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Book Chapter

Dynamic Significance of Primary Structures in the Middle Jurassic Great Oolite Series, Southern England

By
George De Vries Klein
George De Vries Klein
Financial support for this research came from two grants for field work in 1962 and 1963 from the American Philosophical Society, a grant for thin sections from Sigma Xi, and a travel grant for the summer of 1963 from the National Science Foundation.The writer is indebted to W. S. McKerrow for his invitation to study the sedimentology of the Great Oolite Series and for his assistance, suggestions and criticisms during the investigation. Special thanks are extended to Or. McKerrow and Prof. L. R. Wager of Oxford University, and to Dr. J. W. Cowie and Prof. W. F. Whittard of Bristol University for making office facilities available to the writer during the summers of 1962 and 1963 respectively. The writer benefited from personal di8236scussions with Prof. P. C. Sylvester-Bradley, R. Cave, C. Aslin, J. D. Hudson, J. R. L. Allen, L. V. Illing, G. Evans and P. Lee. Aslin, Hudson and Evans are to be thanked in particular for making available unpublished field observations. Finally, the writer is doubly grateful to W. S. McKerrow, H. A. Meyerhoff, G. V. Middleton, H. G. Reading and A. H. Stride for their editorial and scientific criticisms of the manuscript. The writer, however, is solely responsible for any errors that may remain.
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Published:
January 01, 1965

Abstract

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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SEPM Special Publication

Primary Sedimentary Structures and Their Hydrodynamic Interpretation

Gerard V. Middleton
Gerard V. Middleton
McMaster Ulliversity
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SEPM Society for Sedimentary Geology
Volume
12
ISBN electronic:
9781565761421
Publication date:
January 01, 1965

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