Climatic Control of Fluvial-Lacustrine Cyclicity in the Cretaceous Cordilleran Foreland Basin, Western United States

Home , Cyclic sediments, Rupelo Formation

Sedimentology (1996) 43, 677-689

Climatic control of fluvial-lacustrine cyclicity in the Cretaceous Cordilleran Foreland Basin, western United States

CARL N. DRUMMOND," BRUCE H. WILKINSONt and KYGER C. LOHMANNt *Departmentof Geosciences, Indiana University Purdue University, Fort Wayne, Fort Wayne, IN 46805-1 4 99, USA (drummond@sm tplink.in diana.ipfw. ed u) +Department of Geological Sciences, The University of Michigan, Ann Arbor, MI 48109-1063, USA

ABSTRACT Tectono-stratigraphic models of foredeep sedimentation have generally presumed a direct link between changing rates of tectonism and concomitant sedimentological response as manifested by change in thickness, composition or depositional environment of sediment accumulating in adjacent basins. Lacustrine limestone units within the early Cretaceous fluviaMacustrine Gannett Group of western Wyoming exhibit systematic variation in several geochemical proxies of relative rates of precipitation and evaporation, indicating that lakewater chemistry was controlled by variation in regional climate. Change in proportion of allochthonous terrigenous clastic vs. autochthonous carbonate deposition, as well as carbonate Mg/Ca ratio and stable isotopic composition, occurs at two scales. Metre-scale alternation of micritic limestone and argillaceous marl is accompanied by mineralogical and isotopic variation within individual beds, indicating preferential carbonate accumulation during intervals of decreased regional meteoric precipitation relative to lake-surface evaporation. Limestone deposition began during intervals of maximum aridity, and decreased as increased meteoric precipitation-driven flux of terrigenous clastic sediment overwhelmed sites of carbonate accumulation. Similar upsection variation in limestone mineralogy and isotopic composition at a scale of tens of metres reflects the multiple processes of long-term increase in meteoric precipitation and lakewater freshening prior to influx of terrigenous sediment, across-basin fluvial-deltaic progradation, and renewed accumulation of riverine terrigenous units. Such trends suggest that formation-scale alternation between fluvial clastic and lacustrine carbonate deposition was controlled by climate change.

INTRODUCTION Armstrong, 1968; Wiltschko & Dorr, 1983). As such, deposits of gravel wedges within late The stratigraphic architecture of foreland basin Mesozoic and Cenozoic synorogenic sequences of sequences has long been interpreted within a the Cordilleran foreland basin in Wyoming and framework of sedimentary response to changing Idaho have traditionally been attributed to source rate and magnitude of tectonism. In conventional region uplift via episodic motion along thrusts tectono-stratigraphic models, deposition of coarse within the Sevier orogenic belt. The presence of clastic wedges correlates with increased rates of such coarse conglomeratic fluvial sediment is basin subsidence and/or uplift of adjacent source considered primary evidence for constraining regions, while deposition of fine-grained lacus- estimates of the timing and duration of motion trine or marine sediment is assumed to accom- along those thrusts (Wiltschko & Dorr, 1983). pany decreased rates of basin subsidence and An alternative tectono-stratigraphic model with tectonic quiescence (Armstrong & Oriel, 1965; presumed direct but inverse association between 0 1996 International Association of Sedimentologists 677 678 C. N. Drummond et al.

