Modern coastal back-barrier environment: Analog for coal basin or for carbonaceous black shale?

Coastal back-barrier salt-marsh environments are evaluated as possible modern analogs for some coal-forming basins. Analysis of the amount of organic matter in Holocene salt-marsh peats from 158 core samples at Plum Island, Massachusetts, indicates that the organic matter content is too low to ultimately produce coal. C-14 dates indicate depositional rates of 1 mm/yr. Both the high and low salt-marsh peat facies would probably diagenetically alter to a carbonaceous shale. A review of the organic content reported from other nondeltaic back-barrier and coastal salt-marsh environments indicates that they commonly have too much detrital material to classify as coal precursors. Barrier-island systems, being affected by sea-level changes and subject to landward migrations, also lack long-term stability needed for coal formation. These data suggest that modern back-barrier environments are not good analogs for coal-forming basins.