STORM-INFLUENCED DELTAIC DEPOSITS IN THE MIDDLE JURASSIC GAIKEMA SANDSTONE OF THE TUXEDNI GROUP, COOK INLET BASIN, ALASKA

Middle Jurassic strata of the Gaikema Sandstone were deposited about 170 million years ago on a delta located on the western side of the Cook Inlet basin, about 240 km southwest of Anchorage. The delta was built by swift, sediment-laden rivers that flowed southeastward from a mountainous volcanic terrane situated west of the Bruin Bay fault. Upon reaching the edge of the Jurassic sea, the rivers dumped abundant sand, gravel, and mud into a depocenter on the northern Iniskin Peninsula.

We studied a detailed, 34-m-thick measured section of the Gaikema Sandstone on the Iniskin Peninsula at latitude 59.816, longitude -153.168. Sandstone in the measured section is mainly fine grained and occurs in beds that range in thickness from 10-300 cm but are mostly 30-100 cm thick. Nearly all of the sandstone beds have sharp, erosional lower contacts. The lower part of each bed consists of parallel laminated and/or hummocky cross laminated sandstone that grades upward to bioturbated sandstone, which in turn is truncated by a sharp, erosional contact and is abruptly overlain by laminated sandstone of the succeeding bed. Molluscan shell fragments, finely comminuted plant debris, and chunks of wood are common.

By analogy with similar facies that are well-described in the published literature, we suggest the following interpretation for the sandstone in our measured section. Sand carried by rivers to the Gaikema delta was mobilized by storms and redeposited on the shallow marine flank of the delta as beds with parallel lamination and hummocky cross lamination. During periods of fair weather between storms, the upper part of each laminated bed was reworked by sediment-churning organisms to form bioturbated sand. At the onset of the next storm, the bioturbated sand was partly eroded by waves and currents and then sharply overlain by a new layer of storm-deposited, laminated sand.

Petrographic studies indicate that sandstone in the measured section consists of plagioclase feldspar and volcanic rock fragments. These unstable grains are altered to clays and zeolites that occlude nearly all of the original porosity of the sandstone, suggesting that the Gaikema Sandstone has limited potential as a conventional petroleum reservoir but might be prospective for continuous hydrocarbon accumulations if located near petroleum-generating source rocks.