Origin and diagenesis of lacustrine sediments, upper Oligocene Creede Formation, southwestern Colorado

Citation
Db. Finkelstein et al., Origin and diagenesis of lacustrine sediments, upper Oligocene Creede Formation, southwestern Colorado, GEOL S AM B, 111(8), 1999, pp. 1175-1191
Citations number
83
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
GEOLOGICAL SOCIETY OF AMERICA BULLETIN
ISSN journal
0016-7606 → ACNP
Volume
111
Issue
8
Year of publication
1999
Pages
1175 - 1191
Database
ISI
SICI code
0016-7606(199908)111:8<1175:OADOLS>2.0.ZU;2-N
Abstract
Much controversy has arisen over both the depositional and diagenetic envir onments of the lacustrine sediments and the chemical evolution of waters wi thin those sediments that make up the upper Oligocene Creede Formation. Pre vious studies of the neighboring Creede Ag-Pb-Zn district proposed that sal ine and isotopically heavy fluids from the Creede Formation mere involved i n ore deposition. Examination of rhythmically laminated limestones from two Continental Scientific Drilling Program drill cores (CCM-1 and CCM-2) prov ided a unique opportunity to study carbonate sediments of a caldera lake. T he upper parts of CCM-2 (50-148 m depth) are characterized by laminations o f low-Mg calcite containing brine shrimp fecal pellets (with an aragonite p recursor), organic matter (including bacterial mat-like laminae), calcite p seudomorphs after gypsum, and cements of bladed low-Mg calcite and tabular clinoptilolite. The degree of recrystallization and crystal coarsening is g reater in the lower part of CCM-2 (172-309.4 m depth) and in CCM-1 (8.8-155 .2 m depth), These observations suggest that ancient Lake Creede was an evaporative, par tially meromictic, saline to hypersaline, near-neutral, Na-Mg-SO4-Cl or CI- SO4 lake. SO42- and H+ were supplied to the lake by the oxidation of volcan ic SO2(gas), which buffered the alkalinity of the lake, Brine shrimp in the epilimnion are consistent with salinities greater than seawater and with w arm surface temperatures. The lack of bioturbation in rhythmically laminate d carbonate-siliciclastic couplets indicates either meromictic conditions o r saline conditions or both. Bacterial mat-like laminae suggest that lake w aters mere either relatively shallow or clear. Displacively grown gypsum cr ystals indicate times when evaporative ground-water conditions dominated (e .g., during low lake levels), and their absence may indicate changes in wat er chemistry and lake level. Sulfate reduction during burial resulted in th e conversion of gypsum and iron oxides to calcite and pyrite. Maximum estim ates for the duration of the lake range from 132 323 to 76 340 yr and are b ased on rhythmites in CCM-2, Results from this study support the hypothesis of saline and isotopically heavy waters evolving within the Creede Formati on prior to ore deposition.