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LIMNOGEOLOGY |
,1
es4
1 Geological Sciences, Ohio University, 316 Clippinger Laboratories, Athens, Ohio 45701-2979, USA
2 Department of Earth and Planetary Sciences, Northwestern University, Evanston, Illinois 60208-2150, USA
3 Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1063, USA
4 Instituto Pirenaico de Ecologia, Consejo Superior de Investigaciones Cientificas, Apdo 202, E-50080, Spain
ABSTRACT
Provenance studies have used Sr isotopes (87Sr/86Sr) of silicate source rocks as a link to their eroded basinal equivalents. This technique, however, cannot identify the proportional inputs from different watersheds or define 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 87Sr/86Sr 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 influence by volcanism, eolian dust, or deep geothermal waters. The 87Sr/86Sr ratios from the source area are directly comparable to those in the receiving continental basin.
The Sr isotopic signal of the Paleocene–Eocene 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 influence of climate versus tectonics on Flagstaff sedimentation as well as the efficacy 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 87Sr/86Sr 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 87Sr/86Sr isotopic studies targeting carbonate rocks and their presumed source waters are a simple but accurate method for reconstructing the paleohydrology of lake basins.
Key Words: lacustrine • Flagstaff Formation • nonmarine carbonates • paleohydrology • Sevier Belt • Utah
Strontium isotope ratios (87Sr/86Sr) can been used as geochemical tracers of many continental processes, including soil genesis and biochemical cycling (e.g., Herut et al., 1993; Capo et al., 1998), surface-water and groundwater movement (e.g., Neumann and Dreiss, 1995; Johnson and DePaolo, 1997; Jacobson and Wasserburg, 2005), weathering rates (Blum and Erel, 2003, and references therein; Jacobson et al., 2003), paleohydrology of lakes (Benson and Peterman, 1995; Talbot et al., 2000; Rhodes et al., 2002), and determination of marine versus freshwater deposition (e.g., Holmden et al., 1997; Spencer and Patchett, 1997, Becker et al., 2007), in addition to their use in marine stratigraphy (e.g., Veizer, 1989, Capo and DePaolo, 1990). The use of Sr isotopes in provenance studies of sedimentary basins commonly only involves siliciclastic sediments (e.g., Lawton et al., 2003; Weltje and von Eynatten, 2004; Link et al., 2005). Carbonate rocks in source areas can also contribute eroded clasts and dissolved ions into a continental basin through surface erosion (Gierlowski-Kordesch, 1998). Freshwater limestones have great potential for reconstructing the tectonic history of orogenies (Albarède and Michard, 1987; Becker et al., 2007). The potential of Sr isotopic ratios in carbonate rocks to determine continental provenance has not yet been fully explored.
The importance of carbonate weathering to dissolved Sr isotopic ratios in catchments has been investigated in a wide range of settings (Blum et al., 1998; Quade et al., 1997; Galy et al., 1999; English et al., 2000; Jacobson and Blum, 2000; Jacobson et al., 2002a, 2002b; Dalai et al., 2003). Carbonate weathering dominates the Sr isotopic signal of most modern rivers, regardless of the surface-area coverage of carbonate rocks or the proportions of disseminated calcite in a drainage area. For example, in the Raikhot watershed of the Himalayas, carbonate represents only 
1.0 wt% of the bedrock, but its Sr isotopic signature dominates the chemistry of the draining streams (Blum et al., 1998; Jacobson and Blum, 2000). Therefore, Sr isotopic signatures of waters draining carbonates and their surrounding siliciclastics in a source area can be connected with those of basinal sediments in a continental basin. Faure and Barrett (1973) aided in the reconstruction of ancient watersheds by demonstrating that bedrock geology in source areas could be linked to erosional sources through Sr isotope measurements.
Three key properties of strontium isotope chemistry make the system a useful geochemical tracer of erosion in continental systems. (1) Mass-dependent fractionation of 87Sr/86Sr is both negligible and corrected by normalization to 86Sr/88Sr during analysis. (2) Surface and groundwaters derive their Sr isotopic signature directly from the rocks along the transport path. (3) Heterogeneous Sr isotope values of bedrock in basin source areas tend to become homogenized in continental basinal sediments (e.g., Jones and Faure, 1978; Palmer and Edmond, 1992; Neumann and Dreiss, 1995).
