SEDIMENTOLOGY, PALYNOLOGY, AND PETROLOGY OF THE CRETACEOUS MATANUSKA FORMATION, SOUTH-CENTRAL ALASKA: RELATIONSHIPS BETWEEN FOREARC BASIN DEVELOPMENT AND ACCRETIONARY TECTONIC EVENTS

Analysis of the Cretaceous Matanuska Formation (MF) provides information on the timing and nature of forearc basin development and accretionary tectonic events in southern Alaska. Previous studies of the MF were within the context of regional mapping studies. Our new sedimentologic, palynologic, and petrologic data demonstrate that the MF can be subdivided into two depositional facies assemblages. The lower MF overlies a regional Aptian unconformity and comprises a 300- to 500-m-thick transgressive assemblage deposited during Albian-Santonian time. Lowermost strata include lenticular sandstone, paleosols, and carbonaceous mudstone that we interpret as fluvial channel and overbank deposits. Fluvial strata are overlain by tabular sandstones and fossiliferous mudstones interpreted as shoreface and marine shelf deposits, respectively. The transgression documented in the lower MF is attributable to fault-related subsidence in combination with eustatic sea level rise. Lower MF clast compositions are dominated by schist and radiolarian chert indicative of the Chugach subduction complex and metabasalt and tuff indicative of the remnant Talkeetna arc along both the arcward (northern) and trenchward (southern) basin margins. The overlying 2,000-m-thick upper MF comprises a progradational assemblage of Campanian-Maastrichtian graded sandstone, conglomerate, and mudstone interpreted as the product of sediment gravity flows on submarine fans. Proximal-distal facies transitions, paleocurrent data, and slump fold orientations indicate deposition on an unstable, arcward- (northward-) dipping basin floor. Steep depositional gradients were likely maintained by episodic displacement along the Border Ranges fault (BRF), which separates the forearc basin from the subduction complex. Compositional data from the upper MF show a dominance of volcanic, plutonic, and metamorphic clasts that are comparable to the Talkeetna arc in the hanging-wall of the BRF. Similar, age-equivalent submarine fan deposits (MacColl Ridge Fm.) are exposed in eastern Alaska, suggesting widespread instability along the BRF during accretion of the Valdez Group within the subduction complex. MF lithofacies assemblages are displaced by intrabasinal dextral faults, consistent with Late Cretaceous-Cenozoic transpressional tectonics in southern Alaska.