Research about Lake Creede

USGS2000

Hydrologic budget of the late Oligocene Lake Creede and the evolution of the upper Rio Grande drainage system

The filling history, hydrologic budget, and geomorphic development of ancient Lake Creede and its tributary basin are evaluated to determine the factors that controlled its character. The lake filled the Creede caldera that formed in the late Oligocene as a consequence of the eruption of the Snowshoe Mountain Tuff. The caldera's sedimentary fill accumlated to a depth of about 1.26 km and had a volume of about 89 km 3 . The highest lake level was ~3300 m (10,800 ft) present altitude before it drained eastward across a broad volcanic plateau as the ancestral Rio Grande. A tributary canyon several hundred meters deep was cut into hard rhyolite in the north wall of the caldera before the lake was more than half full; its presence demonstrates that ancient Lake Creede filled slowly and thus occupied a long-lived, closed basin. The slow filling rate is incompatible with the present water flux through the Creede caldera basin, because such a flow would fill the basin geologically instantaneously. This mismatch, together with the recognition that the Oligocene climate was similar to that of today, forces the reexamination of the hydrologic and geomorphic history of the caldera. That appraisal shows that the caldera cannot have resurged rapidly immediately after caldera collapse, and that ancient watershed must have been lass than half as large as the present upper Rio Grande basin. The ancient lake had a more or less constant surface area of about 200 km 2 that approximated a steady-state condition between inflow and evaporation. Although the lake level fluctuated with climatic variations, its surface elevation steadily climbed as sediment accumulated, accelerating as resurgance and dome growth usurped spacewithin the basin. It could have had one playa stage early in its development and another after the basin had nearly filled with sediment, but there is no direct evidence for either. At least the lower half of the sedimentary column (the part sampled by the scientific drilling) formed in an euxinic environment. This argues against a persistent early playa, although evaporative accumulation of brine was inevitable. When the rate of resurgance was rapid relative to sedimentary infilling, the lake would have been deep (i.e., bordered by bedrock rather than sedimentary fans). The geomorphic evolution of the Creede caldera and its watershed tracks a two-phase topographic history, the first the Oligocene through Miocene, and the second for Pliocene to the recent. In Oligocene time, the San Juan volcanic field was a hydrologically immature, gently undulating, and outward sloping, constructional volcanic plateau straddling the ancient Continental Divide. West of the Creede caldera, a dendritic drainage discharged northeastward into ancestral Cebolla Creek (a tributary of the ancestral Gunnison River) through an early stage of the Clear Creek graben in the vicinity of Spring Creek Pass. Miocene basalt choked, but did not reconstruct, the drainage. By the end of Miocene time a mature topography of moderate relief developed, exposing some of the higher ores in the Creede district to weathering. In the late Miocene-early Pliocene time the San Juan Mountains were uplifted and titled eastward; the ancestral Rio Grande was revitalized and cut deeply into the older terrain, excavating much of the accessible sediment from the moat of the Creede caldera and exposing successively lowe levels in the Creede district to oxidation. Simultaneously, the southeast end of the Clear Creek graben was reactivated and breached the southwest wall of the Creede caldera. The rejuvenated Rio Grande captured the formerly northeast-directed headwaters of ancestral Cebolla Creek, shifting more than 1000 km 2 from the Pacific-directed drainage to the Atlantic. The water budget for ancient Lake Creede was strictly limited by the early stages of the fist geomorphic cycle; the modern water budget is the product of the second cycle.

