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Testing for Possible Cyclicity in Carbonate Sediments of Middle Ordovician of East Tennessee

M. Ghazizadeh, K. R. Walker

Middle Ordovician carbonate sediments (Chickamauga Group) near Decatur, Tennessee, consist of 450 m of complex tidal-flat and subtidal sediments. Fourteen facies are recognized: (1) cherty dolostone (supratidal environment); (2) green and red silty mudstone (supratidal mud flat); (3) greenish-gray micrite-biomicrite (supratidal mud flat); (4) red silty, intrapelbiosparite-biosparite (intertidal channel); (5) green and red, loosely packed, ostracod-rich pelbiomicrite (intertidal pond); (6) stromatolitic mudstone (intertidal levee); (7) bryozoan-ostracod-brachiopod-rich pelbiomicrite (subtidal lagoon type I); (8) silty, packed pelbiomicrite-pelbiosparite (subtidal lagoon type II); (9) ostracod-gastropod-rich, bioturbated pelbiomicrite (subtidal lagoon type III); (10) biotur ated brachiopod-molluscan-rich biomicrite (subtidal lagoon type IV); (11) bioturbated green and red silty mudstone to silty sparse biomicrite (lagoon); (12) gray-tan mudstone to sparse biomicrite (quiet water, deeper subtidal lagoon); (13) intrapelbiosparite-pelbiosparite (subtidal channel); and (14) Tetradium-rich packstone (subtidal wave baffle).

Using the Gingerich method, facies-transition values of 0.01-0.25 suggest a complex set of transitions, most not interpretable in clear sedimentologic terms. However, values above 0.10 indicate a small set of more meaningful transitions: dolomitic, supratidal-flat sediments are most likely overlain by supratidal mud-flat sediments and least likely by Tetradium wave-baffle sediments; supratidal mud-flat sediments are most likely overlain by intertidal pond deposits, and least likely by intrabasinal lagoon sediments; intertidal pond sediments are overlain by intertidal channel sediments; Tetradium wave-baffle deposits are overlain by subtidal channel sediments; and subtidal channel deposits are overlain by lagoonal sediments.

The modified Markov method identifies only two cells in the same matrix with alpha values of 0.2 or higher (80% significance level). These represent a high probability of (1) subtidal channel deposits overlain by intertidal channel sediments, and (2) intertidal channel deposits overlain by intertidal pond sediments. This more statistically valid technique indicates much lower cyclicity than the Gingerich method and suggests cycles not identified by the latter. The cycles merely result from the topographic relationship between the 4 environments and the lateral migration of facies. If the probability examined is reduced to 60%, a series of statistically insignificant transitions result similar to those identified by the Gingerich method. These results suggest that cyclicity is a rare f ature of this Ordovician platform carbonate sequence.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.