ABSTRACT

The Upper Cretaceous Gammon Shale in the northern Great Plains contains abundant siderite concretions that formed during the early compactional history of the fine-grained, marine sediments. The relative depth of siderite precipitation can be estimated on the basis of textural, mineralogical, and isotopic evidence. Siderite concretions that formed at shallow depths in the sediment column contain a large volume percent carbonate (75-85%), preserve uncompacted sediment textures, and have oxygen isotope ratios similar to those of carbonates precipitated in equilibrium with seawater (¹⁸O e 0 to -2, PDB). In contrast, lower olume percent carbonate values (55-75%), and relative decreases in ¹⁸O/¹⁶O values characterize concretions that formed longer after deposition and (or) at greater depths in the accumulating sediments. Both volume percent carbonate and ¹⁸O values are strongly correlated with independently determined sediment accumulation rates. Concretions in rapidly deposited sediments formed at shallow depths (<10 m), whereas siderite concretions in sediments that accumulated slowly formed at greater depths (>200 m), later in the burial history.

The Gammon served as both source rock and reservoir for accumulations of biogenic methane and, like the siderite concretions, the methane was generated during early diagenesis. Both siderite precipitation and methane accumulation require interstitial pore waters that have sufficiently high bicarbonate activities and that are depleted in dissolved sulfur and free oxygen. Because of chemical requirements, siderite did not precipitate until after the onset of methane generation. Therefore, if the depth of siderite formation is known, the minimum depth for the onset of methane accumulation can be inferred.

The presence of abundant pyrite in slowly accumulated Gammon sediments suggests, by analogy to data from recent sediments, that the depth of siderite precipitation and methane accumulation was controlled by the sedimentation rate. In slowly accumulating sediments, most organic matter was probably consumed by aerobic organisms and sulfate-reducing anaerobic bacteria, whereas in areas of more rapid deposition large quantities of organic matter were available for methane generation.