The Sr concentration and isotopic composition of stream waters are controlled by mixing of Sr derived from carbonate as well from silicate rocks in the catchment (Palmer and Edmond, 1992). Groundwaters inherit the isotopic ratio of dissolving carbonate and siliciclastic minerals in the subsurface (Banner et al., 1994; Barnaby et al., 2004). Dissolution and reprecipitation of carbonate does not affect 87Sr/86Sr values, and coprecipitated dolomite and calcite yield identical 87Sr/86Sr, and therefore do not need to be separated for analysis. The 87Sr/86Sr ratios of carbonate lake deposits preserve the Sr isotopic composition derived from surface runoff and groundwater input into the lake (Rhodes et al., 2002). Sampling of stream waters in the presumed source area catchment in association with the main source carbonates should represent the water chemistry of the ancient source area drainage. Only the input of volcanic ash, discharge of deeper groundwaters flowing through rocks not found in the catchment or shallow subsurface, or a large input of eolian carbonate could affect a change in the basinal lake Sr isotopic values (Palmer and Edmond, 1992; Barnaby et al., 2004; Faure and Mensing, 2005). In this study, we use Sr isotopes for the first time to correlate between the projected source area and basinal nonmarine carbonate rocks of the Paleocene–Eocene Flagstaff Formation in central Utah.
The Flagstaff Formation is exposed in the Gunnison and Wasatch Plateaus and surrounding areas of central Utah, east of the Sevier fold-thrust belt and west of the San Rafael swell and Circle Cliffs uplift (Fig. 1). The Flagstaff Formation is a carbonate lake deposit surrounded by fluvial deposits of the North Horn and Colton Formations (Stokes, 1986, Hintze, 1988; Lawton et al., 1993) associated with a Cretaceous to Paleocene–Eocene foreland basin system (Talling et al., 1994, 1995; DeCelles, 1994; DeCelles and Coogan, 2006). The Sevier fold-thrust belt to the west was the source area, and Laramide basement uplift to the east is theorized to have altered drainage (Fig. 2), allowing the Lake Flagstaff basin to evolve from late Paleocene to middle Eocene time (Stanley and Collinson, 1979; Wells, 1983).
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Spieker (1949) proposed that the middle gypsiferous member of the Flagstaff Formation was the result of weathering of newly exposed diapirs of Jurassic gypsiferous limestones (Carmel Formation–Arapien Shale) in the Sevier fold-thrust belt (for details on the formation see Lawton, 1985; Lawton et al., 1993; DeCelles and Coogan, 2006) and deposition in a greatly expanded lake, in a proposed larger area covered by middle Flagstaff sediments. La Rocque (1960) and Weber (1964a, 1964b) promoted this interpretation with the view that a larger gypsiferous lake could not be the result of climatic effects, because arid phases would tend to decrease the size of the lake. Weber (1964a) attempted to illustrate this tectonic origin with a carbon-oxygen isotopic study of the rocks and fossils of the Flagstaff, showing no evaporative signatures through 18O enrichment, although rocks were analyzed in bulk in centimeter-sized cubes, not as facies-specific samples. Contrary evidence by Gill (1950) suggested that the lake did not expand, but instead contracted. Wells (1977, 1983) and McCullough (1977), through sedimentologic analysis of the carbonates and chert and stratigraphic evidence, determined that the Cove Mountain Member was a playa lake deposited in a contracted lake during an arid phase. This supports a climatic origin for the gypsum and dolomite accumulation. However, dolomite is still an important carbonate phase in the fresher Musinia Peak Member, which overlies the Cove Mountain Member (Wells, 1977, 1983).
Distinguishing between the effects of changing climate versus tectonics is a common problem in lake basin analysis. Bohacs et al. (2000, 2003) outlined the main variables controlling lake size and character, i.e., basin-floor depth, sill height, water supply, and sediment supply, all of which are dependent on both climate (rates of supply of sediment and water) and tectonics (potential accommodation rate and sediment supply). Lake size and stratigraphic extent are controlled by climate and tectonics in a nonlinear way. According to the criteria of Bohacs et al. (2000, 2003), Lake Flagstaff sedimentation may represent balanced-fill to underfilled to balanced-fill lake types in its three members, oscillating between open and closed drainage. Questions about the history of the lake remain, i.e., why the basin alternated between closed and open drainage, continuously accumulating a thick sequence of carbonates, and whether climatic change to more arid conditions promoted evaporation and thus precipitation of gypsum in the middle Cove Mountain Member of the Flagstaff Formation. The answer may be within the carbonate deposits.
