Abstract Eolian dune fields self-organize through a hierarchy of autogenic processes that culminate at the dune-field pattern level. Interactions that occur between flow and grains, flow and dunes, and dunes and dunes define the levels of this hierarchy. These autogenic processes occur within sets of boundary conditions, which impart a uniqueness to each emergent dune-field pattern. The interpretation of allogenic forcing on dune-field patterns and their stratigraphic record requires an understanding of how these external environmental variables are manifested at the dune-field pattern level. The fundamental process in eolian systems is a wind event with basic boundary conditions of sediment supply, sediment availability, and the transport capacity of the wind. It is hypothesized that the basic high-frequency boundary conditions are remade at each level of the hierarchy of autogenic processes or have a cumulative effect over many wind events. The influence of these boundary conditions “trickles up” to and is manifested at the dune-field pattern level. Tectonic, climatic and hydrologic boundary conditions are low frequency and operate over much longer timescales than a wind event. It is hypothesized that these “trickle down” to be remade as high-frequency boundary conditions, which then trickle up. Analysis of the White Sands Dune Field in New Mexico supports these hypotheses by the manifestation of the influence of boundary conditions in the dune-field pattern. The dune field originated by wind deflation of a lacustrine sediment supply, which was made available episodically by climatic forcing that controlled the hydrodynamics of the tectonic basin. Although the dune-field pattern arose through autogenic dune interactions, the morphologies of which are ubiquitous throughout the field, the influence of boundary conditions is evident in the dune morphologies and field-scale pattern heterogeneity.

ABSTRACT Eolian dune fields on Earth and Mars evolve as complex systems within a set of boundary conditions. A source-to-sink comparison indicates that although differences exist in sediment production and transport, the systems largely converge at the dune-flow and pattern-development levels, but again differ in modes of accumulation and preservation. On Earth, where winds frequently exceed threshold speeds, dune fields are sourced primarily through deflation of subaqueous deposits as these sediments become available for transport. Limited weathering, widespread permafrost, and the low-density atmosphere on Mars imply that sediment production, sediment availability, and sand-transporting winds are all episodic. Possible sediment sources include relict sediments from the wetter Noachian; slow physical weathering in a cold, water-limited environment; and episodic sediment production associated with climatic cycles, outflow events, and impacts. Similarities in dune morphology, secondary airflow patterns over the dunes, and pattern evolution through dune interactions imply that dune stratification and bounding surfaces on Mars are comparable to those on Earth, an observation supported by outcrops of the Burns formation. The accumulation of eolian deposits occurs on Earth through the dynamics of dry, wet, and stabilizing eolian systems. Dry-system accumulation by flow deceleration into topographic basins has occurred throughout Martian history, whereas wet-system accumulation with a rising capillary fringe is restricted to Noachian times. The greatest difference in accumulation occurs with stabilizing systems, as manifested by the north polar Planum Boreum cavi unit, where accumulation has occurred through stabilization by permafrost development. Preservation of eolian accumulations on Earth typically occurs by sediment burial within subsiding basins or a relative rise of the water table or sea level. Preservation on Mars, measured as the generation of a stratigraphic record and not time, has an Earth analog with infill of impact-created and other basins, but differs with the cavi unit, where preservation is by burial beneath layered ice with a climatic driver.

Abstract A remarkable range of syndepositional deformation structures are present in eolian and sabkha strata across the Colorado Plateau. In a classification based on scale and style of deformation, these structures include (1) millimeter- to centimeter-scale crinkly and contorted laminae and liquefaction structures; (2) meter- scale folded, contorted cross-strata and liquefied zones; (3) decameter-scale rotated blocks, slumps, and mass-flow deposits; and (4) bed-scale to multiformation-scale clastic pipes. These syndepositional structures record a range of brittle, hydroplastic, liquefaction, and fluidization properties of the sediment at the time of deformation. The pipes are the most enigmatic structures, and these can range from simple forms with structureless fill to complex forms with structureless fill, breccia blocks, and warping or faulting of surrounding and encasing host strata. A disproportionate abundance of syndepositional deformation structures in Jurassic strata of the Colorado Plateau is attributed to (1) the deposition and dissolution of evaporites associated with interdunes, sabkhas, and adjacent shallow seas; (2) a high water table, especially in response to adjacent marine transgressions; (3) dune progradation and loading over saturated, poorly consolidated, marine, and sabkha substrates; and (4) the interbedding of mobile sabkha deposits with bedded and laminated eolian sands capable of recording the overpressurization and deformation of sabkha and eolian deposits. External triggering events may have included catastrophic flooding events, bolide impacts, and seismicity. The range of deformation structures has important implications for understanding syndepositional processes. Furthermore, these studies have applications to interpreting the interconnections of high-permeable injectite conduits across multibed to multi- formational scales.

