Jay P. Skinner and Piret Plink-Björklund
Geology and Geological Engineering, Colorado School of Mines, Golden, CO
The Upper Cretaceous Chimney Rock Sandstone was deposited in near shore deltaic and estuarine environments in the Western Interior Basin. The Chimney Rock Sandstone outcrops in a 15 km long, near continuous exposure near Flaming Gorge Reservoir, Utah. The deltaic portion was originally interpreted as a prograding strandplain succession. However, firstly the presence of distributary channel deposits, and later recognition of two types of delta front deposits allowed the reinterpretation of the prograding clinoforms as river- to wave-dominated delta. The aim of this study is to define specific criteria for recognizing the wave and river-dominated parts. The sand/mud ratio as well as the seaward and lateral extent of sandbodies is distinctly different in the wave and river-dominated parts. This detail study compares the lithology changes, clinoform steepness, sand body geometry, degree of amalgamation, net to gross ratios, and local permeability barriers of the two types of deposits. The River-dominated intervals contain characteristically dirtier sands in the mouth bars that prograded at high angles. The River-dominated delta front consists of heterolithic sediment gravity flow and slump deposits. In the wave-dominated intervals the sands are cleaner, and both the mouth-bar and foreset angles gentler. The wave-dominated delta front is dominated by shoreface to offshore transition deposits. Distributary channel deposits are similar in both delta types. The HST deltas are more aggradational and non-amalgamated with mud separating the sandy clinoform sets and providing permeability barriers. Late HST, FRST, and early LST deltas are vertically amalgamated with the potential permeability barriers provided by river-dominated heterolithic intervals on the delta front, and early-diagenetic cements on the clinoform tops.
AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas