Aspects of the palaeoecology of crinoidal limestones
Blyth Cain, J. D.
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Crinoidal limestones are characteristic of marine epi- continental shelf successions of Lower Palaeozoic to Mesozoic age. Chiefly developed in the lower Carboniferous System, crinoidal limestones are also known in Silurian, Devonian, Upper Carboniferous and Jurassic rocks. Studies by various people show that depositional environment was varied; at one extreme are poorly- sorted, argillaceous, quiet -water sediments (Troell, 1952; Lucia, 1962; Middleton, 1963), and at the other are well- sorted, clean -washed sediments with sparry calcite cement (Lowenstam, 1948) showing cross -bedding (Laudon, 1957; Caldwell & Charlesworth, 1962), torrential bedding, lensing, and scour - and -fill structures attributable to tidal rip- currents (Middleton, 1963).The aim of the present work is to attempt to reconstruct some aspects of the depositional environment of crinoidal limestones. A lame part of any such study obviously involves full consideration of the principal organic component from several points of view. This thesis begins with a review of the ecology of recent crinoids and its bearing on crinoid palaeoecology, and continues with observations on the chemical and crystallographic features of the fossilisable material. The role of crinoid remains as potential constituents of sediments is then dealt with, this leading on naturally to a study of the limestones themselves.For the purposes of this study, crinoidal limestones are defined as biostromal (Cumings, 1932) calcareous sediments in which the principal fossils or fossil fragments are crinoidal; marginal types such as crinoidal sandstones and crinoidal shales are considered, but rocks containing frame- building organisms are excluded.