Stratigraphy and Provenance of Upper Triassic Red Bed Units of the Chulitna Terrane, South-Central Alaska: Tectonic Implications for Sedimentation Prior to Island Arc Collision

The Chulitna terrane of the southern Alaska Range is one of the more enigmatic tectonic terranes in the North American Cordillera and is defined in part by the occurrence of Upper Triassic red bed and carbonate units that occur in association with basalt and serpentinite in the Alaska Range suture zone. Previous studies in this area have provided initial constraint through general stratigraphic descriptions of these units. However, Upper Triassic strata have yet to be placed into a sedimentologic and stratigraphic context.

New geologic mapping, measured stratigraphic sections, and provenance of Upper Triassic strata from the Chulitna terrane reveal a continuous record of competing volcaniclastic, siliciclastic, and carbonate sedimentation. Red bed units are defined by interbedded successions of poorly-sorted and matrix-supported volcaniclastic and siliciclastic sandstone and conglomerate. Individual beds are typically 0.25–2 m thick and exhibit tabular geometries. Well-sorted sandstone occurs sporadically throughout red bed units and is characterized primarily by trough cross-stratification. Red bed units are laterally extensive for up to 150 m and only rarely exhibit evidence of basal erosional scour.

Upper Triassic volcaniclastic, siliciclastic, and carbonate units in the Chulitna terrane were originally thought to represent three distinct successions that were stratigraphically separate from each other. However, measured sections and modal composition reveal that carbonate and red bed successions are interbedded throughout the Chulitna terrane, suggestive of multiple distinct source areas and varying styles of sedimentation. Sedimentation is interpreted to represent a combination of subaerial and submarine gravity flow processes, limited fluvial processes, and carbonate precipitation. These new findings may imply mixed source areas that include a dissected arc and continental margin. Upper Triassic strata of the Chulitna terrane are interpreted to represent sedimentation associated with limited exhumation of a volcanic arc and continental margin and periods of tectonic quiescence where carbonate precipitation persisted.