift flanks are not always underlain by a thermal anomaly. This and other geological evidence indicate that a single mechanism cannot explain the uplift of both the Antarctic and the African margins.