THE MOTIF OF DEPOSITIONAL SEQUENCES ACROSS A CARBONATE-TO-CLASTIC, ACTIVE FORELAND PROFILE: EXAMPLES FROM THE MIDDLE PALEOZOIC APPALACHIAN BASIN

Middle Paleozoic sedimentary successions display carbonate-to-siliciclastic depositional sequences of multiple temporal scales (105 to 106 years) across the Appalachian foreland basin. Vertical facies offsets mark major sequence stratigraphic surfaces: sequence boundaries, maximum starvation surfaces, and forced regression surfaces. Systems tracts between these basin-wide discontinuities exhibit consistent motifs. Skeletal grainstone-rudstone facies typify transgressive systems tracts (TSTs) in shallow cratonic areas. TSTs become increasingly condensed down-ramp, typified by compound hardgrounds, concretion horizons, and bone/conodont beds, but thicken and become increasingly sand-rich, eastward toward siliciclastic sources. Highstand systems tracts (HSTs) are characterized by widespread (> 1000 km cratonward) mud deposition. On the western side of the basin HSTs are dominated by wacke- to packstones, typically containing abundant shaly partings, which impart thin-bedded, rubbly to wavy/nodular weathering. Closer to siliciclastic sources HSTs become increasingly shale-rich and are readily differentiated from surrounding cleaner limestones. These intervals may show very well developed small-scale carbonate-shale cyclicity. Down-ramp this cyclicity fades, as component limestones condense or pinch-out into dark, shale-rich HSTs toward the basin center. Siliciclastic grain-size increases eastward, producing HSTs that are dominated by alternations of mud, silt, and sand. Falling stage systems tracts (FSSTs) occur as sharply based, downstepping packages on both sides of the basin. On the carbonate-dominated flank these deposits are composed of highly abraded and fragmented skeletal material, which typically display distinctive parallel, swaly, and/or flaser-like lamination, indicative of shoreface deposition. On the clastic side of the basin the FSST is sand-dominated and forms a thick, coarse, clastic wedge that extends well out into the basin. In many cases these coarser clastics stand in sharp contrast to underlying shales (HST) and overlying shelly, mineralized sands (TST). The widely correlative sequences that dominate sedimentary successions in the Appalachian basin suggest a predominance of allocyclic processes modified locally by tectonic overprint.