RECOGNIZING EXPOSURE, DROWNING, AND MISSED-BEATS - PLATFORM-INTERIOR TO PLATFORM-MARGIN SEQUENCE STRATIGRAPHY OF MIDDLE ORDOVICIAN LIMESTONES, EAST TENNESSEE

Stratigraphic sequences in a Middle Ordovician platform-interior carbo nate succession (Steinhauff and Walker, in press) are correlated to th e platform margin, Surfaces of subaerial exposure are correlated among seven stratigraphic sections, four from the platform succession and t hree from platform margin rocks. Surfaces of subaerial exposure define stratigraphic sequences within a carbonate succession that shows litt le evidence of cyclicity in the field, Interpretation of carbonate fac ies geometry is based on outcrop observation, petrographic and geochem ical analysis of samples, and consideration of paleobathymetric curves . In the platform succession, most sequences are bounded by exposure s urfaces, but in more continuously subtidal areas periods of shallowing and drowning are discerned in bathymetric corves on the basis of mult iple lines of evidence. Exposure surfaces are evidenced by subjacent m eteoric cements or other features indicating exposure such as truncate d marine cements, vuggy porosity with vadose silt and pendant cements, or mud cracks. At some localities, a marked shallowing event correlat es 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 presenc e of red algae, thin-shelled (eyeless) trilobites, delicate arborescen t bryozoa, and deposit-feeding organisms. Evidence for drowning unconf ormity is in some cases provided by cephalopod-rich, black to blood-re d limestone beds rich in iron and manganese oxides, and occasionally b y corroded surfaces encrusted with these minerals. We have referred to the succession as ''apparently noncyclic'' because evidence for expos ure 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 ar e those that overlie fenestral mudstones that contain exposure feature s, such as crystal silt and calcite cement in vugs lined by reddish cr usts, and are truncated by deeper-water lithologies, For example, seve n pronounced surfaces of subaerial exposure define six third-order seq uences, 10-50 m thick, on the platform near Thorn Hill, Tennessee. Wit hin the lower four third-order subsequences, 19 less pronounced surfac es showing fewer exposure features are present in approximately 400 m of section. These surfaces define fourth-order subsequences that are c orrelative from place to place in the platform succession, Many, but n ot all, of these platform fourth-order subsequences can be correlated to the platform margin. Also, some fourth order subsequences are prese nt only at the platform edge. Fifth-order, ''meter-scale'' packages, o r parasequences, are present within these fourth-order sequences, but are not correlative over wide areas as are the other sequences. Most a re probably the result of migration of lateral environments, Geometrie s unlike those of siliciclastic sequences result where condensed secti ons or deepening events are followed by exposure in this carbonate suc cession. The sequences defined here constitute shallowing-upward packa ges that are, then, a type of cycle, These are used to construct modif ied Fischer plots for both the platform-interim and platform-margin st ratigraphic successions. Surfaces of subaerial exposure are correlated from the platform to the platform margin by matching pronounced expos ure 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 possibl e solutions, the most conservative of which suggests that some sequenc es 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 inte rior but not platform-margin areas until subsidence allows resubmergen ce; and (3) drowning carries platform margin areas to depths precludin g carbonate deposition there, though sediment does accumulate on the p latform. In Cases 1 and 2 the missed ''beats'' would be on the platfor m, but in Case 3 they would be at the platform margin.