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Primary structures observed in the Great Oolite Series include planar, wedge-shape and lenticular cross-stratification, current and interference ripples, micro cross-lamination, dunes, pseudonodules, loadcasts, graded bedding, mudcracks, groove casts, current lineation, prod marks, and bounce casts. These structures are grouped into two environmental and petrographic combinations.
High-energy, channelled, shelly-oolitic limestones (grainstones) are characterized by vertical sequences consisting of a basal zone of imbricated shell fragments, a middle zone of thick sets of planar and lenticular cross-stratification, and an upper zone of interference and current ripples and wavy beds. The cross-strata show two dip directional maxima which are 180° apart, possess a high dip angle (26° average) and contain thin (1-1.5 in.) graded beds. These graded cross-strata consist of a shell chip zone which grades upward into coarse-grained oolite and fine-grained oolite. The shelly base of such a graded bed thickens and increases in particle size downslope. The oolites were deposited by normal flow in a lower flow regime, whereas the shell chips were deposited by cou ter eddies eroding the basal channel shell lag concentrate. The primary structures in the channelled limestone were formed in a lower flow regime operating in intertidal zone channels.
Low-energy, even-bedded, oolitic clayey limestone (packestone and wackestone) are characterized by thick (1-5 ft.) graded beds, load casts and cross-stratified grainstone lenses. These cross-strata are low angle (average dip is 18°) and are current lineated if the limestone contains 15-25% quartz sand. The graded beds grade upward from coarse-grained oolite to clay-size carbonate particles. The top of each graded bed is bored and plastered with oyster shells in life position. These graded beds were formed by periodic sea-level oscillations which transgressed subtidal deposits across low and then high tidal flats. The cross-stratified grainstone lenses represent a beach or barrier bar marginal to the intertidal zone.
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