Mass Movement deposits in the lacustrine Eocene Green River Formation, Piceance Basin, Western Colorado

Abstract

Mass-movement deposits interbedded with laminated sediments are common in lacustrine strata of the Eocene Green River Formation, Piceance Basin, western Colorado, comprising the majority of some stratigraphic intervals. Lake Uinta, a saline lake, formed when much smaller freshwater lakes expanded across surrounding alluvial plains. Mass-movement deposits were rare early in the history of Lake Uinta, but increased in abundance as marginal shelves prograded into the newly expanded lake. Mass-movement deposits accumulated near the base of these expanding shelves. Total organic matter increases in areas where mass-movement deposits accumulated, suggesting that organic matter from marginal areas was entrained along with mineral matter. In addition, intervals that include abundant mass-movement deposits generally contain a higher concentration of organic matter that laminated intervals. Saline Lake Uinta frequently transgressed and regressed across the marginal shelves as water supply varied providing opportunities to erode and transport mineral matter and organic matter toward the center of the lake. In the central part of the lake, mass-movement deposits consisted of massive organic-rich beds that typically contained clasts or “blebs” of mineral matter. Lake Uinta expanded later in its history, and deepwater conditions prevailed over much of the marginal shelves. Mass-movement deposits become rare during this period, and laminated oil shale was deposited over most of the lake. Mass-movement deposits again became common during the infilling stage of Lake Uinta as volcaniclastic sediments progressively filled in the lake from the north.

AAPG Datapages/Search and Discovery Article #90193 © 2014 Rocky Mountain Section AAPG Annual Meeting, Denver, Colorado, July 20-22, 2014