ABSTRACT Discoveries in the 1980s greatly expanded speleologists’ understanding of the role that hypogenic groundwater flow can play in developing caves at depth. Ascending groundwater charged with carbon dioxide and, especially, hydrogen sulfide can readily dissolve carbonate bedrock just below and above the water table. Sulfuric acid speleogenesis, in which anoxic, rising, sulfidic groundwater mixes with oxygenated cave atmosphere to form aggressive sulfuric acid (H 2 SO 4 ) formed spectacular caves in Carlsbad Caverns National Park, USA. Cueva de Villa Luz in Mexico provides an aggressively active example of sulfuric acid speleogenesis processes, and the Frasassi Caves in Italy preserve the results of sulfuric acid speleogenesis in its upper levels while sulfidic groundwater currently enlarges cave passages in the lower levels. Many caves in east-central Nevada and western Utah (USA) are products of hypogenic speleogenesis and formed before the current topography fully developed. Wet climate during the late Neogene and Pleistocene brought extensive meteoric infiltration into the caves, and calcite speleothems (e.g., stalactites, stalagmites, shields) coat the walls and floors of the caves, concealing evidence of the earlier hypogenic stage. However, by studying the speleogenetic features in well-established sulfuric acid speleogenesis caves, evidence of hypogenic, probably sulfidic, speleogenesis in many Great Basin caves can be teased out. Compelling evidence of hypogenic speleogenesis in these caves include folia, mammillaries, bubble trails, cupolas, and metatyuyamunite. Sulfuric acid speleogenesis signs include hollow coralloid stalagmites, trays, gypsum crust, pseudoscallops, rills, and acid pool notches. Lehman Caves in Great Basin National Park is particularly informative because a low-permeability capstone protected about half of the cave from significant meteoric infiltration, preserving early speleogenetic features.

Calcite spar (crystals >1 cm in diameter) are common in limestone and dolostone terrains. In the Guadalupe Mountains, New Mexico and west Texas, calcite spar is abundant and lines small geode-like caves. Determining the depth and timing of formation of these large scalenohedral calcite crystals is critical in linking the growth of spar with landscape evolution. In this study, we show that large euhedral calcite crystals precipitate deep in the phreatic zone (400–800 m) in these small geode-like caves (spar caves), and we propose both are the result of properties of supercritical CO 2 at that depth. U-Pb dating of spar crystals shows that they formed primarily between 36 and 28 Ma. The 87 Sr/ 86 Sr values of the euhedral calcite spar show that the spar has a significantly higher 87 Sr/ 86 Sr (0.710–0.716) than the host Permian limestone (0.706–0.709). This indicates the spar formed from waters that are mixed with, or formed entirely from, a source other than the surrounding bedrock aquifer, and this is consistent with hypogene speleogenesis at significant depth. In addition, we conducted highly precise measurements of the variation in nonradiogenic isotopes of strontium, 88 Sr/ 86 Sr, expressed as δ 88 Sr, the variation of which has previously been shown to depend on temperature of precipitation. Our preliminary δ 88 Sr results from the spar calcite are consistent with formation at 50–70 °C. Our first U-Pb results show that the spar was precipitated during the beginning of Basin and Range tectonism in a late Eocene to early Oligocene episode, which was coeval with two major magmatic periods at 36–33 Ma and 32–28 Ma. A novel speleogenetic process that includes both the dissolution of the spar caves and precipitation of the spar by the same speleogenetic event is proposed and supports the formation of the spar at 400–800 m depth, where the transition from supercritical to subcritical CO 2 drives both dissolution of limestone during the main speleogenetic event and precipitation of calcite at the terminal phase of speleogenesis. We suggest that CO 2 is derived from contemporaneous igneous activity. This proposed model suggests that calcite spar can be used for reconstruction of landscape evolution.

