Watershed reconstruction of a Paleocene-Eocene lake basin using Sr isotopes in carbonate rocks

Abstract

Provenance studies have used Sr isotopes ( 87 Sr/ 86 Sr) of silicate source rocks as a link to their eroded basinal equivalents. This technique, however, cannot identify the proportional inputs from different watersheds or defi ne more precisely sedimentation events due to tectonic or climatic change. Erosion of carbonate rocks dominates the Sr input within basin drainage and potentially can be used through 87 Sr/ 86 Sr ratios to reconstruct paleohydrology of the entire basin and trace watershed inputs and depositional patterns in continental basins. The Sr isotopic ratios from waters of the source area, allowing for the mixing of shallow groundwater and surface water along the transport path, are homogenized in the basinal carbonate sediments. Mineralogy and diagenesis of carbonate rocks generally do not affect the Sr isotopic signal in a near-surface system lacking external infl uence by volcanism, eolian dust, or deep geothermal waters. The 87 Sr/ 86 Sr ratios from the source area are directly comparable to those in the receiving continental basin.The Sr isotopic signal of the PaleoceneEocene Flagstaff Formation (central Utah), a carbonate lake deposit in a foreland basin, is compared to that of projected source waters draining its thrust front, the Sevier fold-thrust belt. Freshwater carbonates compose a large portion of the lowermost Ferron Mountain and uppermost Musinia Peak Members of the formation, whereas gypsum and carbonates predominate in the middle Cove Mountain Member. Previous research had attributed gypsum deposition to the deposition of the middle Cove Mountain Member to either climatic change or unroofing of diapirs of Jurassic gypsiferous carbonates. To examine more closely the infl uence of climate versus tectonics on Flagstaff sedimentation as well as the effi cacy of provenance studies using carbonates, we collected rock samples from the three members of the formation on the Wasatch Plateau of central Utah in addition to sampling stream water associated with Pennsylvanian-Permian and Jurassic carbonate terrains from the nearby thrust front.The 87 Sr/ 86 Sr ratios in carbonates and gyprock belonging to the Flagstaff Formation remained unchanged during deposition, the average Sr isotope composition of the Flagstaff rocks being identical to that of sampled waters draining the projected provenance area. There was little change in source rock weathering as the thrust front evolved. Deposition of gypsum occurred in the basinal lake only during the deposition of the middle Cove Mountain Member, despite its constant exposure in the drainage area, suggesting a changing balance of tectonic and climatic controls during lake sedimentation. The 87 Sr/ 86 Sr isotopic studies targeting carbonate rocks and their presumed source waters are a simple but accurate method for reconstructing the paleohydrology of lake basins.

The effect of drainage reorganization on paleoaltimetry studies: An example from the Paleogene Laramide foreland

“…Results presented here support the thesis that basin development and lake hydrology in the Laramide foreland were characterized by large-scale changes in Cordilleran drainage patterns tapping distal source areas (DeCelles, 1994). Though drainage from the hinterland seems to have been the dominant pattern for much of the Paleogene (Fouch et al, 1983;Gierlowski-Kordesch et al, 2008;Henry, 2008), tectonically mediated drainage rearrangements within the foreland in some cases profoundly influenced the developing Laramide basins and their O isotope records. Unrecognized, such drainage rearrangements might easily confound studies of isotope paleoaltimetry.…”

“…The isotopic signature of Sr in lacustrine carbonates has recently been recognized as a valuable method for reconstructing lake paleohydrology (Pietras et al, 2003;Gierlowski-Kordesch et al, 2008 Sr ratio of waters is dictated by contact with rocks in the drainage area, and especially carbonate rocks (Palmer and Edmond, 1992;Jacobsen and Blum, 2000). Heterogeneities in the Sr isotope ratios of lithologies present in the drainage basin are homogenized in lake water such that when carbonate precipitates, its Sr isotope composition reflects the weighted average of isotopically distinct inflows to the lake.…”

“…Lack of covariance of C and O isotopes (Fig. 3) (Blum et al, 1998;Jacobsen and Blum, 2000;Gierlowski-Kordesch et al, 2008) and (2) the fact that few Precambrian basement rocks, which might have imparted a more radiogenic signature, were ever exposed south of the Uinta Basin except for the Front Range, which lay more than 300 km to the southeast (Foster et al, 2006). We interpret the increase in 87 Sr/ 86 Sr ratios in carbonate samples from Lake Uinta at~53 Ma as the result of a large-scale reorganization of the lake's drainage system with new and substantial inflows from Lake Gosiute and the catchments of the Greater Green River Basin that eventually extended into the Challis Volcanic Field at~49 Ma (KentCorson et al, 2006;Carroll et al, 2008 Sr ratios of samples from the Laney Member of the Green River Formation (~50 Ma) in the Greater Green River Basin (~285 km from our sampled section over the Uinta Uplift) give a mean of 0.71245 ± 0.00014 (n = 22, p b 0.05) (Rhodes et al, 2002).…”

“…As a case study, we examine the O, C and Sr isotopic composition and Sr/Ca ratios of authigenic carbonates formed in Late Cretaceous to late Middle Eocene lakes in the Laramide foreland. Laramide segmentation of the foreland impounded large lakes (N20,000 km isotopes (Norris et al,1996;Dettman and Lohmann, 2000;Davis et al, in press) and Sr isotopes (Rhodes et al, 2002;Gierlowski-Kordesch et al, 2008) suggest that these lakes preserve a record of reorganizing drainage patterns attendant with rise of mountains. However, whether such reorganization occurs locally (Norris et al,1996;Dettman and Lohmann, 2000) or on a more regional scale (Davis et al, in press) is unknown.…”

A proposed Laramide proto-Grand Canyon

“…Strontium isotope values for the Pink and White Members of the Claron measured 0.710424, 0.711472, and 0.70923 (n = 3, mean = 0.71038, this study); while values for speleothems deposited from Redwall karst aquifer water measured 87 Sr/ 86 Sr = 0.712106-0.723185, n = 8, mean = 0.714598 (V. Polyak, University of New Mexico, personal communication, 2008). The strontium values for the Claron Formation may reflect an input from the Sevier thrust zone to the west, as modeled by Gierlowski-Kordesch et al (2008). However, also possible is a proto-Grand Canyon river that could have supplied higher strontium-ratio carbonate to the Claron Formation in its southeastern part (where we sampled).…”