Abstract

Clinton ironstones (mid-Silurian) on the southeastern margin of the Appalachian Foreland Basin in central Pennsylvania consist largely of skeletal grainstone storm beds whose skeletal fragments are extensively impregnated and replaced by ferric oxide. Other forms of iron mineralization consist of ooids, superficial ooids, cavity fillings and cements composed of ferric oxide and/or chamositic clay, and two generations of iron-rich carbonate spar. True oolite is rare. Small proportions of ferruginized grains are present in coarse lags at tops of some small-scale, 1- to 5-m coarsening-upward sequences, which are the record of mid-shelf shoals from which the skeletal grainstone storm beds were derived.

Ooids and superficial ooids are likely to have formed in dysaerobic conditions where the pycnocline of a highly stratified sea intersected shoal flanks. Ferrous iron from anaerobic deeper waters precipitated on quartz and skeletal fragment nuclei in weakly reducing conditions with episodic gentle agitation. Several stages of early diagenetic ferruginization affected skeletal grainstone storm beds and sequence-capping coarse lags: chamositic clay precipitated in pores and cavities of skeletal fragments, within quartz sand adjacent to crinoid ossicles, and as bed-parallel connectors of large skeletal fragments. As diagenesis progressed with further burial, many skeletal fragments were extensively replaced by ferric oxide, most chamositic clay altered to ferric oxide, other chamositic clay matured from berthierine to chamosite, and clusters of bladed specularite replaced parts of primary grains and secondary carbonate cements. Paleomagnetic and fluid-inclusion analysis indicates that many late diagenetic changes took place at elevated temperatures during deen burial, in an early phase of the Alleghanian orogeny.

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