Sedimentary petrology and authigenic mineral distributions in the Oligocene Creede Formation, Colorado, United States


The Oligocene Creede Formation is 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 and mineral-water interaction processes following caldera collapse. The authigenic mineral distributions also provide information regarding the role of the Creede Formation in the ancient Creede hydrothermal system. The basal part of the Creede Formation is characterized by interbedded calderawall-derived debris-flow breccias, alluvial, and shallow lacustrine 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 basin-wide correlation. Carbonate minerals were deposited as travertine at spring orifices and as suspension-fallout (micrite and micritic peloids) laminae across the lake bottom. The travertine accumulations circumscribe the margins of the moat basin and probably outline the structural margin of the caldera. Most of the detrital sediment within the Creede strata was derived from reworking of Fisher Quartz Latite fallout ash and erosion of the caldera walls. Calcareous and tuffaceous siltstone intraclasts are common in most coarse-grained lacustrine lithologies, especially in beds deposited after the emplacement of the H fallout tuff. Most of the identifiable ash-flow tuff lithic fragments in the coarser grained beds can be ascribed to one of the Carpenter Ridge Tuffs or the Wason Park Tuff. Fragments from a variety of intermediate composition lavas are also common in most beds. Fragments of crystal-rich ashflow tuff units are generally less abundant, although clasts of Snowshoe Mountain Tuff and Fish Canyon Tuff are locally present in various parts of the caldera. Clasts of tuffs associated with formation of the San Luis caldera may be present in the lithic composition of Creede sediments, although confirmation awaits more definitive petrographic analysis of the Creede Formation and San Luis caldera ash-flow tuffs. The relative rarity of Snowshoe Mountain Tuff in the Creede Formation, even in the upper part of the section, is surprising and alludes to the unusual character of this ash-flow tuff unit. Hydrolysis and dissolution of the ash are interpreted to have led to formation of smectite, phillipsite(?), clinoptilolite, erionite, potassium feldspar, and quartz during burial diagenesis under a high geothermal gradient. The formation of phillipsite is inferred from pseudomorphic structures and may reflect increased alkalinity in the lake waters during the latter part of the lake history. The effects of two major low-temperature hydrothermal events are superimposed on diagenesis. The Antlers Park event resulted in replacement of the smectite and zeolite diagene tic assemblage by analcime,chlorite, and chlorite/smectite mixed-layered clay in the northwestern part of the moat. The Creede hydrothermal event is interpreted to have produced various silica minerals, illite, and potassium feldspar observed above 400 m in the formation in the northeastern part of the basin. In both events, the patterns of alteration indicate that faults and coarse-grained deltaic and lacustrine fan deposits served as pathways for fluid movement through the formation. The alteration associated with the Creede hydrothermal event seems to have resulted from reactions associated with conductive cooling of the hydrothermal fluid and mixing with diagenetic fluids. The pattern of alteration suggests that the hydrothermal plume flowed laterally into the Creede Formation from the faultcontrolled Creede mineral district to the north of the caldera. Diagenetic reactions in the tuffaceous strata probably resulted in saline, alkaline fluids, especially in the upper part of the formation. Initiation of faulting and hydrothermal fluid flow in the Creede hydrothermal system could have allowed diagenetic pore waters from the Creede Formation to recharge the hydrothermal flow cell. The chemical evolution of the diagenetic fluids in the hydrothermal system has not been investigated, but is required to address the problem further.

Publication Title

Special Paper of the Geological Society of America