Southeastern Westcentral sediment supply and/or subsidence during initial Idaho Wyoming phases of Sevier deformation. Inferred linkage between styles of sedimentation and intensity of liil Bear River Formation tectonism is in general agreement with docu- a mented regional stratigraphic relations within the Gannett Group at a scale of individual fluvial and Smoot Formation lacustrine formations. However, such an explanation presupposes that rate of sediment supply was solely controlled by tectonic uplift in the chler Formation adjacent orogen, and that other factors related to denudation, such as climatically controlled variation in fluvial discharge, were less significant in altering sediment supply. An alternative scenario for the development of foreland basin depositional cyclicity holds that orogenic uplift and/or basin subsidence may Stump Formation play a minor role in development of fluvial-to- lacustrine depositional systems, and presumes that climate change, reflected as changing fluvial m 0 0 discharge and terrigenous clastic sediment deliv- Limestone Sandstone Shale ery to basins of deposition, is the dominant factor Fig. 1. Stratigraphic column of units exposed in south- controlling lateral and temporal lithological eastern Idaho and west-central Wyoming. The Lower variation. A similar explanation of Cordilleran Cretaceous Gannett Group is composed of two lacus- foreland basin depositional cyclicity has been trine and three fluvial formations, the lowest of which (the Peterson Limestone) represents initial deposition proposed for younger marine units (Pratt, 1984; of continental sediment into the newly formed Cordill- Arthur et ~l.,1985; Barron et al., 1985; Arthur & eran foreland basin. The Gannett Group lies uncon- Dean, 1991; Kirkland, 1991; Sethi & Leithold, formably above Upper Jurassic marine sandstones of 1994). Here, rhythmic bedded limestone-shale the Stump Formation and conformably below Creta- couplets within the Turonian Greenhorn cycle ceous marine sandstones and shales of the Bear River have been interpreted to record episodic climate Formation. Modified from Glass & Wilkinson (1980), change during accumulation of shale-rich layers. Brown & Wilkinson (1981) and Wanless et al. (1955). Regardless of the importance of tectonism and/or climate change in the generation of litho- clast size and fault motion has also been proposed logical cyclicity, fluvial to lacustrine transitions by Beck & Vondra (1985), Blair & Bilodeau (1988) in foreland basin sequences must largely reflect and Heller et ~l.(1988). Within this scenario, the interplay between changes in rates of subsid- rapid subsidence during thrusting confines coarse ence and change in rate of terrigenous sediment sediment deposition to regions adjacent to the supply. Because elemental and isotopic compo- thrust (and associated development of lacustrine sitions of lake systems are sensitive to variation in or marine systems along the basin axis), while regional meteoric precipitation relative to evap- intervals of tectonic quiescence allow for the oration in lake basin drainages, the lithological, progradation of coarse clastic material across the mineralogical and geochemical composition of basin axis. Additionally, Flemings & Jordan authigenic carbonate within lacustrine units of (1990) have shown that both progradational and the Gannett Group potentially contain an excel- retrogradational stratigraphic frameworks can be lent record of climate change in western North associated in time and space with episodic thrust- America during the early Cretaceous. ing, and that specific characteristics within a Such an interpretation of episodic climate particular basin vary as a function of patterns of change controlling the secular distribution of tectonic load, flexure and sediment supply. lacustrine deposition is certainly not new. Olsen Regardless of specific relations between progra- (1986) used facies-dependent ranking of inferred dational portions of foreland basin sequences and lakewater depths to calculate periodicities of timing of thrusting, deposition of formation-scale sediment accumulation (that were generally fluvial-lacustrine couplets within the lower equivalent to orbital forcing functions) within Cretaceous Gannett Group of the western USA early Mesozoic rift sediment of eastern North (Fig. 1) undoubtedly record alternation of rates of America, and subsequent studies have more

