MINERALOGY AND PARAGENESIS OF LARGE DOLOSTONE SEPTARIA IN THE UPPER DEVONIAN HURON SHALE, NORTH-CENTRAL OHIO

Both early diagenetic and burial diagenetic histories of large septarian concretions in the lower part of the Huron Shale, Huron River, are recorded in the depositional sequence of their matrix and vein minerals. Ranging from 0.3 to 4 m in diameter, the septaria are concentrated in two laterally extensive horizons that are a few meters thick and separated by 10 m of black shale. The septaria are of two general shapes: a spherical-type that is slightly flattened in the plane of bedding, and a flattened-type with a typical height to width ratio of 1:2. Bedding planes can be traced through the concretions, which formed in highly porous organic-rich argillaceous sediment a few meters below the sediment-water interface. At an early stage, the sediment that enclosed organic nuclei such as arthrodire remains was cemented with calcite. Limestone concretions were first replaced by fine-grained ferroan dolomite under conditions of low dissolved sulfate concentrations, and then fractured while the sediment was still soft. Smaller concretions frequently are rimmed with a thin layer of radiating fibrous pyrite that predates fracturing. Fractures are most abundant in flattened-type septaria, forming polygonal patterns in sections parallel to bedding.

The dominance of replacement dolomite in the matrix and the appearance of ferroan dolomite as the earliest vein mineral suggest that early dolomite was deposited soon after fracturing. Early dolomite also is finer-grained and more abundant than vein phases deposited later. A generalized sequence of deposition is as follows: early brown dolomite, barite, dark brown rhombohedral dolomite, brown calcite, whewellite, light brown rhombohedral dolomite, colorless calcite, quartz, chalcedony, and late light brown dolomite. Small amounts of pyrite also appear at various positions in the sequence. Whewellite is associated with arthrodire remains. Greenish-gray geopetal sediment, which is comprised of quartz and 2M₁ illite with some kaolinite, is found in small pockets.