The Oligocene Creede Formation represents an exceptionally well-preserved intracaldera sedimentary sequence within a large, resurgent caldera. The tuffaceous, epiclastic, and limestone deposits observed in surface exposures and Continental Scientific Drilling Program (CSDP) core provide a record of depositional processes and basin evolution following caldera collapse. The basal part of the Creede Formation is characterized by interbedded caldera wall–derived debris-flow breccias and ephemeral lake deposits. This unit is succeeded by deep-water lacustrine beds that constitute the bulk of the Creede Formation. Interbedded fallout tuffs from intracaldera volcanic eruptions significantly affected lacustrine sedimentation patterns and provide a means of basinwide correlation. Prominent sublacustrine travertine accumulations circumscribe the margins of the moat basin and probably outline the structural margin of the caldera. Deposition in a cool, montane climate is indicated by the presence of pseudomorphs after ikaite in the travertine and lake beds, and corroborative paleofloral analyses. Distinctive lowstand (shallow lake) and highstand (deep lake) facies-association distributions were deposited. Changes in the proportion and character of lowstand versus highstand sequences imply an evolution from a narrow, deep, steep-walled basin to a broad, shallow, incised basin through time.
Carbonate minerals were deposited in the lake as travertine at spring-orifices, and as suspension-fallout laminae across the bottom. Little evidence for shelly or fish fauna is preserved in the lake beds. The character of the laminated strata and abundance of depositional carbonate suggest that the lake evolved into a permanently stratified, bicarbonate lake. The volume and mineralogy of depositional carbonate changed through time, but evidence for development of hypersaline, alkaline lake-water compositions is not observed. The level of basin-fill and geomorphic considerations preclude control of lake level by a hydrologic sill; thus, the basin hydrology appears to have been controlled through a combination of climate and groundwater seepage.
The deposits of the Creede Formation closely resemble those in modern and ancient caldera and crater lake basins. The effects ofvolcanism on detrital lacustrine sedimentation are profound; however, the influences on lake chemistry and biology are less clear. The extensive travertine in the Creede Formation appears to be related to the volcanic setting, although the origin is as of yet unclear.