0 1996 International Association of Sedimentologists, Sedimentology, 43, 677-689 Climatic control of fluvial-lacustrine cyclicity 679 rigorously described Milankovitch-hand fre- indicate carbonate accumulation in lake systems quencies of lake depth and area within these of considerable size (15 000 and 20 000 km’, sequences (Kominz & Bond, 1990; Kominz et al., respectively; Glass & Wilkinson, 1980; Brown & 1991). Analogous investigations have docu- Wilkinson, 1981). As such, any change in lake- mented the presence of thousand-year fre- water composition inferred from upsection vari- quencies and presumably climate-driven ation in carbonate mineralogy or composition oscillations within Cretaceous marine sediment could reflect climate change at a regional scale. as well (Fischer & Schwarzacher, 1984; Ditchfield & Marshall, 1989; Park & Ogleshy, 1991). Depositional setting If fluvial-lacustrine cyclicity within the Gannett Group does reflect secular variation in In order to document regional patterns of carbon- regional climate, such change should also have ate accumulation, 13 stratigraphic sections were influenced the flux of meteoric water via precipi- measured throughout the Wyoming-Idaho Over- tation and inflow, as well as water flux from the thrust belt (Fig. 2). Because post-depositional basin via evaporation and outflow. In that minera- uplift and erosion have removed the Peterson logical, trace elemental and isotopic variation in Limestone and coeval units along the western lacustrine sediment has been recognized as a margin of the lake basin, measured outcrops potentially useful record of regional climate record basin centre to eastern margin lake (Eugster & Hardie, 1978; Talbot, 1990; Talhot & deposition. Kelts, 1991), compositional trends within the Stratigraphic sections parallel and perpendicu- Peterson Limestone (see below) may record the lar to the basin axis record spatially and tem- importance of Cretaceous climatic variation in porally discontinuous episodes of carbonate south-eastern Idaho and west central Wyoming, accumulation. In a context of spatial variation as well as the degree to which resultant variation down the basin axis, relatively pure limestone is in fluvial discharge influenced formation-scale the principal component of stratigraphic sections lithological cyclicity within the Gannett Group. north of the lake centre, while grey argillaceous marl and shale predominate to the south. This GENERAL SETTING pattern is taken as evidence for a generally southern fluvial source of terrigenous sediment, The Peterson Limestone is the lowest lacustrine an interpretation in agreement with palaeocurrent formation within the Gannett Group, which con- studies of coeval fluvial sediment that indicate sists of five terrestrial (three fluvial and two lacus- south-to-north axial drainage within the foreland trine) formations bracketed between late Jurassic basin (Eishacher et al., 1974; Hopkins, 1985). marine units and latest early Cretaceous deltaic Sedimentary characteristics commensurate with marine sandstone and shale (Fig. 1). palustrine deposition (e.g. Platt, 1989) only occur in the lower several metres of stratigraphic sections near transitions from underlying fluvial Stratigraphy lithofacies. The Gannett Group represents the first continental sediment shed into the Cordilleran foreland Petrology and palaeontology basin formed by thrust loading during the Sevier orogeny. In ascending order, the five formations of The Peterson Limestone consists of metre-scale the Gannett Group are the basal fluvial Ephriam interbeds of light grey to white micrite and grey Conglomerate, the lacustrine Peterson Limestone, to red calcareous shale. Limestones generally the fluvial-floodplain Bechler Formation, the consist of homogeneous mudstone with rare lacustrine Draney Limestone and the Smoot For- soft-sediment deformation structures, indicating mation, a poorly exposed alluvial siltstone and occasional slumping and transport of semicon- shale (Eyer, 1964). Outcrops of the Gannett Group solidated sediment (Glass, 1980). Limestones are are similar to a sequence of foreland basin sedi- not varved, and generally lack any distinguish- ments that occurs to the north as the Kootenai able sedimentary structures. Mineralogically, the Formation of western Montana (Stokes, 1952; micrites consist of generally subequal percentages Wanless et al., 1955; Peck, 1957; Peck & Craig, of anhedral calcite and uniformly dispersed 1962; Suttner, 1969; Holm et al., 1977). euhedral microdolomite (5-15 pm). Although the Outcrops of the Draney and Peterson Lime- amount of each component varies up section and stones occur throughout the Overthrust Belt, and throughout the outcrop area, all occur with near

$2 1996 International Association of Sedimentologists, Sedimentology, 43, 677-689 680 C. N. Drummond et al.

Fig. 2. Isopach map (contour interval=25 m) showing regional distribution of Peterson Limestone outcrops within the Overthrust belt of western Wyoming and south-eastern Idaho, and the location of 13 measured stratigraphic sections. SV=Swan Valley; MC=McCoys Creek; FR=Freedom; AB=Auburn; SP=Salt Creek Pass; LC=Lost Creek; SR=Snake River; AS=Astoria; BB=Blind Bull Creek; TC=Turkey Creek; SH=Sheep Creek; WC= Willow Creek; EF=East Fork. Inset: location of the state of Wyoming relative to the Cordilleran foreland basin and the I I rest of North America.

