52 Structural Geology and Tectonics II

Friday, 5 April 2002
1:00 PM - 5:00 PM

TECTONIC IMPLICATIONS OF SLOPE FACIES IN THE CONASAUGA FORMATION (MIDDLE CAMBRIAN) IN THE SOUTHERN APPALACHIANS, ALABAMA

Paper number: 52-8
Program: Technical Sessions
Time: 2:00 PM – 2:20 PM

Authors

Abstract

The Conasauga Formation in the southern Appalachians comprises several different facies associations, recording a heterogeneous mosaic of paleoenvironments and complex basin architecture that allows reconstructing the Laurentian margin from late Early to early Late Cambrian. Mixed carbonate-siliciclastic facies associations ranging between pure-carbonate and pure-siliciclastic end members represent low-gradient depositional systems, including tidal flats, intertidal and subtidal environments, intrashelf depocenters, and mixed ramp and shelf-margin oolitic shoals. Locally, in the Jones Valley thrust sheet in Alabama, a unique facies of the Conasauga records unstable depositional settings in high-gradient-slope environments, as well as base-of-slope environments. Evenly stratified thin-bedded platy dark muddy limestones and laminated black argillaceous limestones show pinch-and-swell, pull-apart structures, slumped beds, and pseudobreccias that indicate downslope creep and sliding. Boudinage, slumping, buckling, and brecciation are typical slope-failure processes. Lenticular polymictic breccias and massive to graded packstones and grainstones indicate incisive channeling across the slope. Reworked ooids, as well as calcimicrobe and metazoan associations, characteristic of inner-shelf shoals and shelf-margin buildups, are found in turbidites and grain-flow deposits, suggesting slope bypassing; however, no clasts of biohermal facies have been recognized. Slope instability and local deep-water carbonates indicate local slope breaks within the shelf. Agnostoid trilobites confirm the deep-water association and indicate a Cambrian (latest Marjuman-earliest Steptoean) age.

Local steepening of slopes and increased subsidence suggest active faulting within the southern Laurentian shelf. Paleoenvironmental reconstructions include local steep slopes along intrashelf fault blocks in Alabama, in contrast to gently sloping ramps in Virginia and Tennessee bordering an intrashelf clastic depocenter inboard of the passive-margin carbonate bank facing Iapetus. Regional facies distribution indicates greater structural-topographic relief south of the Conasauga intrashelf basin and near the Alabama-Oklahoma transform margin of southern Laurentia.