Print this pageStratal Architecture of a Transgressive Coarse-Grained Tidal Flat: Cutler Group
KATTAH, S. S. and W. E. GALLOWAY
Three tectono-stratigraphic sequences compose the Cutler Group, Pennsylvanian/Permian, Paradox basin. The last sequence records the most extensive progradation of non-marine facies into the Paradox Basin. As result of different scenarios of accommodation distribution, this sequence can be divided into highstand, lowstand and transgressive systems tracts. These systems tracts consist of higher-order genetic sequences defined by progradation/aggradation/retrogradation episodes. In contrast to highstand and lowstand, the transgressive scenario favored a strong influence of tidal processes on deposition and preservation of genetic sequences. The great and constant supply of coarse-grained sediments to the coastal plain, the strong onshore wind-driven currents and the periodic high fluvial diffusivity contrast this depositional context with that of typical tidal-influenced coastal areas.
An ideal genetic sequence within the transgressive system tract contains six facies associations: (1) extensively bioturbated shoreface sandstones and transgressive lags, (2) interstratified sandstones and siltstones of tidal sand flat environments with occasional crevasse-splay and minor channelized sandstones, (3) cross-bedded sandstones deposited in tidal-influenced fluvial channels, (4) sandstones with medium to large-scale lateral accretion surfaces of tidal-channels and tidal sand bars, (5) paleosoils and root horizons, and (6) coastal eolian facies. Transgressive ravinement surfaces at the base of transgressive lags, regressive ravinement surfaces between lower shoreface sandstones and coastal eolian dunes and fluvial incision represent key stratigraphic surfaces for establishing genetic sequences.
Despite the tabular external geometry of facies association, each is characterized by great lateral and vertical variability reflecting the complex interaction of intrinsic processes and environments.