Depositional environments and resource potential of cretaceous coal-bearing strata at Chignik and Herendeen bay, Alaska Peninsula

Abstract

Cretaceous (Campanian to Maestrichtian) coal-bearing strata occur on the Alaska Peninsula southwest of Wide Bay and extend to Pavlof Bay. The depositional succession consists of the lower member of the Chignik Formation, the Coal Valley Member (middle member, Chignik Formation), the upper member of the Chignik Formation, and the Hoodoo Formation. Previous interpretations of the depositional environments of these formations suggest that deposition of nonmarine sands of the Coal Valley Member through the nearshore sediments of the Chignik Formation and the deep-water marine Hoodoo Formation represents a marine transgression. Other researchers have interpreted these units as coeval sedimentary facies deposited in different environments: (1) alluvial fan to flood plain (Coal Valley Member); (2) inner-neritic (upper and lower shoreface) continental shelf (Chignik Formation); and (3) outer-neritic continental shelf to bathyal continental slope (Hoodoo Formation). Additional field work by the authors suggests that both previous interpretations should be combined. Upper and lower shallow-marine facies of the Chignik Formation represent transgressive events separated by the nonmarine Coal Valley Member, a regressive phase. The interrelationship of these units indicates that the Coal Valley Member was deposited simultaneously with individual marine facies of the Chignik and Hoodoo Formations in three different environments. This simultaneous, intermediate depositional phase was followed by onlap (transgression) by the upper member Chignik and Hoodoo Formations. This interpretation is supported by field evidence of lateral interfingering of the sediments and by gradational vertical contacts. The depositional environment for the sediments of the Coal Valley Member is nonmarine flood plain and alluvial fan. There are indications that marine conditions were nearby and salt-marsh and delta deposition may have taken place. If depositional environments were the same or similar elsewhere in the Late Cretaceous, thick coal accumulations probably cannot be expected to occur in covered areas on the Alaska Peninsula. Coal resource estimates were computed for two areas within the Coal Valley Member trend. These areas, Herendeen Bay and Chignik Bay, contain most of the known potential for commercial coals on the Alaska Peninsula. Resource estimates approach 360 × 10 6 MT in beds 0.9 m or more in thickness.