constant abundance within any hand sample ostracodes (Eyer, 1964, 1969; Peck, 1957; Peck & (Drummond et al., 1993). Calcite composition Craig, 1962). Macrofossils are uncommon but as determined by X-ray diffraction (XRD) include Unio, Planorbis, Viviparus, Goniobasis (e.g. Goldsmith et al., 1961) is typically and Physa (Mansfield, 1927, 1952). All taxa Ca,.,,Mg,.,,CO,; dolomite is slightly calcian at indicate deposition in nonmarine waters. Ca,.,,Mg,.,,CO,. Detailed mineralogical analysis The depositional origin of the matrix micrite is of several stratigraphic sections indicates that difficult to determine due to textural disruption dolomite compositions range from 0 to 100% of during diagenetic stabilization. However, the total carbonate. Completely calcitic micrites com- presence of abundant Chara fragments within prise over 20% of all samples, pure dolostone calcite-rich beds indicates that at least some pre- makes up ~157'0,and the remaining 65% fall cipitation was driven by photosynthetic activity within a range of 10-60% dolomite (Fig. 3). of aquatic macrophytes. Dolomite-rich beds are Fossil remains are generally small and rare, but interpreted to have formed in more saline waters, include several varieties of charophytes and probably through direct chemical sedimentation.

0 1996 International Association of Sedirnentologists, Sedhentdogy, 43, 677-689 Climatic control of fluvial-lacustrine cyclicity 681

30 1 0 sI8o v) 30 2 a5 n = 200 v) 20 8 c z '0 w a

I 0 20 40 60 80 100 PERCENT DOLOMITE -1.5 -1.0 0.5 1.0 1.5 Fig. 3. Distribution of dolomite abundance within -05 0.0 samples of micritic limestone. Note that the greatest Micro Sample Value - Mean Hand Sample Value (PDB) proportion of Peterson units is calcitic limestone Fig. 4. Homogeneity of micrite isotopic composition ranging in dolomite content up to about 60%. Very within kilogram-sized hand samples of Peterson Lime- few samples exhibit dolomite contents between this stone. This is a frequency plot of differences between composition and that of pure dolostones. microsample isotopic composition (!LPDB) and mean sample composition from multiple PO and hT3C analyses of 50 hand samples. Approximately 86% of all Carbonate geochemistry samples have a 6l8O variance of less than I%, and 90% of all samples have a variance less than 0.5% in S'T. Geochemical analysis of the Peterson Limestone has centred on documenting lateral and stratigraphic variability in total carbonate content, compositions. Corrections for potential differ- dolomite/calcite ratio and stable isotopic compo- ences in the C02/C03=acid fractionation between sition within a context of observed limestone/ calcite and dolomite have been made by applying shale patterns discussed above. X-ray diffraction a 0.8% correction (Sharma & Clayton, 1965) nor- and stable isotopic analysis indicate that minera- malized to the dolomite content of individual logical and isotopic variability within hand samples. Such a procedure possibly over-corrects samples is small compared with the magnitude of for fractionation at 73T, a factor which should variability in a stratigraphic section. Replicate decrease with increasing temperature. stable isotopic analyses of microgram-sized Carbonate stable isotopic compositions exhibit samples, taken from kilogram-sized hand- strong covariance between 6I80 and 613C (Fig. 51, specimens of micritic carbonate from several with 6l80 ranging from about - 13%0 (PDB) to stratigraphic sections, demonstrate that 86% of all - 3%0(PDB) and 613C ranging from - 5%0(PDB) to samples have less than l%o variance in 6l80 and -2% (PDB). In addition, 6l80 values linearly 90% have less than 0.5%0variance in 613C (Fig. 4). correlate with dolomite content in all strati- In preparation for isotopic analysis, samples graphic sections (Fig. 6); calcitic limestone has were placed in stainless-steel boats and roasted at the most 180-depleted compositions and dolos- 380°C under vacuum for 1h to remove volatile tone has the most 180-enriched compositions. contaminants. Samples were then placed in indi- Textural and compositional attributes of dolo- vidual borosilicate reaction vessels and reacted at mite and calcite within lacustrine micrite from 72f2"C with three drops of anhydrous phos- the Peterson Limestone indicate that these phases phoric acid for 12 min in a Finnigan 'Kiel' extrac- co-precipitated during early stabilization of pre- tion system coupled directly to the inlet of a cursor magnesian calcite carbonate mud in a Finnigan MAT 251 isotope ratio mass spectrom- largely closed diagenetic system (Drummond eter. Isotopic enrichments were corrected for acid et al., 1993). Uniform difference in the isotopic fractionation and 170contribution (Craig, 1957) compositions of these phases over the entire and are reported in per mil notation relative to the mineralogical range of whole-rock compositions PDB standard. Precision was monitored with reflects equilibrium partitioning of l80 and 13C standards, bracketing the sample suite at the between dolomite and calcite. Diagenetic dolo- beginning, middle and end of each day's run. mite is enriched in I80 by 4-4o/oOand in 13C by Measured precision was maintained at better 1.8%0relative to coexisting calcite, values that are than 0.1%~for both carbon and oxygen isotope discernibly different from expected open-system 0 1996 International Association of Sedimentologists, Sedimenfology, 43, 677-689 682 C. N.Drummond et al.

