High-resolution surface and subsurface sequence stratigraphy of late middle to late Ordovician (late Mohawkian-Cincinnatian) foreland basin rocks, Kentucky and Virginia
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The late Middle to Late Ordovician (late Mohawkian to Cincinnatian) supersequence in Kentucky and Virginia was deposited in a tectonically active foreland basin during a transition from the 'ice-free' Early Ordovician to the glacial Late Ordovician. This sequence was deposited in less than 12 m.y., is 250-500 m thick, and is composed of four large third-order sequences (each 40-150 m thick) that are regionally correlative in outcrops, cores, and gamma-ray logs. Smaller scale third-order parasequence sets (up to 20 m thick) and component parasequences (1-8 m thick) make up the larger sequences and are only locally correlative. Subtidal-dominated parasequences comprise the basal part of each sequence, whereas shallower subtidal- or peritidal-dominated parasequences compose upper parts of sequences. Each large, third-order sequence is asymmetric and marked by lowstand, transgressive, and highstand systems tracts with unique lithologic and gamma-ray log response characteristics. Ramp margin wedges (RMW) are poorly developed and consist of marine siltstone or grainstone/packstone sheets extending into deep ramp settings; these sheets have high gamma-ray values in their base and lower, blockier gamma-ray responses in their tops. Transgressive systems tracts (TST) are thin; composed of high-energy, locally phosphatic grainstone and deeper ramp, interbedded skeletal packstone and shale; and become shalier upsection with a corresponding increase in gamma-ray values to maximum flooding surfaces. Maximum flooding surfaces (MFS) occur within shaly 'zones' in outcrop and cores, but are evident on gamma-ray logs as a sharp turnaround between increasing gamma-ray values below and decreasing gamma-ray values above. Highstand systems tracts (HST) are thick and composed of aggraded skeletal grainstone or prograded peritidal facies (restricted skeletal wackestone/packstone and fenestral lime mudstone or silty dolomite). Sequence boundaries that developed on peritidal facies are low-relief exposure surfaces below which underlying facies are mottled by paleoweathering. Sequence boundaries on subtidal facies tend to be conformable and marked by basinward shift in overlying facies. Parasequences in all systems tracts except late HST are predominantly subtidal. Parasequences in the lowstand systems tracts (LST) and early HST show a shallowing-upward trend, but in the TST parasequences progressively deepen upward to the MFS. Late HST parasequences are shallow subtidal or peritidal carbonates that also progressively shallow upward. Juxtaposition of facies within the subtidal cycles of the TST suggests moderate-amplitude (up to 15 m or more), relative high-frequency sea level fluctuations, but facies in peritidal parasequences of late HST indicate lower amplitude, relative high-frequency sea level fluctuations. The stratigraphic packaging was likely caused by complex thrust-induced subsidence and uplift interacting with long- and short-term (a few million years to tens of kiloyears) eustasy during onset of Gondwana glaciation. Reservoir analogs on this ramp are subtidal skeletal grainstone sheets occurring in all systems tracts, and skeletal buildups in the supersequence STS. Moderate-amplitude eustatic fluctuations produce repeated drownings that vertically compartmentalize these facies. Dolomitic tidal-flat facies in the supersequence HST are also potential reservoir facies; however, the high silt content in these units makes them a less desirable analog.
