ABSTRACT: Genetic Sequences of the Lower Minturn Formation (Middle Pennsylvanian), Southeastern Eagle Basin, Colorado
John A. Karachewski
Facies analysis of siliciclastic and minor carbonate rocks indicates that the lower Minturn Formation was deposited in alluvial fan, braidplain, Gilbert-type delta, paralic, shoreline, and offshore-marine environments. At least 12 genetic sequences, defined as paired transgressive-regressive units bounded by offshore-marine or paralic flooding surfaces, were recognized within clastic units B (uppermost part), C, and D of Tweto and Lovering, an interval of approximately 450 m. Genetic sequences are asymmetric and characterized by thin transgressive units and two types of thick regressive units. Gradational regressive units (about 25% of total) shoal upward from offshore-marine to shoreline or Gilbert-type delta deposits and formed by progradation during relative stillstand of sea level. In contrast, nongradational regressive units (about 75% of total) are characterized by the abrupt juxtaposition of offshore-marine or paralic deposits with overlying alluvial-fan or braidplain deposits. Nongradational regressive units probably formed in response to eustatic base level changes associated with Gondwanan glaciation or by debouching of alluvial fans or braidplains into paralic and shoreline environments.
Genetic sequences 1 through 9 range in thickness from 6 to 45 m, whereas genetic sequences 10 through 12 range in thickness from 76 to 100 m. The thickening-upward trend of genetic sequences probably records increasing rates of subsidence in the southeastern Eagle basin.
AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990