Sedimentary History and Economic Geology of San Juan Basin

Abstract

The San Juan basin contains up to 15,000 feet of sedimentary rocks ranging in age from Cambrian to Recent. Beginning with the Cambrian transgression, at least 10 major events in the sedimentary history of the basin area can be recognized. The earliest development of the area as a sedimentary basin or trough apparently took place in Pennsylvanian time and the basin was maintained, with changing rates of subsidence and filling, through the remainder of geologic time. During the early Paleozoic, sedimentation was dominated by marine transgressions across the northwestern flank of the regional Transcontinental arch. The late Paleozoic history was strongly influenced by tectonism related to development of the Ancestral Rocky Mountains uplifts and associated downwarping. Domina tly cyclic marine carbonate deposition during the early phases of this event preceded the infilling of the trough with coarse clastics. The early Mesozoic is characterized by fluvial and eolian environments, interrupted periodically by thin marine transgressive deposits of nearshore redbeds. The final Mesozoic event was the widespread Upper Cretaceous marine transgression which deposited a thick cyclic sequence of marine gray shale and sandstone, with interbedded coal beds. Final withdrawal of marine waters in Late Cretaceous time was associated with the rise of major uplifts on the northern and eastern sides of the basin and infilling of the basin area with stream, lacustrine, and paludal deposits during early Tertiary time. Late Teritary regional uplift and resulting volcanism were acc mpanied by a regional dissection of the area by stream systems that evolved into the present-day drainage pattern of superposed streams. The sedimentary history is directly related to the occurrence of economic deposits of the basin. Major reserves of petroleum are in Cretaceous and Pennsylvanian sections, coal in Cretaceous, and uranium in Jurassic and Cretaceous.