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Petrology of Lower Jurassic Chamosite-Siderite Ironstones from Northeast England

Timothy M. Chowns

The Cleveland Ironstone Formation of northeast England is a condensed sequence (up to 27 m thick) consisting mainly of shale with chamosite-siderite ironstones developed at six different horizons. The ironstone seams are thin, sheetlike, isochronous deposits that invariably form diastems in the sequence. The entire formation is marine with the ironstone occupying the highest energy, shallowest water environments. Petrographically, the various seams are similar: grain or mud-supported chamosite ooids in a matrix of chamosite-siderite mud, and siderite cement. Other allochems include shell debris, phosphatic intraclasts, and fecal pellets. Although these ironstones are texturally almost identical to limestones, several lines of evidence indicate that chamosite is the primar mineral of the ooids; most important the perfect oolitic envelopes and flaxseed shape are characteristic of chamosite ooids. This shape is a phenomenon of growth and not the result of compaction as often interpreted.

In the main seam, which was worked extensively for iron ore, chamosite ooid grainstones pass laterally into sparsely oolitic siderite mudstones and then into shales with siderite mudstone concretions. The process of iron mineralization began with the formation of chamosite ooids and muds on the sea floor, continued with siderite replacement of grains and matrix during early burial, and ended with early diagenetic cementation by siderite. The anomalous association of ferrous iron chamosite in well-sorted, oolitic textures may be explained if bacteria maintained individual growing ooids in a reduced state within a generally oxidizing environment.

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