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USGS2000

Stable isotope evolution and paleolimnology of ancient Lake Creede

The lacustrine carbonate and travertine (tufa) deposits of ancient Lake Creede preserve a remarkable record of the isotopic evolution of the lake. That record indicates that the δ18O of the lake water, and by analogy its salinity, evolved through evaporation. Limited and less reliable data on hydrous minerals and fluid inclusions in early diagenetic carbonates indicate that the δD of the lake waters also evolved through evaporation. The isotope data place restrictions on models of the physical limnology of the lake and its evolution. The closed-basin Lake Creede formed shortly after collapse of the 26.9 Ma Creede caldera. Throughout most of its history it occupied the northern three quarters of the moat between the resurgent dome and wall of the caldera. The Creede Formation was deposited in the basin, dominantly as lacustrine sediments. Travertine mounds interfinger with Creede Formation sediments along the inner and outer margins of the lake basin. An estimated one-half of the original thickness of the Creede Formation has been lost mainly to erosion although scattered remnants of the upper portion remain on the caldera walls. Two diamond core holes (CCM-1 and CCM-2) sampled the uneroded portion of the Creede Formation as part of the U.S. Continental Drilling Program. Volcaniclastic material, including tuff units deposited directly into the lake and ash washed in from the watershed, compose the main lithologies of the Creede Formation. These volcaniclastic strata were produced by episodic ring-fracture volcanism. Lacustrine carbonates make up about 15% of the section sampled by drill core. They occur as 1 mm to 2 cm low-Mg calcite laminae alternating with siliciclastic laminae in scattered intervals throughout the preserved section. The carbonate laminae are accumulations of 5–20 µm crystallites (microsparites) and brine shrimp fecal pellets (peloids) composed mainly of microsparite particles. Low-Mg calcite also occurs as an early diagenetic replacement of gypsum or ikaite (CaCO3 ·6H2O) crystals grown displacively in the muds and silts near the water-sediment interface (rice grains). Other studies indicate that aragonite was the original CaCO3 precipitate forming the microsparite and peloidal laminae and that it converted to calcite during burial diagenesis. Samples from CCM-2 and nearby outcrop do not appear to have undergone significant isotope exchange during recrystallization. Samples from CCM-1 and nearby outcrop, however, appear to have undergone extensive oxygen isotope exchange with meteoric water-dominated fluids possibly during a local 17.6 Ma hydrothermal event. The δ18O-δ13C data set produced by microsampling of individual carbonate lamellae and rice grains is exceptional in several aspects and provides important clues concerning the evolution of limnologic structure of the lake and its chemical and isotopic composition. Travertine and ikaite pseudomorphs in travertine deposits extend the record an additional 330 m above the collar of CCM-2. The δ18O values on CCM-2 samples range from 10.4‰ to 37.3‰ and δ13C values range from –10.8‰ to 9.6‰. The data fall into two distinct groups, a covariant group and an invariant group. The covariant group shows a strong negative covariance and a large range of δ18O and δ13C values. The negative covariance is opposite that normally reported for lacustrine carbonates. The large range of δ18O and δ13C values requires that the carbonates precipitated from waters have a large range of temperature and carbon and oxygen isotopic composition. The invariant group has a narrow range of large δ18O values (35‰ ± 2‰) and a wide range of δ13C values (–10.8‰ to 9.6‰), indicating precipitation from waters with a narrow range of temperature and δ18O but a wide range in δ13C of aqueous carbon. The ranges of isotope values for microsparite and peloid samples are virtually identical; two-thirds are in the covariant group. By contrast, the values for almost all rice grain samples are in the invariant group. The range in δ18O for all samples reflects precipitation from waters having varying proportions of deep, cold evaporated lake water and shallow, warmer meteoric water. The range for δ13C reflects varying proportions of organic carbon and carbon of volcanic or atmospheric origin, probably dominantly volcanic, in the aqueous carbon. Changes in the detailed carbon-oxygen isotope systematics with stratigraphic position define three periods of isotopic evolution of Lake Creede. Period I is represented by the lowest ~200 m of Creede Formation core in CCM-2. Analyses of individual microsparite and peloidal carbonate laminae within single thin sections of samples from that interval are tightly grouped. The data set as a whole shows a negative covariance. Rice grains are not found in this interval. Period II is represented by the succeeding 120 m of core in CCM-2. In that interval, δ13C-δ18O values for individual microsparite and peloidal carbonate laminae within single thin sections show strong negative covariance, and the set of values for the entire interval also shows strong negative covariance. Rice grains occur near the top of the interval. Period III is represented by the upper 225 m of CCM-2 core. In this interval, rice grains are abundant and δ13C-δ18O values for microsparite and peloidal laminae as well as rice grains fall in the invariant group. During Period I the lake was well mixed and the oxygen isotopic composition of the lake in the productive zone was only slightly influenced by short-term (e.g., annual) variations in the water budget of the lake. In Period II the lake was stratified, possibly with annual overturn. The productive zone included the mixolimnion and the isotopic composition of the microsparites and peloids reflected mixtures of shallow surface (meteoric) water containing volcanic or atmospheric CO2 (epilimnion) and cold underlying waters, the oxygen isotopic compositions of which had evolved through evaporation and were dominated by CO2 produced by the oxidation of organic matter (hypolimnion). During Period III the lake remained stratified. The productive zone was in the hypolimnion, probably due to a thinning of the epilimnion resulting from an increase in the surface area of the lake or a decrease in input waters reflecting a climate change. An upsection increase in values of δ18O for the heaviest samples during Periods I and II indicates evaporative concentration of 18O and, by analogy, salinity in the hypolimnion. The δD-δ18O evolution of the lake is inferred on theoretical evaporation trends, comparison to Mono Lake, and measurement of the δD in fluid inclusions in a calcite pseudomorph after ikaite. The δD-δ18O composition of the lake water followed a curved path that eventually hooked over at a nearly constant δ18O value for the lake of 2‰ ± 2‰ Travertine (tufa) mounds formed along the inner and outer margins of the lake in a zone of mixing of warm, volcanic CO2-bearing, meteoric waters and lake water. Ikaite crystals formed on the mounds from unmixed saline lake water, probably below the thermocline. As the position of the thermocline deepened, likely following the spring runoff, the ikaite was replaced by calcite and the resulting “pearls” were covered with travertine deposited from mixed meteoric and lake waters. The upsection increase in δ18O values of the carbonates, the long period of invariance of large δ18OH2O values, the presence of brine shrimp fecal pellets, and the inferred hooked δD-δ18O path are consistent with evidence from other studies that Lake Creede obtained significant salinity rather early in its history and certainly by the time the lake became permanently stratified.

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