Dynamic Significance of Primary Structures in the Middle Jurassic Great Oolite Series, Southern England
George De Vries Klein, 1965. "Dynamic Significance of Primary Structures in the Middle Jurassic Great Oolite Series, Southern England", Primary Sedimentary Structures and Their Hydrodynamic Interpretation, Gerard V. Middleton
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Primary structures observed in the Great Oolite Series include planar, wedge-shaped and trough cross-stratification, current and interference ripple marks, micro-cross-lamination, sand waves, pseudonodules, load-casts, graded bedding, mudcracks, groove casts, parting lineation, prod marks and bounce casts. These structures are grouped into two environmental and petrographic combinations.
Shelly, oolitic channel 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 trough cross-stratification, and an upper zone of interference and current ripple marks and wavy beds. The cross-strata show two dip directional maxima which are 180° apart, possess a high dip angle (24° average) and contain thin (1 to 1½ inches) graded beds. These graded cross-strata consist of a shell chip zone which grades upward into a coarse-grained oolite and into fine-grained oolite. The shelly base of such a graded cross-stratum thickens and coarsens downslope. The graded oolites were deposited by normal flow whereas the shell chips were deposited by foreset avalanching. The primary structures in the channel limestone were formed in tidal flat channels under a lower flow regime. Bank caving and deposition of the shell lag concentrate took place under an upper flow regime.
Oolitic, clayey, flat-bedded limestones (grainstone, packstone and wackestone) are characterized by thick (1 to 5 feet) graded beds, load casts and cross-stratified grainstone lenses. The graded beds range upward from coarse-grained oolite to calcilutite, and their surface is bored and plastered with oysters in life position. These graded beds were formed by regressions of low tidal flat sediments over high tidal flat sediments. The cross-strata are low angle (average dip is 18°) and show parting lineation where the limestone contains 15 to 25 percent quartz grains. These cross-stratified grainstone lenses represent fossil sand waves on tidal flats. Their presence in limestone suggest that hydraulic processes and principles governing the formation of sand waves in flumes and streams are equally applicable to bottom-scouring marine currents.
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This volume contains papers presented as part of a symposium held in Toronto on May 18, 1964. These papers are mainly designed to assist the geologist, who is interested in the hydrodynamics of formation of sedimentary structures but who has little or no training in hydraulics, to become familiar with the extensive body of research which has been undertaken by hydraulic engineers interested in sediment problems.