0 I FREEDOM SWAN n -1 - VALLEY rn = 8.67 3 b = -1 1.48 z r = 0.91 m

~ ASTORIA 5- n -5 v rn = 3.27 I. -10 I b = -8.77 1. I m r = 0.48 -13 -12 -11 -10 -9 -8 -7 -6 -5

d80( PDB) nM SNAKE RIVER Fig. 5. Cross-plot of carbon and oxygen isotopic analyses of micritic limestones from the Swan Valley section (Fig. 2). Note the pronounced covariance between PO Go ,I m = 5.67 and 6% (r=0.89), a compositional pattern typical of b = -1 0.32 arid-region lakes where carbonate precipitation and accumulation occurs in closed basins with relatively long residence times (Talbot, 1990). Outliers to the LOST CREEK trend represent replicate analyses of two organic-rich limestone samples. Their extremely enriched carbon isotopic compositions possibly reflect organically derived methane-sourced carbon incorporation during $0m -10 rn = 6.17 diagenesis (e.g. Talbot & Kelts, 1991). b = -9.49 r = 0.90

SWAN VALLEY mixing trends (Drummond et al., 1993). Both stable isotopic and major element chemistry is preserved within micritic carbonate samples and, as such, micrites retain direct records of variation m in the composition of Cretaceous lakewater during accumulation of the Peterson Limestone. Stratigraphic variation in geochemical par- 20 40 60 80 PERCENT DOLOMITE ameters occurs at two scales. Dolomite and PO contents of lacustrine micrite units generally Fig. 6. Paired 6"O and dolomite contents of carbonate micrites from five Peterson Limestone stratigraphic decrease progressively from the base to the top of sections (Fig. 2). Note that all sections exhibit simi- each limestone bed (Fig. 7), while upsection lar slopes of compositional covariance (m=about €I), through any individual stratigraphic section but with intercept values ranging from about -9.0 samples become more depleted in l80. (Astoria) to - 11.7 (Swan Valley). Similarity of slopes is interpreted as representing chemical (Mg/Ca) and isotopic evolution of the same body of water, while INTERPRETATION differences in intercept values (PO value of pure calcite limestone in each section) possibly reflect the position of each stratigraphic section relative to inlets Given the presence of metre-scale micritic carbon- and outlets of water (e.g. Talbot, 1990). Note that ate to argillaceous marl alternation within the sections with slopes differing from 6 do not contain Peterson Limestone, upsection lacustrine carbon- pure dolostone end-members, indicating that slopes ate to fluvial clastic cyclicity at the formation- calculated for those sections may be inaccurate. scale, and strong correlation between stable isotopic and mineralogical compositions of Origins of dolomite lacustrine micrite, it is possible to interpret both metre-scale and formational lithological and It has long been recognized that the presence of geochemical transitions in a context of climatic dolomite in lacustrine sediment indicates evap- rather than tectonic forcing. orative evolution of lakewaters and attainment of

