Recognizing exposure, drowning, and "missed beats"; platform-interior to platform-margin sequence stratigraphy of Middle Ordovician limestones, East Tennessee

Abstract

Stratigraphic sequences in a Middle Ordovician platform-interior carbonate succession (Steinhauff and Walker, in press) are correlated to the platform margin. Surfaces of subaerial exposure are correlated among seven stratigraphic sections, four from the platform succession and three from platform-margin rocks. Surfaces of subaerial exposure define stratigraphic sequences within a carbonate succession that shows little evidence of cyclicity in the field. Interpretation of carbonate facies geometry is based on outcrop observation, petrographic and geochemical analysis of samples, and consideration of paleobathymetric curves. In the platform succession, most sequences are bounded by exposure surfaces, but in more continuously subtidal areas periods of shallowing and drowning are discerned in bathymetric curves on the basis of multiple lines of evidence. Exposure surfaces are evidenced by subjacent meteoric cements or other features indicating exposure such as truncated marine cements, vuggy porosity with vadose silt and pendant cements, or mud cracks. At some localities, a marked shallowing event correlates to exposure elsewhere. In subtidal sediments, drowning is indicated by the lack of shallow-water physical sedimentary features and water-depth curves based on multiple lines of evidence including the presence of red algae, thin-shelled (eyeless) trilobites, delicate arborescent bryozoa, and deposit-feeding organisms. Evidence for drowning unconformity is in some cases provided by cephalopod-rich, black to blood-red limestone beds rich in iron and manganese oxides, and occasionally by corroded surfaces encrusted with these minerals. We have referred to the succession as apparently noncyclic because evidence for exposure and shallowing is subtle and is not always observable in the field. Nonetheless, these strata show fourth-order subsequences that can be grouped into third-order sequences by the existence of pronounced and less pronounced exposure surfaces. Pronounced surfaces of exposure are those that overlie fenestral mudstones that contain exposure features, such as crystal silt and calcite cement in vugs lined by reddish crusts, and are truncated by deeper-water lithologies. For example, seven pronounced surfaces of subaerial exposure define six third-order sequences, 10-50 m thick, on the platform near Thorn Hill, Tennessee. Within the lower four third-order subsequences, 19 less pronounced surfaces showing fewer exposure features are present in approximately 400 m of section. These surfaces define fourth-order subsequences that are correlative from place to place in the platform succession. Many, but not all, of these platform fourth-order subsequences can be correlated to the platform margin. Also, some fourth-order subsequences are present only at the platform edge. Fifth-order, meter-scale packages, or parasequences, are present within these fourth-order sequences, but are not correlative over wide areas as are the other sequences. Most are probably the result of migration of lateral environments. Geometries unlike those of siliciclastic sequences result where condensed sections or deepening events are followed by exposure in this carbonate succession. The sequences defined here constitute shallowing-upward packages that are, then, a type of cycle. These are used to construct modified Fischer plots for both the platform-interior and platform-margin stratigraphic successions. Surfaces of subaerial exposure are correlated from the platform to the platform margin by matching pronounced exposure surfaces and deepening events shown in water-depth curves for the platform with pronounced exposure surfaces and deepening or shallowing events at the platform margin. This analysis provides several possible solutions, the most conservative of which suggests that some sequences or cycle beats are missing. Beats are missed in three ways: (1) sea level remains near the platform margin, causing deposition to be restricted to that area; (2) tectonic uplift exposes the platform interior but not platform-margin areas until subsidence allows resubmergence; and (3) drowning carries platform margin areas to depths precluding carbonate deposition there, though sediment does accumulate on the platform. In Cases 1 and 2 the missed beats would be on the platform, but in Case 3 they would be at the platform margin.