Preliminary stratigraphic interpretation of the Naknek Formation—Evidence for Late Jurassic activity on the Bruin Bay fault, Iniskin Peninsula, lower Cook Inlet

摘要: The northeast-trending Bruin Bay fault is a major structure bounding the western side of Cook Inlet where it generally separates Early Jurassic plutonic and volcanic rocks of the Talkeetna arc on the northwest side from Middle and Late Jurassic forearc sedimentary rocks on the southeast side (Detterman and Reed, 1980). Despite the significance and size of this fault system, which can be traced for> 400 km, its history remains poorly understood (fig. 1, see Bruin Bay fault section, this paper). To evaluate the possibility of Jurassic activity on the fault, we conducted stratigraphic studies of the Middle and Upper Jurassic units exposed on the southwestern Iniskin Peninsula, nearest the mapped trace of the fault system (fig. 36). LePain and others (2011) reported stratigraphic evidence from the upper Tuxedni Group that suggested that activity on the Bruin Bay fault was likely initiated by late Middle Jurassic time. The following discussion focuses on our observations from the Upper Jurassic Naknek Formation (fig. 37).We measured detailed stratigraphic sections along the shores of Iniskin and Oil bays (fig. 36), focusing specifically on the Chisik Conglomerate Member and the Northeast Creek Sandstone Member (formerly called the “lower sandstone member”; Detterman and Hartsock, 1966; Detterman and others, 1996). This work has led to an improved understanding of the map-scale distribution of the coarse-grained Chisik, restricting the unit to exposures along Iniskin Bay (fig. 36) where it is dominated by approximately 100 m of poorly organized pebble, cobble, and boulder conglomerate (fig. 38), interpreted as fan delta deposits. In sharp contrast …