Use of Sedimentary Structures in Determination of Depositional Environments, Mesaverde Formation, Williams Fork Mountains, Colorado

Abstract

ABSTRACT Rock-stratigraphic units composing the Mesaverde Formation of northwestern Colorado were deposited in offbeach, beach, lagoon, swamp, and floodplain environments of deposition. The vertical distribution of these rock-stratigraphic units reflects the lateral distribution of environments at the time of deposition. In a similar manner, the vertical distribution of sedimentary structures within the rock-stratigraphic units reflects the lateral distribution of processes at the time of deposition. Therefore, an understanding of modern processes and the distribution of their resultant sedimentary structures will permit prediction, through the principle of uniformitarianism, of the vertical distribution of stratification within the depositional products of ancient environments. It is possible to subdivide the products of the various nearshore marine and nonmarine deposits into a series of subenvironments based on types of stratification. In a regressive suite of rocks, beginning with the offbeach environment, the rocks are characterized by parallel laminae and low-angle cross-laminae reflecting deposition from suspension and from low-energy traction currents. Landward, through the beach environment, varying processes and energy levels result in a sequence of sedimentary structures characteristic of the offshore-beach, submarine-bar, and foreshore-beach subenvironments. The beach zone may be overlain by rocks of the lagoonal, swamp, or floodplain environments. The lagoonal environment comprises sedimentary structures indicative of processes in tidal-delta, tidal-chumel, lagoon-pond, and tidal-flat subenvironments. The swamp environment is charactmzed predominantly by lithology—coal and carbonaceous sediments—rather than by stratification. The floodplain environment comprises stratification characteristic of the channel and interchannel subenvironments.