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
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.