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 and mineral-water interaction processes following caldera collapse. The sedimentology and authigenic mineral distributions also provide information regarding the role of the Creede

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... ion in the ancient Creede hydrothermal system. 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 basin-wide correlation. Carbonate minerals were deposited as travertine at springorifices and as suspension-fallout (micrite and micritic peloids) lamina across the bottom. The travertine accumulations circumscribe the margins of the moat basin and probably outline the structural margin of the caldera. Remains of invertebrate and fish fauna are not observed in the lake beds. Deposition in a cool, montane climate is indicated by the presence of pseudomorphs after ikaite in the lacustrine deposits and corroborative paleofloral analyses. Distinctive lowstand and highstand facies-association distributions were deposited; the transition between which is recorded by fining-upward, alluviallacustrine sequences around the caldera margin and in paleovalleys. 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. The character of the lacustrine lamina and travertine suggests that the lake evolved into a meromictic lake with bicarbonate-rich waters. Although the basin was topographically closed, no evidence for hypersaline conditions is observed. 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. Hydrolysis and dissolution of the ash are interpreted to have led to formation of smectite, clinoptilolite, erionite, potassium feldspar, and quartz during burial diagenesis. 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 diagenetic assemblage by analcime and chlorite-smectite mixedlayered 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 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 appears to have resulted from reactions associated with conductive cooling of the hydrothermal fluid and mixing 1 with diagenetic fluids. The pattern of alteration suggests that the hydrothermal plume flowed laterally into the Creede Formation from the fault-controlled Creede mineral district to the north of the caldera. Diagenetic reactions in the tuffaceous strata probably resulted in moderately 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 Na-bicarbonate diagenetic fluids in the hydrothermal system has not been investigated, but is required to address the problem further. example of the complex interplay between volcanism, tectonism, sedimentation, and hydrology that exists in volcanic settings. METHODS OF STUDY The distribution of sedimentary facies and structural features was mapped at a scale of 1:25,000, and 23 detailed stratigraphic sections were measured (Larsen, 1994a) . Parts of CSDP cores CCM-1 and CCM-2 (Fig. 1 ) were described in detail. The complete stratigraphy of the cores is described in detail elsewhere (see Hulen, 1991). The sections and cores were sampled every 3 to 5 m for petrographic, X-diffraction, and geochemical analyses. Soluble carbonate content was determined gravimetrically on 150 samples by dissolution in 1 N Na-acetate solution (buffered at pH = 5). Mole % magnesium was determined in 16 bulk carbonate samples by X-ray diffraction using the method of Goldsmith and others (1961). TOC (total organic carbon), HI (hydrogen index), and OI (oxygen index) were determined by Joel Leventhal and Ted Daws at the U. S. Geological Survey (Denver) using a Leco carbon apparatus and Rock-Eval anhydrous pyrolysis. The authigenic mineralogy was investigated through a combination of X-ray diffraction, petrographic, scanning electron microscope, and electron microprobe analysis. The authigenic mineral composition of 270 samples was determined by Xray diffraction analysis of semi-random glass-slide mounts of the < 20 (im (fine silt) and oriented glass-slide mounts of the < 2 (im (clay ) fractions (see Larsen, 1994a) . The carbonate-free, fine-silt fraction was used because volcanic rock fragments, pyrogenic crystals, and soluble carbonate dilute the proportion of authigenic mineral phases and produce peak interferences with authigenic silicates. The mounts were subjected to Cu Koc (30 to 35 mA, 45 keV) radiation using a Scintagg PAD V X-ray diffractometer. Spectra were obtained continuously through a range of 2 to 50° 20 for the fine-silt fraction and 2 to 35° 20 for the clay fraction. Selected clay-fraction samples were analyzed using the thermal treatment of Boles (1972) in order to assess the composition and structure of clinoptilolite. Peak intensity data from the fine-silt fraction were used to calculate weight proportions of the constituent mineral phases by the Chung method (Chung, 1974a and 1974b) . Mineral standards were prepared with corundum, and were mounted and analyzed using the same procedure as for the fine-silt fraction (see Larsen 1994a). The mineral-standard peak intensities were used to calculate reference intensity ratios (RIR) for each mineral as required by the Chung method. The precision of the analysis was determined by multiple analyses of five samples to be between ± 10 to 20 % (1 standard deviation). The accuracy of the analyses was determined using synthetic mixtures of mineral standards to be within 10 to 20 % of measured values (Larsen, 1994a) . The authigenic mineral paragenesis was determined using a combination of polarized-light and scanning electron microscopy. Approximately 150 thin sections were described; of which, 60 were point-counted using a Swift automated point STRATIGRAPHY AND STRUCTURE OF THE CREEDE FORMATION Although exposures of the Creede Formation are found throughout the caldera moat, no more than 100 m of section is exposed at any location. Furthermore, extensive faulting (Fig. 1 ) commonly prohibits correlation across major drainages. Cores CCM-1 and CCM-2 provide the most complete sections of Creede Formation. Correlation between the cores (Fig. 2 ) was accomplished using a combination of lithostratigraphy, tephrostratigraphy, and borehole geophysical data (Nelson and Kibler, Chapter P, the Open-File Report). Core-to-surface correlation was completed using lithological and mineralogical characteristics of the fallout tuffs (labeled A through M2 in Fig. 2 ) and lithologic characteristics of the depositional and early diagenetic carbonates (Larsen, 1994a) . The base of the Creede Formation in core CCM-2 is chosen as a datum because of the great thickness represented by this core. Assuming that no fault exists between the CCM-2 site and measured sections 0.5 km to the north, approximately 700 m of Creede Formation are present in the northcentral part of the moat. The Rio Grande graben structure is particularly well-developed where it coincides with the apical graben of the Bachelor caldera (Steven and Ratte, 1965) . Significant displacements (up to 200 m) are also present in the Creede Formation to the south, as indicated by surface mapping and the correlation in Figure 2 . Travertine accumulations are observed around the margins of the moat basin, but also commonly follow the trace of faults. The moat-margin travertine distribution does not correspond to a lithologic discontinuity or a specific elevation, but rather probably reflects the position of the caldera ring-fracture system (Heiken and Krier, 1987) and pre-existing structures (Larsen, 1994a) . Evidence for syndepositional movement (buried scarps, travertine talus, etc.) is not observed. Thus, many of the present-day faults with associated travertine accumulations may have existed as fractures or faults of minimal displacement during Creede Formation deposition. FACIES AND FACIES ASSOCIATIONS Stratified Sedimentary Facies The stratified sedimentary facies include lamina and beds composed of pyroclastic and epiclastic detritus, depositional carbonate, intraclasts, and minor organic material (Table 1 ). Low-magnesium calcite is the only carbonate mineral identified by X-ray diffraction of bulk samples (Table 2 ). Carbonized organic material is commonly preserved along partings, especially in laminated facies. Impressions of conifer needles, shrub leaves, and deciduous tree leaves are common; insect impressions are rare. Remains of shelly or vertebrate fauna are nearly absent in the beds; gastropod shells have been reported (E. Evanoff, personal communication, 1993) and diatoms are present in some travertine deposits (P. M. Bethke, personal communication, 1994).