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Abstract Within the Mannville Group (largely Aptian-Lower Albian) of the Alberta foreland basin, two distinct types of alluvial facies occur in different sequence stratigraphic contexts. One is relatively clean, braided-stream conglomerates and sandstones with prominent unconformities identifiable by abrupt lithologic changes; the other is comprised mainly of mudstones with thin coals and subsidiary (less than 25%) fine sandstones. Basal Lower Mannville sediments were deposited during a period of very low (<10 m/my) subsidence rates or even during uplift of the basin and orogen, because overthrusting in the Cordillera had not been initiated. These relatively thin braided-river deposits fill paleotopographic depressions on a long-period (second order) tectonic unconformity. They consist of at least 90% sandstones and/or conglomerates, with one or more unconformities or disconformities within them. Internal unconformities are marked by channelled surfaces which separate non-marine (and some marginal marine) units with different lithologies, facies, and directions of transport. The mudstone-dominated facies in the Upper Mannville was deposited during a period of high subsidence rates (>40 m/my) caused by active overthrusting in the associated Cordillera. Small-scale relative sea-level (RSL) fluctuations are documented in equivalent transgressive/regressive shoreline sequences. Deposition of the volumetrically dominant and widespread mudstones, thin (to 8 m) sandstones and coals, all interpreted as low-energy anastomosed stream and floodplain sediments, occurred during periods of coastal transgression. Each maximum flooding surface is correlative to a muddy and/or coaly zone immediately south of the shoreline. The allostratigraphic units defined by these surfaces contain thin sands, variable amounts of mudstone and thin coals, and can be correlated on well logs at least 350-km inland from the shorelines. They are generally sheet-like in geometry and slope seaward at a very low angle, with an offlapping stratal pattern. Episodes of falling sea-level caused incision of 20-m-deep valleys (fourth-order unconformities) through these deposits, however, rendering them difficult to correlate. In the early stages of the next RSL rise, the valleys were infilled by braided-river sands, with some estuarine sands in proximity to the shoreline. The major fluvial sandstones are therefore not contemporaneous with the mudstones, coals, and minor thin sandstones laterally adjacent to them. In interfleuve areas between valleys, the subaerial unconformities cannot be correlated on well logs or easily recognized visually because they lack lithologic expression. The differences between the unconformity-related, sheet-like braided stream sandstones/conglomerates and the fine-grained meandering to anastomosing alluvial facies result from their sequence stratigraphic settings, controlled largely by the tectonic subsidence rates of the basin.

Abstract Relative Role of Eustasy, Climate, and Tectonism in Continental Rocks - The renaissance in stratigraphy over the last two decades has been largely driven by the belief that stratigraphic packaging is determined by allocyclic controls. An understanding of the controls on stratigraphy allows us to make better predictions

Abstract The renaissance in stratigraphy overthe last two decades has been largely driven by the belief that stratigraphic packaging is determined by allocyclic controls. An understanding of the controls on stratigraphy allows us to make better predictions about the nature and geometry of strata within areas of basins where data is more limited. From an economic standpoint one can better estimate the potential for oil gas coal or mineral accumulations. Ten years ago discussions concerning the importance of driving mechanisms was polarized with many stratig raphers tending to emphasize the importance of a single allocyclic control Cyclostratigraphers emphasized the importance of climatic cycles Event stratigraphers studied the role of major events such as meteorite or comet impacts volcanic episodes or widespread oceanic anoxia Tectonostratigraphers examined the role of tectonism Sequence stratigraphers who divided strata on the basis of time significant surfaces such as unconformities and marine flooding surfaces demonstrated the importance of eustasy. Although some zealots remain most stratigraphers now recognize the importance of taking a more holistic approach to understanding allocyclic controls. A special session was convened at the 1994 Denver annual meeting of the AAPG SEPM entitled Allocyclic controls on nonmarine stratigraphy. This session was enthusiastically attended and featured papers that demonstrated a wide range of approaches to developing an understanding of alluvial architecture. This volume represents a collection of these papers and as such brings together the results of research where authors have examined the relative importance of eustasy climate and sediment supply in determining the nature of lithologies and the style of packaging of continental strata.