(3 1996 International Association of Sedimentologists, Sedimentology, 43, 677-689 Climatic control of fluvial-lacustrine cyclicity 683

Fig. 7. Oxygen isotopic composition, SWAN VALLEY dolomite abundance and carbonate 6 "0 (PDB) PERCENT DOLOMITE PERCENT CARBONATI content within individual beds of -12 -10 -8 -6 -4 0 20 40 60 80 50 70 Peterson Limestone. Limestones 25 become more 180-depleted, more calcitic and of greater purity upsection. In a similar manner, most individual beds (lightly shaded] h 20 vE exhibit abrupt and asymmetric (I) compositional variations in which cn 15 dolomite and l80-enriched bases Y pass vertically into more calcitic and 0 I80-depleted tops (heavily shaded). I + 10 These higher-frequency patterns of micritic limestone/marly shale alternation, as well as changes in 5 limestone composition are interpreted to record short-term transitions from arid to humid climatic conditions superimposed on a similar formation-scale pattern 1UMID ARID HUMID ARID HUMID ARID of change manifest over tens of LIT;gI-IF[HIC metres of stratigraphic section. ARGILh;LAYOUS rn EE elevated Mg/Ca ratio (Hardie & Eugster, 1970; unusual distribution of dolomite abundance Hardie et al., 1977; Eugster & Hardie, 1978). in the Peterson Limestone is additional evidence Muller et al. (1972), for example, recognized mod- for the episodic development of evaporitic lake- ern and near-recent dolomite in a number of lake waters (Fig. 3). Dolomite contents from 0 to 60% deposits to be the product of post-depositional of total carbonate (a range encompassing most diagenetic alteration of originally high-Mg calcite Peterson Limestone samples) originated during sediment through a process of dissolution of the in-situ stabilization of precursor Mg calcite metastable high-Mg calcite and reprecipitation of (Drummond et al., 1993) with compositions that thermodynamically stable dolomite and low-Mg ranged from pure calcite to calcites with a com- calcite phases. Other studies have demonstrated position of about Ca,,.,Mg,.,. Those stratigraphic that an elevated Mg/Ca ratio and the presence of intervals containing pure dolomicrite probably post-depositional diagenetic dolomite generally represent development of extremely evaporitic corresponds to periods of relatively arid con- lakewater and the atypical precipitation of dolo- ditions, as corroborated by other proxies of palaeo- mite as the primary sedimentary carbonate phase. climate such as positions of former shorelines, The fact that pure dolostone is rare compared abundance of different pollen taxa and variation in with dolomite-calcite mixtures indicates that the stable isotopic signature of autogenic phases such extremely evaporitic conditions were (e.g. Muller &Wagner, 1978) infrequently achieved. Thus, the abundance of dolomite within Peterson limestone micrite is interpreted to be a Carbonate stable isotopic compositions proxy of lakewater Mg/Ca ratio. More calcitic micrites record a regionally positive hydrological Carbon and oxygen stable isotopic composition of balance between precipitation and evaporation, primary lacustrine carbonate has been used exten- resulting in the development of a large flux of sively as a direct indicator of changes in climate fluvial waters from surrounding catchments into during sediment deposition (Stuiver, 1970; Fritz the lake basin, and establishment of a through- et al., 1975; Schoell, 1978; Stiller & Kaufman, flowing, hydrologically open lake system. Con- 1985; Kelts & Talbot, 1986; McKenzie & Eberli, versely, increased dolomite abundance records 1987; Suchecki et al., 1988; Kelts & Talbot, 1989; climate change to more negative hydrologica Talbot, 1990; Drummond, 1995). While com- 1 balances, progressive decrease in fluvial flux and pletely unequivocal evidence that lacustrine chemical evolution of lakewaters to more saline carbonate forms in isotopic equilibrium with conditions, coupled with an increase in lakewater lakewater has yet to be produced, some studies Mg/Ca through carbonate precipitation. The have suggested this to be true (Turner et al., 1983;

