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

Abstract

Using multiple isotope systems, we examine the complex effects of drainage reorganization in the Laramide Foreland in the context of stable isotope paleoaltimetry. Strontium, oxygen and carbon isotopic data from lacustrine carbonates formed in the southwestern Uinta Basin, Utah between the Late Cretaceous and late Middle Eocene reveal a two stage expansion in the drainage basin of Lake Uinta beginning at ~ 53 Ma culminating in the Mahogany highstand at 48.6 Ma. A marked increase in ⁸⁷Sr/⁸⁶Sr ratios of samples from the Main Body of the Green River Formation is interpreted as the result of water overflowing the Greater Green River Basin in Wyoming and entering Lake Uinta from the east via the Piceance Creek Basin of northwestern Colorado. This large new source of water caused a rapid expansion of Lake Uinta and was accompanied by a significant and rapid increase in the O isotope record of carbonate samples by ~ 6‰. The periodic overspilling of Lake Gosiute probably became continuous at ~ 49 Ma, when the lake captured low-δ¹⁸O water from the Challis and Absaroka Volcanic Fields to the north. However, evaporation in the Greater Green River and Piceance Creek Basins meant that the waters entering Lake Uinta were still enriched in ¹⁸O. By ~ 46 Ma, inflows from the Greater Green River Basin ceased, resulting in a lowstand of Lake Uinta and the deposition of bedded evaporites in the Saline Facies of the Green River Formation.

We thus show that basin development and lake hydrology in the Laramide foreland were characterized by large-scale changes in Cordilleran drainage patterns, capable of confounding paleoaltimetry studies premised on too few isotopic systems, samples or localities. In the case of the North American Cordillera of the Paleogene, we further demonstrate the likelihood that (1) topographic evolution of distal source areas strongly influenced the isotopic records of intraforeland basins and (2) a pattern of drainage integration between the hinterland and foreland may correlate in space and time with the southward sweep of hinterland magmatism.

Introduction

During the past decade, numerous stable isotopic studies have reconstructed the paleoaltimetry of mountain belts worldwide (Chamberlain and Poage, 2000, Garzione et al., 2000, Rowley et al., 2001, Poage and Chamberlain, 2001, Kohn et al., 2002, Takeuchi and Larson, 2005, Graham et al., 2005, Kent-Corson et al., 2006). These studies use the O isotope composition of authigenic minerals as a proxy for past altitudes, and often assign isotopic shifts of these minerals over time to the growth of local topography. There are, however, other factors such as evaporation, temperature, and diagenesis that can influence the O isotope composition of authigenic minerals. These are generally taken into account in paleoaltimetry studies. Not often considered in these studies is the regional drainage reorganization that occurs during mountain building events. Large-scale drainage reorganization and stream piracy can strongly influence the O isotope composition of water in basins. Such changes may confound paleoaltimetry estimates because: 1) drainage reorganization can occur on time-scales of 10⁵ yr (Hilley and Strecker, 2005), whereas tectonism generally occurs on time-scales of 10⁶ yr (although removal of the lower lithosphere can be faster Garzione et al., 2006); and 2) the expanded drainage basin can tap waters with different O isotope values either as a result of draining areas with different atmospheric source regions or waters that have undergone evaporation.

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 (> 20,000 km²) whose sedimentological history suggests that their hydrology coevolved with accommodation space over millions of years (Pietras et al., 2003, Surdam and Stanley, 1980, Carroll et al., 2006, Smith et al., 2008). Studies using O 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.

Our combined isotopic study shows that drainage reorganization has occurred in response to developing topography, both locally and as much as 1000 km away. Specifically, we show that large O isotopic shifts (6–7‰) are primarily the result of changing hydrologic regime of Lake Uinta. By combining data from O and C isotopes and Sr/Ca ratios with Sr isotope ratios, we are able to evaluate the role of drainage reorganization and suggest that lake hydrology was responding to both local and distal tectonic forcing.