Stages in Evolution of Paleozoic Carbonate Platform and Basin Margin Types--Western United States Passive Continental Margin

Harry E. Cook, Michael E. Taylor

Late Precambrian rifting along the western edge of North America established a passive continental margin that became the site of 5,000 m of platform and basin carbonate sediments over a 150-m.y. interval (Cambrian-Devonian). This megaplatform evolved through several stages: (1) Cambrian-Silurian, distally steepened nonrimmed ramp with base-of-slope fan (Hales Limestone) to homoclinal ramp (Hanson Creek Formation); to (2) Silurian-Devonian, rimmed platform (Lone Mountain Dolomite) having low-angle depositional slopes and slope aprons (Roberts Mountains Formation) and basinal debris sheets (Tor Limestone); to (3) Devonian, rimmed platforms having high-angle bypass slopes, slides, and base-of-slope aprons (McColley Canyon Formation and Devils Gate Limestone).

The position of the rifted continental margin controlled the overall trend of the platform-slope break. Postrift subsidence with superimposed eustatic sea level changes allowed the platform to accumulate 5,000 m of sediment. The stratigraphic progression from nonrimmed ramps in the Cambrian to rimmed platforms with high-angle bypass slopes in the Devonian was a function of both the gradual steepening of the slope, as the platform margin built up and prograded seaward, and the evolution of reef and bank-building organisms through time. Evolution of adjacent basinal carbonates was strongly influenced by slope declivity and relative sea level changes. As slope declivity increased through time, sedimentary processes on the slope changed from small-scale sediment gravity flows that accreted on the slope (ex: Roberts Mountains Formation slope apron) to large-scale sediment gravity flows that deposited debris in base-of-slope settings (ex: Devils Gate Limestone base-of-slope apron). These submarine sedimentary processes were dramatically punctuated several times during eustatic sea level lowerings, which caused slope failure and sediment gravity flows in the Cambro-Ordovician (i.e., forming the 500-m thick Hales Limestone submarine fans and slumps) and platform-margin collapse in the Early Devonian that resulted in immense volumes of Lone Mountain Dolomite shoal-water carbonates being transported tens of kilometers into basinal settings by sediment gravity flows (i.e., producing the 150-m thick Early Devonian Tor Limestone).

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.