Abstract The carbonate ramp to rimmed shelf transition is a well-known depositional pattern in carbonate platforms, but the nature of the stepwise evolution, the evolution of facies and facies tract patterns from sequence to sequence, and the intrinsic and extrinsic controls that drive the transition are less well known. The Guadalupian carbonate platform system in the southwestern US provides an example of a ramp-to-rim transition in a carbonate to mixed siliciclastic-carbonate succession during a second-order progradational supersequence. The sequence framework of the Guadalupian section includes 30 high-frequency sequences deposited over an 11 Myr time span. Compositing data from exposures across a 50 km strike width and 30 km dip width area provides the control needed to document this transition. Early Guadalupian San Andres sequences exemplify low-angle (<2°) foreshore-shoreface-offshore profiles (ramps) with 60 to 80 m of topset to toeset relief and as much as10 km of slope width (distance from shelf inflection to slope inflection point). Extensive lower slope-basin hemipelagic mudstones of the Cutoff Formation were shed from this active mud factory, and this balance of accumulation rates in the platform and slope maintained a low-angle profile. A distinct lack of sedimentgravity-flow features is noted from this profile. After the protracted Brushy Canyon lowstand event, upper San Andres shelf carbonates show cessation of low-angle 2 to 4° ramplike profiles, where active carbonate factories on the shelf produced thick margins that tapered downdip into less-developed lower-slope hemipelagic mudstone aprons. Dilute toe-of-slope turbidites and upper-slope fusulinid banks add heterogeneity to this slope profile. Uppermost San Andres and Grayburg sequences mark the beginning of a forced regressive mixed siliciclastic-carbonate pattern in which siliciclastics constitute 30 to 50% of the platform and slope. Uppermost San Andres clinoforms prograded between 5 and 10 km, range in slope angle from 4to15°, and have 100mof topset to toeset relief. A combination of siliciclastic and mixed carbonate-siliciclastic turbidites accumulated on the lower slope, and the upper slope consists of fusulinid-rich packstones with dips up to 20°. No debris deposits developed in the uppermost San Andres sequence. Rare patch reefs occupied shelf-margin promontories between reentrants that were dominated by siliciclastic bypass. The reduced (<5 km) carbonate factory width of the uppermost San Andres results in absence of a hemipelagic mudstone lower-slope apron, which accentuates oversteepening of the slope. The lower Grayburg-upper Cherry Canyon sequence can be considered a lowstand wedge complex within the longer-term succession. This Grayburg-Cherry Canyon interval prograded 5 km and is characterized by 5-to 25°-dipping mixed siliciclastic-carbonate clinoforms, 65 m of topset to toeset relief, and minor debris flows. As for the G9 high-frequency sequence, there is a narrow shelf carbonate factory (<5 km) and absence of a hemipelagic mudstone lower-slope apron. Progradational and aggradational stacking of the middle Grayburg through lower Queen Formations most likely contains the record of the first reef-rimmed platforms with massive bedding developed near the shelf-slope break. A180-m-relief collapse scar at the margin as exposed at Bush Mountain on the Western Escarpment removed critical evidence of potential reef-margin development for the middle Grayburg-lower Queen sequences. Following this major collapse, the upper Queen (shelf)-Goat Seep (reef)-South Wells (basin) section represents the first preserved reef-rimmed platform margin. Clinoformheights of200m, a massive reefal margin-upper slope, and a debris-dominated foreslope with dips at angle of repose signal the final transitional stage into a Capitan-like system. Remaining Seven Rivers-Yates-Tansill-Capitan-Bell Canyon sequences continued to build relief to 500 to 600 m as they prograded an additional 5 km basinward. Slope complexes within the Capitan-equivalent section consist of compensationally stacked ponded and backfilling geometries separated by through-going collapse events. Three primary but interrelated drivers are visualized for the ramp-rim transition recorded in Guadalupian strata of the Guadalupe Mountains outcrops. First, initial siliciclastic influx and concomitant reduced export of fine-grained carbonates to the slope during uppermost San Andres deposition led to basin starvation, margin steepening, and an increase in sediment bypass and early stage margin colonization. Second, middle Guadalupian Grayburg aggradational stacking of shelf margins drove progressive oversteepening and ultimately led to large-scale collapse of the margin, at least locally. This headwall became the site of the first preserved reefal margin in the Guadalupes. Finally, a marked increase in shelf accommodation space was created by flexural subsidence of Leonardian shelf strata over the buried Late Leonardian shelf margin. In other parts of the Delaware Basin, the steep Leonardian margin break was the single dominant control on the development of Guadalupian margins.