Abstract Outer shelf carbonates in the Tansill Formation in Dark Canyon include the following inferred depositional environments landward of the Capitan reef: (1) subtidal: bioclastic wackestones, packstones, and graded and locally cross-stratified grainstones; and biostromes; (2) peritidal flats: fenestral and locally desiccated mudstone; and (3) shoreface: admixed subtidal and peritidal deposits. Two sequences are recognized in the section based on biotic diversity and parasequence thickness and facies-stacking patterns, and the boundary between them is at or near the base of the Ocotillo Member. Maximum flooding of the platform occurred during deposition of sequence 1 in the lower part of the middle Tansill. Environments were biostromes, mainly high energy shallow subtidal packstones and grainstones, and associated peritidal islands in the early HST in this sequence. Precipitation of abundant marine cements in these rocks may have been promoted by active seawater pumping through the sediments on this wide and shallow shelf. Microbial activity in the grainstones may have been promoted by restricted circulation around associated peritidal islands. In contrast, environments in the late HST of this sequence and in sequence 2 were more restricted in terms of shelf width and energy, and the rocks are dominantly micritic and contain little marine cement. The principal difference between depositional facies in the Carlsbad Embayment and along the Reef Escarpment appears to be the presence of patch reefs in the former area, which were deposited on an outer shelf that was wider and of more open circulation than to the south. Inferred marine cements are dominated by prismatic calcites, with microdolomite inclusions and some recognizable radiaxial-fibrous habit, interpreted as former high-Mg calcite. The 8 I8 0 and 8 13 C compositions of the least-altered of these cements (−1.6 %c and 5.8 %o PDB, respectively) suggest precipitation from marine pore fluids. Former aragonitic cement of similar isotopic composition is a volumetrically minor, first-generation marine cement in these rocks. Dominance of inferred former high-Mg calcite cement contrasts that in coeval platform-margin patch reef facies, the Capitan reef, and in pisolites on the shelf crest in which former aragonite marine cement dominates. Mean 8 18 0 composition of the dolomite that replaced peritidal deposits (0.1 %c PDB) suggests that it precipitated from ambient marine fluids of elevated salinity. Mineralogic stabilization of earlier diagenetic phases likely attended precipitation of equant calcites (spar 1) in the rocks, which is interpreted to have occurred in a meteoric phreatic system during the latest Guadalupian and/or early Ochoan. A second generation of equant cements (spar 0) is inferred to have precipitated later, during the Ochoan or later, in a deeper meteoric-dominated groundwater system. Replacement of syndepositionally precipitated evaporites by poikilotopic calcite is the last diagenetic phase present and, based on stable oxygen-carbon isotope data, appears to have attended meteoric dedolomi- tization during the Tertiary.

Abstract: Model arkoses containing K-feldspar (or Na-feldspar) + kaolinite + quartz (or silica glass or boehmite) were reacted in solutions of a variety of compositions in the KCl-NaCl-H 2 O system, and in sea water at 200 to 350°C and 500–1,000 bars in rapid-quench, cold-seal pressure vessels. Experiments reveal that: (1) mixed-layer illite/smectite (I/S) with or without discrete illite are the major neoformed phases for all runs with low fluid/rock ratio; (2) the precipitation rate for neoformed clays is similar for near-neutral solutions of varying compositions, but the expandability of the I/S depends strongly on solution composition at the same temperature and pressure; (3) discrete illite appears only in solutions rich in K*; (4) sea water dramatically retards the formation of illite layers in I/S; (5) illitic clay with fibrous habit grows more effectively in solutions with silica oversaturated with respect to quartz; illitic clays formed in initially alkaline and acidic solutions tend to be platy; and (6) illitic clays form in near-neutral solutions at much slower rates than those in alkaline solutions. The experimental study verifies that the nonequilibrium mineral assemblage feldspar + kaolinite + quartz serves as a control for fluid composition, which favors precipitation of illitic clays in rock-dominant systems. The diagenetic products of a feldspar-bearing sandstone are determined primarily by the presence or absence of kaolinite and by the flow rate, and secondly by the initial fluid composition or temperature. The initial fluid composition becomes crucial only in the fluid-dominant system, whereas temperature is more important in controlling the kinetics than in shifting the thermodynamics of illitic-clay formation. The commonly observed illitization of kaolinite-bearing arkosic sandstones during burial diagenesis is attributed more to the reduction of the flow rate of the existing fluids than to the influx of new illitization fluids, or to the increase of temperature alone.