Nonmarine carbonate deposition has been attributed to more arid to seasonal conditions (i.e., Cecil, 1990, 2003), but carbonates can accumulate in all climate zones in continental settings (Platt and Wright, 1992). Thick, basin-wide, nonmarine carbonate accumulations must be connected to a carbonate-rich source area (Kelts and Talbot, 1990; Gierlowski-Kordesch, 1998), not solely attributed to reducing clastic input or isolating an entire basin floor (Franczyk et al., 1991; Zaleha, 2006). Spring deposits cannot produce basin-wide carbonate accumulation (e.g., Smoot, 1978), and carbonate cannot precipitate on siliciclastic floodplains (e.g., Bowen and Block, 2002; Dunagan and Turner, 2004) without a sedimentologic mechanism for excluding siliciclastics (see Gierlowski-Kordesch, 1998). Sediment accumulation adjacent to mountain thrust fronts normally results from erosion by surface water (Fraser and DeCelles, 1992), the clast mineralogy of deposits providing provenance evidence. Paleozoic and Jurassic carbonates (Figs. 1 and 2) were present in the watershed draining into Lake Flagstaff (Stanley and Collinson, 1979; Talling et al., 1994; DeCelles et al., 1995). Overland transport of carbonates is evidenced by the presence of carbonate clasts in the interbedded units of the associated alluvial North Horn Formation along the fold-thrust belt (see Talling et al., 1995, for details).
In an attempt to understand the origin of Lake Flagstaff carbonate sediments, Neat et al. (1979) carried out a 87Sr/86Sr study of the carbonate rocks from the Flagstaff exposures in Fairview Canyon (see Fig. 1—sample locality FC). Members of the formation represented in this area are not known, and the mineral composition of the carbonates ranged from pure dolomite to pure calcite. The range of 87Sr/86Sr values from 0.70890 to 0.71260 was attributed to modulation from periodic input of volcanogenic detritus of felsic composition. Although Neat et al. (1979) stated that the Sr isotopic composition is "determined primarily by the geology of the basin," (p. 271) no analysis of the source was performed.
The depositional setting of the Flagstaff Formation within a foreland basin at the foot of a fold-thrust belt limits the possible source areas, with the main drainage into the proximal area limited to the thrust front (Talling et al., 1994, 1995; DeCelles and Mitra, 1995; Horton and DeCelles, 2001). Streams draining a thrust belt normally retain their location over long periods, even as the thrust front advances (Fig. 3). Drainage networks commonly expand headward into the highlands watershed, capturing streams across the thrust front and developing a mostly planform morphology. In a mature landscape, only a few streams discharge sediment eroded from the entire belt into the foreland basin, and their input can eventually become minimally affected by geometric alterations within the growing fold-thrust belt. This drainage stability over long periods of time aids in reconstructing the source area from the remaining original source rocks.
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In the source area, water samples of streams were collected for Ca, Rb, and Sr concentration and Sr isotopic analyses (n = 14). Water samples were collected from streams that drain modern watersheds (see Fig. 1) containing marine carbonate source rocks and associated siliciclastics (exact sample locations in Table A1). These streams run through two separate areas containing Paleozoic and Mesozoic marine carbonate rocks. Sampling within the fold-thrust belt was done in areas containing the targeted source carbonates of marine origin and geographically nearest to the Flagstaff basinal samples (Figs. 1 and 2). We made the assumption that all areas targeted have a consistent 87Sr/86Sr value along the entire thrust front, inherited from the marine carbonate rocks and their associated siliciclastics at the time of Flagstaff deposition. The exact areal extent of the watershed that fed the foredeep area of the Flagstaff basin area is not precisely known, so the sample areas are projected to be the closest in lithologic composition and location. The foredeep is defined as the area between the structural front of the fold-thrust belt and a high or forebulge area farther from the structural front where sediments accumulate in a depositional low (DeCelles and Giles, 1996).
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50–100 mg) samples (n = 38) were removed using a microdrill to target different microfacies.
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The 87Sr/86Sr ratios of all the samples (n = 38) from the basinal Flagstaff Formation (Table A2), regardless of member, mineralogy, or texture (Fig. 4), represent one normally distributed population (mean = 0.709995 ± 0.000262) at the 95% confidence level (Fig. 5) in both parametric and nonparametric statistical tests. The nonparametric Kruskal-Wallis Test has the hypothesis that the median of Sr isotopic ratios of rock samples from the three members of the Flagstaff Formation (n = 38) shows that all samples are drawn from one population (H = 0.854 and 
2crit at the 95% confidence level is 5.99).
