The Barranca Formation contains important deposits of coal and, where metamorphism was more intense, some of North America’s most valuable graphite reserves. Information derived from petroleum exploration supports a Tertiary model for the Cook Inlet basin that applies to the Barranca. Since a fore-arc basin has existed near Cook Inlet through the present time, the modern Cook Inlet could also be an analog of the Barranca basin.
The Kenai Group in the Cook Inlet area consists of fluvial-deltaic, swamp, and estuarine sediments about 4,600 m thick that represent three sedimentation phases: (1) the Oligocene to Miocene West Foreland, Hemlock, and lower Tyonek formations contain conglomerate, graywacke, siltstone, tuff, basalt flows, and coal; (2) siltstone, shale, carbonate, and coal of the Miocene upper Tyonek and lower Beluga formations indicate lower energy deposition; and (3) the Pliocene upper Beluga and Sterling Formations are mostly coarse clastic rock.
The three members of the Barranca Formation, which is over 1,500 m thick, indicate gross changes from high to low to high-energy deposition. The upper and lower members contain sandstone, conglomerate, minor shale, and coal, and enclose a middle member that consists of coal, nonmarine to shallow marine shale and siltstone, and some conglomerate and impure sandstone. The rocks within each member, and mainly within the middle one, indicate that shallow marine incursions interrupted fluvial and swamp sedimentation.
Paleogeographic and tectonic similarities between the two areas suggest that detailed aspects of Cook Inlet sedimentary environments apply to the Barranca Formation.