Lower Cretaceous Lacustrine Deposits and Paleosol Development

Christos Mantzios, Carl F. Vondra

The Little Sheep Mudstone Member of the Lower Cretaceous Cloverly Formation in the Big Horn basin, Wyoming, is predominantly a montmorillonite-rich lacustrine deposit. It is informally subdivided into lower and upper units. The lower unit is a playa-lake deposit rich in bentonite derived from the alteration of volcanic ash which had its origin to the west. Horizons of silcretes and septarian nodules are common. The latter show irregular lenticular cracks and are filled with coarse calcite and barite crystals. They formed by desiccation of a clay-gel during dry periods. Silcretes are diagenetic products that formed due to the lowering of silica solubility along with decreasing of the pH value. Desiccation cracks on the playa surface were filled with chalcedony which later nderwent replacement by calcite.

The upper unit is a perennial saline lake deposit similar to that accumulating in Lake Magadi of the Eastern Rift Valley, Kenya. Chert nodules covered with calcium carbonate form stratigraphically persistent horizons. This chert is the product of the magadiite-kenyaite-chert transformation of Hay. Lenticular devitrified tuffs occurring at various stratigraphic positions up to 3 m thick were deposited in depressions or swales. Locally the tuffs show evidence of mass movement.

Pedogenic features in both units indicate paleosol development. Tree trunks, plant roots, burrows, clay-rich zones, and organic-rich A and more iron-rich B master horizons are recognized. These paleosols resemble modern-day Vertisols. The lower unit is not as extensive areally as the upper unit which is present throughout the Big Horn basin, indicating that extensive lakes occurred during the Early Cretaceous in the Sevier foreland basin.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.