Allostratigraphic Control Of The Occurance Of Chert: Lower Devonian, Pennsylvania

Abstract

Chertification in the 4th-order (400 ky) sequence of the Lower Devonian New Creek and Corriganville Formations is primarily controlled by high-frequency sea-level fluctuations. The New Creek Formation (one or two 5th-order sequences) is divisible into six small-scale allocycles, in which chert occurs at cycle boundaries. The Corriganville Formation (two 5th-order sequences) is composed of nine laterally correlative small-scale cycles, in which chert occurs at cycle boundaries as well as at sea-level-fall surfaces and in high-stand portions of the cycles. The position of chert in the allocyclic framework is primarily controlled by stratigraphic processes, while inhomogeneities in sediments and supply of diagenetic ingredients are secondary controls. For example, chert morphology, which varies from bedded to nodular, seems to be controlled by inhomogeneities in the sediments. Stratigraphic processes act through control of sedimentation rates; in these shallow, below-wave-base environments, seclimentation of carbonate and terrigeneous matter ceased at cycle boundaries in the New Creek and Corriganville Formations and at sea-level-fall surfaces in the Corriganville Formation, and was intermittent during the high-stand portions of cycles in the Corriganville Formation. Silica accumulated undiluted during cycle boundary times; the silica from these primary locations of accumulation migrated later to a nearby host. In addition to controlling concentrated accumulation of siliceous sediments, stratigraphic processes enhanced chert formation by promoting diagenetic reactions in the sulphate reduction zone. In this manner the occurrence of chert in the New Creek and Corriganville Formations corresponds to the allostratigraphic framework and was controlled by stratigraphic processes.