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The problem of matching Sr isotopic signals from continental basinal sediments to their provenance requires the identification of possible source areas. It is not practical to directly sample all of the possible rock units in a source area, and it is difficult to estimate the relative contributions of the various units to dissolved runoff. Collection of water samples within representative watersheds draining probable carbonate source rock units and their associated siliciclastics may be the best way to measure Sr output of a proposed ancient watershed. The Sr isotopic ratios of the waters draining the projected source area within the Sevier fold-thrust belt can be directly matched to the basinal 87Sr/86Sr ratios of the Flagstaff Formation.
During thrusting and folding of the Sevier belt (Talling et al., 1994, 1995), the most likely carbonate rocks to be exposed (as shown in Figs. 1 and 2) would have been the Pennsylvanian–Permian rocks at the top of the Paleozoic carbonate sequence and the gypsiferous Jurassic Carmel Formation–Arapien Shale, all extensively eroded along the thrust front directly west of the sampled foredeep zone (Hintze, 1988). Stream waters running over and through these formations may be the modern equivalent of the ancient watershed Sr input from the source rock carbonates into Lake Flagstaff.
A mixing calculation for the water samples associated with the streams draining these two areas indicates that a mixture of 67% Sr from Pennsylvanian–Permian spring waters (Oquirrh Formation spring water; weighted average of W9, W10 = 0.711001) and 33% Sr from the Jurassic bedrock area (Chicken Creek, W15 = 0.707490, Carmel Formation–Arapien Shale) produces the average Flagstaff 87Sr/86Sr ratio of 0.709995. A range of 24%–41% Sr from Jurassic bedrock waters and 59%–76% Sr from Pennsylvanian–Permian bedrock waters can explain Sr isotopic ratios across two standard deviations (±0.000262) from the given Flagstaff mean value. Mixing of waters from these two main source areas can readily explain the Sr isotopic composition of Lake Flagstaff. The Sr isotopic ratios from the lower Paleozoic carbonate watershed (Table A1) (W1–W8, W11–W14) are lower than the Flagstaff Formation average, but could have contributed Sr to the Flagstaff Formation.
The Sr isotope ratios through the three members of the Flagstaff Formation (Figs. 4, 5, and 6) in the sampled area of the foredeep zone of the Sevier foreland basin (Wasatch Plateau), including the gypsiferous middle member, show that the isotopic input of Sr was consistent throughout deposition of the Flagstaff Formation. This supports the conclusion of Horton and DeCelles (2001) that drainage areas along fold-thrust belts can be stable over long periods of time and drain the entire tectonic front. Carbonates as well as gyprocks produced similar Sr isotopic ratios in the Flagstaff samples (Fig. 4; Table A2).
A plot of 87Sr/86Sr versus Ca/Sr ratios of the Flagstaff samples (identified by member designation) versus Ca/Sr (Fig. 6) illustrates the independence of these isotopic ratios with respect to Sr concentrations as well as mineralogy, whether carbonate (dolomite or calcite) or gypsum. No patterns of Sr isotopic variation among stratigraphic levels within Flagstaff member samples can be detected, illustrating isotope homogeneity in the receiving sediments. It is clear that no outside source contributed large amounts of Sr of a different isotopic composition into the fold-thrust belt drainage during deposition of the Flagstaff Formation. These Sr isotopic ratios appear to represent homogenization from a well-defined and stable watershed draining a fold-thrust front.
Variation of 87Sr/86Sr ratios observed by Neat et al. (1979) in the Flagstaff Formation exposed in Fairview Canyon north of the study area (see Fig. 1—sample locality FC) may not require periodic volcanic input of felsic composition, but may instead be explained by drainage from other catchments associated with the northern terminus of the Flagstaff basinal area. For example, axial drainage within the Sevier foreland basin was recognized in Late Cretaceous age fluvial sandstones in a petrographic provenance study along the Sevier fold-thrust belt in southern Utah (Lawton et al., 2003). Recognition of other drainage systems within the Flagstaff lake basin could be further clarified by analysis of Flagstaff samples in other portions of the basin, including areas east and south of the study area.
