Valley Widening and the Composite Nature of Valley Margin Sequence Boundaries: Evidence from the Nielson Wash Complex Valley Fill, Cretaceous Ferron Sandstone, Utah

Incised valleys can provide critical information on a region’s geologic history, are important hydrocarbon reservoirs, are key features to identify when making sequence stratigraphic interpretations, and are pathways for bringing marine sequence stratigraphic interpretation onto the continents. Current research has suggested that the sequence boundary that makes up the floor and walls of an incised valley is constantly being modified throughout the entire relative sea level cycle. This would imply that valley enfolding sequence boundaries are composite surfaces created over a number of erosional events. New models of incised valley evolution show that incised valleys continue to widen as they are being filled. This is in disagreement with existing models illustrating that valleys stop widening as soon as backfilling commences. There are many different autogenic and allogenic controls on valley incision and excision. The Cretaceous Ferron Sandstone outcrop in Nielson Wash, located west of Hanksville, Utah, contains a superbly exposed valley fill. The Ferron Sandstone in the study area was deposited as part of the Notom Delta, which is the southern lobe of the Southern Utah Deltaic Complex, that prograded into the Henry Mountains Basin from the southwest. The complex valley system was formed during the latest in a series of relative sea level fluctuations, each lasting ~ 100,000 years. The 100,000 year duration of these cycles suggests that they are driven by volume changes in the global ice sheets caused by eccentricity. These high frequency cycles are overprinted on the higher order regression of the Greenhorn Sea. Thirteen measured sections were completed and hundreds of digital photographs were taken along the outcrop. From this data a detailed bedding diagram of the fluvial and facies architecture of the fill was completed. It was found that the Nielson Wash incised valley was created by up to five high frequency cut and fill events. Deposits found within the valleys range include esturine, inclined heterolithic strata, strongly to weakly tidally influenced fluvial, and floodplain. The idea that incised valleys widen as they are being filled was explored by analyzing the shape of the valleys with respect to the fluvial architecture as well as relationships of valley margin deposits (e.g. terraces). It was discovered that there is evidence that valleys do indeed widen as they are filling.

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