0 1996 International Association of Sedimentologists, Sedimentology, 43, 677-689 684 C. N.Drummond et al. McKenzie, 1985; Gasse & Fontes, 1987; Fritz et al., variance (Fig. 7) to indicate coupled mineralogi- 1987). cal and isotopic evolution of sediment generated The assumption that carbonate forms in equi- within each stratigraphic section. librium with lakewater (or that the magnitude of Talbot (1990) interprets the compositional the disequilibrium is nearly constant) allows for a position of any individual sample along such direct evaluation of changes in carbonate isotopic paired 6l80 and 613C trends to reflect the degree of composition in terms of inferred hydrological evaporative evolution experienced by the lake- fluxes. The most '*O-depleted samples probably water in which that carbonate was precipitated, represent completely open hydrological con- an evolution that in theory could proceed through ditions with through-flow dominating over sur- time with progressive basin closure, or in space as face evaporation, while 180-enriched samples water moves via axial flow within an arid-region represent progressive lake closure and isotopic lake (Muller & Wagner, 1978). Differences in evolution (Fig. 7). This process of evaporative calcitic end-member isotopic compositions (y- enrichment of lakewater in l80is well understood intercepts of compositional covariance; Fig. 6) (Fontes & Gonfiantini, 1967; Gat & Gonfiantini, could reflect the location of individual strati- 1981; Gonfiantini, 1986; Fritz et al., 1987), and graphic sections relative to inlets and outlets of has been documented in a number of modern Lake Peterson and/or regional variation in the evaporative lake systems (Schoell, 1978; Spencer isotopic composition of inflowing meteoric fluids. et al., 1984; McKenzie, 1985; Talbot & Kelts, 1986; Regional variation of 6l80 intercept values from Hillaire-Marcel & Casanova, 1987; Halfman et al., the sections along the western Sevier orogen 1989). (Freedom, Idaho; - 11.48), to the lake centre Conversely, a complete understanding of the sections (Snake River, Wyoming, - 10.32), to sec- carbon isotopic budget of carbonate producing tions along the eastern cratonward lake margin lakes has not as yet been achieved. For some (Astoria, Wyoming; - 8-77), do suggest a pre- modern systems, carbonate 613C has been shown dominance of more depleted inflow, presumably to be directly related to relative rates of biologi- from higher western source regions, and indicate cally mediated production of reduced organic that even under hydrologically open conditions carbon and subsequent oxidation of that organic such as must have existed during calcitic micrite carbon in the water column (McKenzie, 1985). accumulation, some isotopic variation probably Although this mechanism dominates in eutrophic did occur within the lake basin. However, error in and hypereutrophic lake systems, large changes the estimation of the y-intercept associated with in carbonate 613C in generally oligotrophic arid- this regression analysis decreases the absolute region lakes is thought to be controlled by iso- amount of supposed intralake isotopic variation topic partitioning during CO, outgassing (Talbot (Fig. 6). It should be noted that the possibility & Kelts, 1991). Therefore, direct coupling between exists that variation in intercept values represents carbonate 6l80 and 613C (Fig. 5) is generally the isotopic evolution of a number of lakes considered typical of evaporitic systems (Talbot, arranged in series. However, little or no other 1990; Talbot & Kelts, 1991). evidence exists to support this hypothesis. Indeed, isopach data support the presence of only a single lake basin (Fig. 2). Isotopic-mineralogical covariance Isotopic and mineralogical data describe strong positive linear covariance between 6l80 and Implications for climate change percentage dolomite in all stratigraphic sections. Perhaps one of the most significant observations Talbot (1990) has noted that paired 6la0 and 613C to be drawn from distributions of mineralogical data from other lacustrine carbonate sequences and isotopic variation in the Peterson Limestone commonly describe slopes of isotopic covariance is that upsection transitions from more carbonate- similar to those seen in sections of the Peterson rich to more clastic-rich lithologies are accom- Limestone (Fig. 5), and has argued that these panied by a progressive decrease in dolomite trends record isotopic evolution of lakewater abundance and depletion in l80. This pattern is under conditions of increasing evaporation. repeated both at a scale of individual micrite beds Because increased rate of evaporation relative to and at a formational scale between the lacustrine freshwater flux should influence lakewater Mg/Ca Peterson Limestone and the overlying fluvial ratio as well as isotopic composition, we interpret Bechler Formation. At both scales of consider- slopes of 6l80 and percentage dolomite co- ation, more dolomitic and 180-enriched intervals