Clues about the tectonic history of the lacustrine phase within the Sevier foreland basin, including mechanisms forming closed or open drainage, can be assessed. The consistent Sr isotopic ratio from the Flagstaff sample data in the foredeep zone shows that erosion of marine carbonates, gyprocks, and their associated siliciclastics from the source areas did not change appreciably during the life of Lake Flagstaff in the study area. Input of dissolved sulfate derived from gypsum deposits in the Mesozoic carbonates, therefore, occurred throughout Flagstaff time (Paleocene–Eocene), but gypsum only accumulated in the sedimentary foreland basin during the time of deposition of the Cove Mountain Member (La Rocque, 1960; Stanley and Collinson, 1979; Wells, 1983). This is contrary to the earlier interpretation of Spieker (1949), La Rocque (1960), and Weber (1964a, 1964b), that unroofing of Mesozoic (Jurassic) carbonates allowed the deposition of gypsum within the middle member of the Flagstaff Formation. According to DeCelles and Coogan (2006), in their detailed study of the Sevier thrust front, the Jurassic carbonates were exposed by the Late Cretaceous.
The precipitation of evaporites in a lake basin requires closed drainage and a low precipitation/evaporation ratio. This can happen in any climate, because salt lakes occur at all latitudes (Carroll and Bohacs, 1999; Bohacs et al., 2003). For example, evaporite deposition in the Wilkins Peak Member of the Green River Formation (Eocene of Wyoming) has been linked to differential subsidence, inferring that basin accommodation and drainage patterns are primary controls for the accumulation of evaporites (Pietras and Carroll, 2006). Closed drainage hydrology resulting from changes in the eastern foreland basin margin linked to the Laramide uplift of the San Rafael swell (Talling et al., 1994, 1995) could have allowed gypsum accumulation during Cove Mountain Member deposition. The climatic change to warmer, more humid conditions at the Paleocene–Eocene thermal maximum in the U.S. mid-continent (Norris and Röhl, 1999; Bowen et al., 2004; Clechenko et al., 2007) may not have influenced local climate directly in the Flagstaff lake basin. Tectonic uplift can also affect regional climatic patterns through rain-shadow effects, as noted for Eocene climate change in the southwestern United States (Norris et al., 2000), producing local dry conditions for gypsum accumulation.
The 87Sr/86Sr ratios of streams draining areas containing source carbonates and their associated silicate rocks in a continental basin can be used as geochemical tracers to reconstruct the path of water and material transport into the basin center, in essence reconstructing basin paleohydrology. If there are no other inputs by volcanism, eolian dust, or deep geothermal waters, the Sr isotopic ratios from the source area can be directly transferred to the receiving basin where the signal is homogenized. Mineralogy and diagenesis generally do not affect the Sr isotopic signal in a near-surface system because shallow groundwater and surface water within the watershed derive their isotopic signal from the same rocks. Because all carbonate and gypsum textures at all stratigraphic levels in the Flagstaff Formation produced similar Sr isotopic values, interpretation by this method is simple.
The Sr isotope ratios of the Paleocene–Eocene Flagstaff Formation in the sampled area of the foredeep zone (Wasatch Plateau) of the Sevier foreland basin remained largely unchanged throughout Flagstaff Formation time (Paleocene–Eocene); thus, the watershed feeding into this area of the foredeep drained similar rocks throughout that time. Measurement of 87Sr/86Sr values in different areas of the Flagstaff Formation across the Sevier fold-thrust belt in Utah might help define different watersheds feeding sediments into the basin, allowing a more complete reconstruction of basin evolution, including possible lake subbasins. The deposition of gypsum in the middle member of the Flagstaff Formation was probably due to hydrologic closure of the basin because of a change in subsidence in the basin in relation to the Laramide uplift to the east, coupled with regional or local climate change.
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APPENDIX
ACKNOWLEDGMENTS
We thank A. Klaue, K.C. Lohman, and L. Wingate for laboratory assistance at the University of Michigan and S. Judge for library research assistance at Ohio State University. This manuscript was greatly improved by comments from our reviewers, Michael Talbot and David Finkelstein. We thank Dina L. López and E. Troy Rasbury for clarifying geochemical concepts to Gierlowski-Kordesch. Funding was provided to Gierlowski-Kordesch by the National Science Foundation (POWRE-0074647).
FOOTNOTES
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RECEIVED FOR PUBLICATION 6 June 2006
REVISED MANUSCRIPT RECEIVED 13 April 2007
MANUSCRIPT ACCEPTED 16 April 2007
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). For details on statistical tests, see text.
