The Impact of Buffer Zone Changes on Architecture of the Paleozoic Lower Cutler Beds of Utah

The Shafer basin fluvial units, as being part of the Paradox fold and fault belt basin, received sediments from alluvial fans through mixed braided-meandering river systems that eroded the clastic rocks of the Uncompahgre highlands to the northeast and flowed southwestward through aeolian dune fields to an open sea during Pennsylvanian to early Permian time. The best exposed rocks of the Lower Cutler beds were photographed and investigated along the Colorado River in the Shafer and Canyonland basins. The Architectural Element Analysis technique was used to categorize bounding surface orders, and to identify lithofacies and fluvial elements such as lateral accretion bars, channels and overbank deposits. Both merged photographs and outcrop panel drawings were used for this project. The objective of the method was to dissect fluvial sand-sheets and to characterize their origin and the depositional environment under which they were deposited. Therefore, Architectural Element Analysis was applied to the FL2 interval of the Permian Cutler Group in two Shafer basin locations (Potash and Goose Neck) and to the FL3 interval in the Canyonland basin location of Utah. In the Goose Neck fluvial unit, a ~thirty five meters long and ~five meter deep valley incision surface bisects the section into two intervals of mixed meandering and braided bars. Moreover, numerous mid-channel bars on the top of the FL2 strata are indirect evidence of a transition toward braided systems at the top. Similarly, the ~seven meters thick Potash section is bisected by multiple nested-valley surfaces bound by a regional sequence boundary. The multiple valleys are filled by several channel belt (CB), and channel fill elements such as downstream accretion bar element (DA), overbank, etc. that are produced by progressive transitions between braided and meandering paleo-rivers.

The base of all of the fluvial units is punctuated by a regional sequence boundary. The fluvial sequence boundary was created through paleo-river re-washing and removing of dry aeolian and deeper buried interdune sand-sheets in places. The sequence boundary records a hiatus at the base of the FL2 interval; however, it is not punctuated by erosion, but rather by a facies transition and aggradation of the Canyonland rocks, the FL3 strata landward. This discrepancy may be explained by the strata’s location relative to the paleo-sea level shoreline, subsiding basin, and sediment source. The change in the rivers profile related to cycles in sediment/discharge ratio caused incision and aggradation at a higher frequency than the rate of subsidence. This process resulted in cut and fill of buffer valleys concurrent with an overall trend of aggradation tied to continued subsidence. The result is aggradation punctuated by periods of incision during sand-sheet deposition. This result contrasts with traditional models of fluvial aggradation and deposition of low-accommodation sandstone sheets that presume these sheets are accumulated by linear progressive stacking. The Lower Cutler deposits in the Shafer basin thus represent aggradation by stacking of channel deposits and their incision by high order valley surfaces that shows regional changes in paleo-river dynamics. Permian fluvial strata are marked by higher-order surfaces of re-incision in what otherwise appear to be progressively stacked channel belts.

The last phase of FL3 deposition is marked by severe seismic activity. The liquefaction of bars at the top of the interval confirms that the basin was active at the time of FL3 deposition and that a high frequency earthquake greater than 6.5Mo likely occurred.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California