Abstract The Cambro-Ordovician shelf was initiated during Late Precambrian rifting with breakup at 600 ± 25 Ma. The passive-margin carbonates developed over Early Cambrian rift to marine-shelf clastics. Subsidence rates were low (1 to 10 cm/ka), and driven by exponentially decreasing fhermo-tectonic subsidence coupled with flexural loading. Variation in subsidence rates formed depocenters in Tennessee, Pennsylvania, and Vermont, separated by arches in New Jersey, Virginia, and Alabama. Calculated platform slopes per cycle for the carbonate sequences were 2 to 4 cm/km. Initially, an early Cambrian bank-fringed ramp developed. By Middle Cambrian, this ramp had formed into a high-relief reef-rimmed margin fringed by thick periplatform talus deposits. This rimmed shelf persisted into the Late Cambrian. Middle Cambrian rifting formed the Rome trough inboard of the passive margin, while redbeds were deposited over much of the shelf. With sea-level rise, a huge intra-shelf basin formed within the Tennessee depocenter. Both the Rome trough and the intra-shelf basin had ceased to exist by the Latest Cambrian. The rimmed shelf developed into a ramp during initial subduction in the Early Ordovician. During Middle Ordovician collision, a widespread unconformity developed, the ramp foundered, and its leading edge was deformed. At the formation level, the stratigraphy reflects third-order (1 to 10 m.y.) sea-level fluctuations. At a smaller scale, platform facies are mainly 1 to 5 m thick, peritidal carbonate cycles that reflect 20- to 100-ka (Milankovitchian) sea-level fluctuations with less than 10-m amplitudes. These low amplitudes allowed the shelf to track sea-level fall as it continued to subside during regressions; thus, soils or regoliths are lacking. Dolomitization during this phase was intense, however. Regional thick (30–300 m) subtidal limestones punctuate the shelf sequence and likely were due to third-order sea-level rises. Long-term sea-level falls periodically caused widespread deposition of eolian and coastal clastics over the shelf at tops of carbonate cycles. Intra-shelf basin Grand Cycles reflect gradual shallowing during third-order sea-level rise/fall coupled with superimposed Milankovitch-scale fluctuations.

Abstract This chapter deals with the evolution of the various parts of the Appalachian orogen in the United States prior to the onset of the formative deformational episodes of the orogen in the early Paleozoic. The general discussion that follows is intended to add perspective to the more detailed discussions within each section.

Abstract The Middle Ordovician limestone sequence, Virginia Appalachians, formed on a carbonate ramp that lay northwest of a subsiding foreland basin. Large carbonate buildups (I to 60 km (.6 to 37 mi) wide and 20 to 250 m (66 to 820 ft) thick) occur in the lower (onlap) part of the sequence. The buildups are underlain by peritidal and lagoona! carbonates, and they occur as isolated buildups in two well-defined belts on the ramp. Shallow ramp buildups (Rockdell, Ward Cove Limestone) were buried by argillaceous limestone deposited during regional drowning of the shallow ramp. Downslope buildups (Effna, Murat Limestone) occur on the deep ramp and basin slope. They were killed by anoxic bottom waters (indicated by local sulfide and/or phosphorite crusts on buildup crest) and by burial beneath aggrading basinal shale and/or limestone. The middle Ordovician buildups have similarities to late Paleozoic Waul-sortian-type buildups. They commonly have cores of stromatactoid-bearing mud-stone and/or wackestone and minor boundstone. Cores are underlain, overlain and flanked by bedded, pelmatozoan-bryozoan grainstone and packstone. Buildups generally lack robust frame-builders. They are best described as carbonate banks that formed by high rates of skeletal carbonate accumulation under a transgressive setting, baffled by pelmatozoans and ramose bryozoans, and widespread marine cementation. Buildups were initially lithified by three major marine cement fabrics. These include turbid isopachous neospar cement and isopachous cement with local botryoidal habit, turbid rim cement which is an inclusion-rich syntaxial cement on echinoderm grains, and bladed cement which is turbid to clear with a well-defined crystal habit. These syndepositional cements were of profound importance in constructing and stabilizing the non-reefal buildups. Remaining spaces were filled by nonferroan, clear rim and equant cement together with solid hydrocarbon (in downslope buildups), and ferroan dolomite cement. The clear cements are zoned, as defined by cathodoluminescence, and consist of nonluminescent (oldest), bright and dull (youngest) cements; the zonation relates to increasingly reducing conditions of pore waters. The zonation is simple, reflecting progressive burial of the buildups by 3000 m (9843 ft) of Middle Ordovician and Mississippian sediments.