Doctoral Abstract

The outcrop stratigraphic Framework, the facies architecture,

and superior temporal resolution sedimentation ESTIMateS

FOR THE WALL CREEK MEMBER, FRONTIER FORMATION,

POWDER RIVER BASIN, WYOMING

    The research presented in this dissertation draws upon recently developed sequence stratigraphic concepts.  Detailed sedimentology is integrated with ichnology to suggest that the Wall Creek sandstones and mudstones are more proximal and shoreline related, as opposed to distal shelf deposits.  The Wall Creek Member contains top-truncated mixed influenced deltas and shorefaces.  Non-marine delta topsets are nearly absent indicating substantial truncation and sediment ravinement during subsequent transgression.  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 paleo-topographic highs.  The regional Wall Creek stratigraphic framework is built from measured sections collected at outcrop within dip and strike oriented correlation corridors.  Intra-parasequence bedset scale facies architecture is correlated and mapped across 500 m of a continuously exposed 15 m thick cliff face.  Dip oriented correlations show several offlapping shingles, all of which are truncated.  These shingles are interpreted as distinct delta lobes.  High in the section, delta lobes contain clinoform sandstone simple 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-currents, and eventually inter-finger with and pinch out into the underlying mudstones.  The presence of normally graded, flat stratified to structureless sandstone beds interpreted as Bouma Sequences suggest they are 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 upward coarsening facies succession from clay prone mudstones and siltstones to mud free fine- to medium-grained sandstones is conformable.  Thus, 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.  Quantitative estimates for the duration of short term sedimentation are problematic.  Previous work suggested that the Wall Creek was formed as a basin-distal prodelta shelf plume, and that the individual sandstone beds 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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