Abstract Sediment supply to continental basins is determined at first order by the rate at which source areas are eroded. Factors govering the rate of denudation are investigated using data from 97 intermediate and large drainage basins around the world. By multiple regression, five variables are selected from a larger set of climatic, topographic and hydrological estimators as being most effective at explaining global variance in specific sediment yield. The five are drainage area, maximum height of catchment, specific runoff, mean annual temperature and temperature range. These factors may be most adequate at describing the influence of weathering, hillslope erosion and fluvial transport on the rate of denudation, but they only explain half of the observed variance of that rate. It is demonstrated that rate of uplift of rock is a control on regional denudation rates of an importance similar to, or greater than that of climatic and topographic factors. Sediment supply reflects the way in which surface processes have interacted with the tectonic input of material to the regolith. Estimates of sediment supply can be based on knowledge of precipitation, relief and runoff but should be refined using information on the rate of uplift of rocks. Alternatively, the volume of sediment supplied may be estimated from the tectonic setting and size of the source area of the material.

Abstract The sequence stratigraphic model, with its initial emphasis on eustatically driven controls on sedimentary sequences, has generated considerable interest in the ultimate controls on alluvial successions. Arguments regarding controls on alluvial sequences have been ongoing for many decades. Proposed controls include global (eustatic) or local base-level fluctuation, climate, tectonics, and sediment supply. Since sediment supply is in general a function of one or more of the other three controls, the possible number can be reduced to three. Most models and explanations for fluvial successions are too simplistic. They attempt to explain these successions on the basis of a single controlling factor. Additionally the models commonly fail to take into account modern geomorphic concepts of complexity and ignore the fact that controls other than base-level fall can produce incision and that base-level lowering does not always result in incision and rejuvenation of a fluvial system. They also fail to realize that base-level fluctuations may have their greatest effect only in the lower reaches of a fluvial system and that the amounts of sediment produced by incision alone cannot account for the volume of sediment observed in most stratigraphic sequences. For these reasons it is difficult to justify the application of systems tracts designations to upstream portions of fluvial valley-fill successions. Field and analog experiment studies demonstrate the difficulty of distinguishing various controls in fluvial systems because (1) similar erosional and depositional features and sequences can be produced by different processes and vice versa and (2) different levels of sensitivity may result in a minor, a major, or no response of a system to an extrinsic perturbation.

Abstract The Texas Gulf Coastal Plain consists of a series of low-gradient, fan-shaped alluvial plains emanating from each major river valley. The majority of alluvial plain surfaces have been mapped as Pleistocene Beaumont Formation or younger unnamed strata, and interpreted to represent eustatically-controlled deposition during the oxygen isotope stage 5 and modern interglacial highstands. Reevaluation of preexisting data combined with reexamination of Beaumont and younger strata of the Colorado River suggests the stratigraphic and geochronologic framework needs revision, and processes of alluvial plain deposition are more complex than previous interpretations have inferred. As a result, Beaumont and younger strata provide an opportunity to examine alluvial plain construction within a sequence-stratigraphic framework and discuss some key characteristics and the heirarchal nature of eustatically-controlled versus climatically-controlled components of alluvial plain depositional sequences. Mapping from satellite imagery, field documentation of geomorphic and stratigraphic relationships, consideration of the stratigraphic significance of surface and buried soils, and a number of radiocarbon and thermoluminescence ages suggests that Beaumont and younger alluvial plains consist of multiple cross-cutting and/or superimposed valley fills of widely varying age, and may represent the last 300-400 ky or more. Valley fills become partitioned by initial lowering of sea-level below interglacial highstand positions, when channels rapidly incise and valley axes become fixed in place as they extend across the subaerially-exposed shelf. While shorelines remain basinward of highstand positions, the remainder of the alluvial plain is characterized by non-deposition and soil development. During this time, multiple episodes of lateral migration, aggradation, degradation, and/or flood-plain abandonment with soil formation occur within incised and extended valleys in response to climatic controls on discharge and sediment supply. This creates a composite basal valley fill unconformity, as well as multiple smaller-scale allostratigraphic units within the valley fill. With late stages of transgression and highstand valleys fill at paces set by upstream controls on sediment delivery. As valley filling neaxs completion, veneers of flood basin sediments spread laterally, which buries soils developed on downdip margins of the alluvial plain. Complete valley filling during highstand is one of several processes that promotes avulsion, with relocation of valley axes before the next sea-level fall, such that successive 100-ky valley fills have a distributary pattern, and successive increments of geologic time occur lateral to each other.