ABSTRACT: Variations in Fluvial-Deltaic Architecture Related to Preservational Trends in a Cretaceous Clastic Wedge, Ferron Sandstone, Utah

GARDNER, MICHAEL H., Bureau of Economic Geology, University of Texas, Austin, TX

The dimensions, diversity, proportions, and arrangement of bedforms, facies associations, and facies tracts within fluvial-deltaic genetic sequences change as a function of the stratigraphic position of a genetic sequence in a clastic wedge. Changes in these architectural elements record both intrinsic depositional processes described by Walther's law, and extrinsic preservational processes related to temporal and spatial changes in accommodation space (the space available for sediment to accumulate) and base level (the energy of sediment flux). A comparative analysis of architectural elements in different genetic sequences reveals systematic and predictable changes in the: (1) degree of facies offset between successive genetic sequences; (2) proportions of sediment accumulated during regressive and transgressive hemicycles producing different cycle symmetries and thicknesses; (3) internal and external facies geometries; (4) facies tract lithology ratios; (5) sediment volume partitioning in different facies tracts; and, (6) bedform diversity in geomorphologically identical depositional environments.

The Ferron Sandstone comprises seven genetic sequences that each record an episode of regression and transgression (base-level transit cycle) under conditions of rising relative sea level. Each genetic sequence contains a complete spectrum of marine-shelf, shallow marine, and coastal-plain facies tracts. Genetic sequences are bounded by time-significant surfaces represented by condensed sections in marine-shelf facies tracts, marine flooding surfaces in

shallow-marine facies tracts, and a change from organic-poor/sand-rich to organic-rich/sand-poor lithologies in coastal-plain facies tracts. These genetic sequences occur in a geometric stacking pattern of seaward-stepping, vertically stacked, and landward-stepping facies tracts.

In the shallow marine facies tract of seaward-stepping, genetic sequence 2 (time equivalent to the Gallup A Sandstone in the San Juan basin), discrete delta lobes are characterized vertically by an upward-coarsening sandstone succession and laterally by gently inclined sandstone wedges, 30 to 40 m in height and 1 to 3 km in length. Internally, amalgamated distributary mouthbars produce numerous, inclined bounding surfaces that are hundreds of meters in length and width. Amalgamated distributary mouthbars are replaced along strike at a spacing of a few kilometers by mud-rich interdistributary bay facies. Coeval distributary channelbelts consist of discrete, erosive-based compound barforms or macroforms. Macroform diversity is low, dominated by highly interconnected cut-and-fill types t at are typically 2 to 5 m in height and tens of meters in width and display low bedform diversity. Macroforms are separated by erosive surfaces covered by thin sand-rich matrix channel lags and by numerous, shorter length reactivation surfaces.

By contrast, in landward-stepping genetic sequence 5, the shallow marine facies tract contains more steeply inclined, seaward-dipping surfaces that segregate the delta front into compartments that are 20 to 30 m thick and less than 1 km in length. The paucity of distributary mouthbars and interdistributary bay facies reflects the dominance of wave and storm processes on the delta front. Coeval distributary channels contain a diverse assemblage of moderately interconnected, low sinuosity, high sinuosity, and abandonment-fill macroforms that are typically 3 to 10 m in height and hundreds of meters in width. Macroforms display a high bedform diversity reflecting more complete preservation of depositional bedforms and facies successions. Basal channel lags are thick, mud-rich, and hundred of meters in length, with subordinate bounding surfaces of equal length represented by depositional surfaces associated with lateral and downstream channel migration.

The most heterogeneous facies tracts (seaward-stepping shallow-marine and landward-stepping coastal-plain facies tracts) record the position of greatest accommodation space and lowest sediment flux, which produces facies tracts where time is more completely represented by strata. By contrast, the most homogeneous facies tracts (landward-stepping shallow marine and seaward-stepping coastal-plain facies tracts) record the position of lowest accommodation space, greatest amalgamation and cannibalization, and highest sediment flux, which produces facies tracts where time is more completely represented by stratal surfaces.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)