Sedimentary structures on a delta-influenced shallow shelf, Norton Sound, Alaska

Abstract

Sedimentation in an epicontinental sea influenced by deltaic progradation is exemplified by the Norton Sound-Yukon Delta region. Norton Sound is a large embayment of more than 24,000 km2 with water depths of less than 25m. The Yukon Delta. on the south side, is a major North American source of sediment that enters the Sound. Progradational deposits on the seaward part of the delta are highly reworked by storm waves and currents, and serve as a model for a depositional sequence that encroaches on a shallow shelf. To describe the primary physical and biogenic sedimentary structures of the several facies in this embayment, we utilized X-ray radiographs, relief casts, and grain-size analyses of 83 box cores. Primary physical sedimentary structures are best developed in and adjacent to the Yukon Delta and include parallel- and ripple-laminated sand and silt and crossbedded sand. Biogenic sedimentary structures are found throughout Norton Sound and, in the northern part, completely obliterate physical sedimentary structures. Bioturbation close to the northern shoreline suggests that rates of sedimentation there are low. Dominance of physical structures near the delta results from (1) increased wave and current energy in this very shallow water, (2) reduced biological activity in brackish water, and (3) increased rates of deposition. As a result, the Holocene progradational sequence in Norton Sound consists of basal beds with well developed physical structures deposited during lower eustatic sea level, a thin middle interval of bioturbated mud and a thick upper section of structured beds deposited by the prograding delta.