A UNIFIED HIGH-RESOLUTION SEQUENCE STRATIGRAPHIC MODEL FOR FORELAND BASIN SYSTEMS: ADVANCES FROM ANALYSIS OF MIXED CARBONATE-SILICICLASTIC STRATA

A unified model of sequence stratigraphy for mixed siliciclastic-carbonate fills of foreland basins has been derived from comparative analyses of Paleozoic Appalachian basin deposits in eastern North America (Taconic, Acadian, and Alleghenian orogenies) and Mesozoic deposits in the Western Interior Basin (Sevier orogeny). Critical to development of this model was: 1) the recognition of repeating stratigraphic patterns common to all foreland basin systems studied, 2) detailed analysis of mixed carbonate-siliciclastic strata, which yielded the most sensitive record of environmental change and critical connection between the well studied pure siliciclastic and carbonate end members; 3) assumption that load induced subsidence rates and sedimentation rates on the siliciclastic-dominated side (SDS) of the foreland basin are much greater than on the carbonate-dominated side (CDS) of the foreland basin. Key aspects of this model are a) differential development of transgressive systems tracts as relatively thin lags on the SDS and as thicker and relatively clean skeletal limestones on the CDS; b) identification of a sharp surface of maximum sediment starvation (MSS) particularly well developed on the CDS as a mineralized corrosion surface, associated with maximal rates of sea level rise and drowning of the carbonate factory, as opposed to a more diffuse maximum flooding zone in overlying muddy deposits and in some cases separated by up to several meters from the MSS; c) identification of a sharp forced regression surface (FRS), and in some cases, an overlying condensed shell rich bed (“precursor bed”) at the base of d) distinctly shallowing upward falling stage (regressive) deposits, typified by progradation of coarser sediments on the SDS, and skeletal sands on the CDS. The regional nature of shallowing trends coupled with local coarse wedges indicates that the progradation is a response to, rather than the cause of shallowing. Three scales of cyclicity observable in most outcrop sections share a common stratigraphic architecture, suggesting similar forcing mechanisms at a variety of temporal scales (~40Kyr, ~400Kyr, ~1Myr). The shortest duration cycles are not recognizable in the deposits of the Alleghenian orogeny and may represent the moderating effects of large continental ice sheets during that time.