Sedimentology, facies architecture and stratigraphic framework of the UPPER-CRETACEOUS TURONIAN WALL CREEK MEMBER, FRONTIER FORMATION, SOUTHWESTERN POWDER RIVER BASIN, WYOMING, U.S.A.
The research presented in this dissertation draws upon recently developed sequence stratigraphic concepts. Detailed sedimentology is integrated with ichnology and indicates that the Wall Creek Member sandstones and mudstones of the Frontier Formation are more proximal and shoreline related, as opposed to being preserved distal shelf deposits. The Wall Creek Member found in the study area contains top-truncated mixed influence deltas, delta front sandstones and mudstones, as well as associated shoreface clastic deposits. Remnant non-marine delta topsets are rare, only found locally, and elsewhere are not preserved indicating substantial top-truncation and sediment reworking during subsequent transgression and ravinement. Regional sandstone body distribution and stratigraphic correlations show progradation of several variously proportioned mixed-, river-, storm-, tide- and wave-influenced delta lobes that grew around and over local paleo-topographic highs. The regional Wall Creek stratigraphic framework presented is built from measured sections collected at outcrop and presented in dip and strike oriented correlation corridors. Intra-parasequence bedset-scale facies architecture is described, correlated and mapped across 500 meters of a continuously exposed 15 meter thick cliff face. Dip oriented correlations show several offlapping basinward stepping shingles, all are top-truncated. These shingles are interpreted as being preserved remnants of distinct delta lobes. High in stratigraphic section, delta lobes contain clinoform simple sandstone bedsets that dip basinward and correlate laterally with increasingly heterolithic sandstone and mudstone composite bedsets. Bedsets dip toward the south, the same direction as paleo-current indicators, and eventually inter-finger with and ultimately pinch out into the underlying delta front mudstones. The presence of normally graded, flat stratified to structureless sandstone beds interpreted as Bouma Sequences suggest sediments were deposited as delta front turbidites. Simple and composite bedsets are interpreted as preserving distal terminal distributary mouth bar deposits and off-axis crevasse and delta front splays. The sharp-based upward coarsening facies successions from clay prone mudstones and siltstones to mud free fine- to medium-grained sandstones is conformable. The succession preserves a quasi-continuous record of basinward progradation of a single deltaic parasequence. Periods of rapid sediment emplacement and relatively slow quiescent accumulation can be qualitatively identified in the rock record by integrating sedimentology with sediment faunal interactions. Quantitative estimates for the duration of short term sedimentation are problematic. Previous work suggested that the Wall Creek Member was formed as a basin-distal prodelta shelf plume, and that individual sandstone beds described at outcrop were deposited by geostrophic flows linked to large scale, 1,000 year storm events. Comprehensive sedimentology and ichnology of a mixed-influenced delta lobe is presented and yields decadal, annual, and possibly seasonal depositional rates.
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