A COMBINED OUTCROP AND CORE STUDY OF LOWER SILURIAN STRATA FROM EASTERN NORTH AMERICA: IMPLICATIONS FOR A UNIFIED MODEL OF FORELAND BASIN SEQUENCE STRATIGRAPHY

Sequence stratigraphic models for foreland basins have emphasized outcrop based analysis, but to date have largely been developed from study of the siliciclastic-dominated margin of the Cretaceous Sevier Basin in the western US. Here we present one of several ongoing studies from middle Paleozoic strata of eastern North America that expand on existing models to also encompass the basin center and carbonate margins of foreland basins. Lower Silurian rocks of the Cincinnati Arch and Appalachian Basin were deposited during the Taconic and Salinic orogenies. The strata of these two great upheavals are separated by a regionally angular unconformity. The 4th-order depositional sequence immediately overlying this unconformity has been traced in outcrops and cores along the Cincinnati Arch (800 km parallel to depositional strike) and eastward across the Appalachian Basin (400 km parallel to depositional dip), providing an example of sequence stratigraphic patterns across an active foreland basin from the siliciclastic to carbonate margin. Along the Cincinnati Arch a thin, hardground-rich, coral-bearing grainstone (~0.1-3 m) rests directly on the unconformity. It thins basinward into a lenticular (~0-10 cm) phosphorite. Toward the siliciclastic margin it thickens into an ironstone (~0.25-1.5 m). This highly condensed interval represents the lowstand and transgressive systems tracts. The overlying highstand systems tract transitions from argillaceous dolostone on the carbonate margin (~2 m), into organic-rich shale near the basin center (~50 m), and silty and sandy mudstone near the siliciclastic margin (~100 m). The uppermost systems tract of this sequence, the falling stage, is dominated by fine-grained, argillaceous calcarenite on the carbonate margin (~0-3 m); grading into mixed shale, siltstone, and sandstone (~3-10 m) near the basin center; and is dominated on the siliciclastic margin by argillaceous sandstone (~30 m). Lateral facies change is typically gradual, whereas vertical facies change is generally sharp and marks sequence stratigraphic surfaces. The highly condensed nature of depositional sequences on the carbonate margin allows for easy study even in relatively small exposures, while the greatly expanded sections of the basin center and siliciclastic margin require much larger exposures.