0 1996 International Association of Sedimentologists, Sedimentology, 43, 677-689 Climatic control of fluvial-lacustrine cyclicity 685 commonly occur at the base, with progressive depletion in l8O content and decreasing dolomite Humid Phase Inflow + Precipitation > Evaporation abundance upsection (Fig. 7). I 1 Metre-scale observations show that most Precipitation Evaboration OuNOw 1 limestone beds exhibit asymmetric patterns of tnflnw - mineralogical and isotopic variation. Initially "0-enriched dolomitic layers record accumulation under hydrologically closed arid conditions that change upsection into 180-depleted Water 180 = -1 1 oloo (SMOW) Fluid MqICa = 0.5 calcitic layers recording carbonate accumulation I Sediment 180 = -13 o/oo (PDB) Sediment MglCa = 0 07 I in open lake systems before evolving to settings Arid Phase in which deposition was dominated by the Inflow + Precipitation accumulation of clastic material. I t These patterns indicate that carbonate deposition was initiated during relatively evaporative periods when fluvial inflow and clastic flux were at a minimum, and when rates of tectonic sub- Water 180 = -2 oloo (SMOW) Fluid Mq/Ca = 12.0 sidence were able to outstrip rates of sediment Sediment 180 = -4 oloo (PDB) Sediment MgICa = 0.4 accumulation. With time, regional rates of precipitation gradually increased relative to mean ~ ~~ Fig. 8. Conceptual model of arid and humid phase evaporation and fluvial flux into and through the carbonate deposition. (A) During humid phases, rates lake basin. During intervals of increased meteoric of inflow and precipitation exceeded rates of evapor- precipitation, the net effect of lake-surface evap- ation, producing a hydrologically open system. Lake oration and lakewater "0 enrichment diminished water isotopic composition most closely reflected to the point that lakewater POapproached the meteoric water composition (- ll%oSMOW] and car- mean isotopic composition of regional meteoric bonate precipitated from the lakewater was at its most water, while ionic strength and Mg/Ca ratio of "0-depleted value [ - 13%0PDB). Likewise, lakewater Mg/Ca ratio was close to the riverine value, and low-Mg lakewater decreased. calcite was precipitated. (B) During arid phases, inflow Metre-scale intraformational variation in 6l80 and surface precipitation was equal to or less than (Fig. 7) probably records episodic evaporation evaporation, and a hydrologically closed lake system and lakewater enrichment from a relatively con- developed. Lake water underwent evaporative enrich- stant meteoric fluid-derived calcite with 6l80 ment in l8O, which was reflected in the composition around - 13%0 (PDB), followed by a return to of carbonate sediment. Continued precipitation of cal- more humid conditions and lakewater depletion cium carbonate in the closed lake system led to progressive increase in the Mg/Ca ratio of lakewater, which in in "0. Increased meteoric inflow was accom- turn gave rise to deposition of progressively more panied by a greater flux of allochthonous argil- Mg-rich carbonate sediment until autogenic dolomite laceous sediment, manifest as generally abrupt was precipitated. transitions from pure carbonate accumulation to the formation of more shale-rich beds. Formation-scale transitions from fluvial to limestone-to-shaly micrite transitions. The total lacustrine to fluvial deposition within the Gannett duration of continental foreland basin deposition Group are also accompanied by changes in min- in western Wyoming and eastern Idaho is poorly eralogy and isotopic composition of lacustrine constrained, and it is therefore difficult to pre- limestone (Fig. 7). These trends suggest that long- cisely calculate cycle period at either scale. Based term changes in climate also served to control the on contained freshwater fauna, available data indi- flux of water and fluvial sediment to the subsiding cate that the Gannett Group deposition occurred foreland basin. If observed cyclicity of fluvial to during the early Cretaceous. Peck (1957) suggested lacustrine accumulation was related to climate, that Ephraim Conglomerate deposition occurred only one hemicycle (arid to humid) is recorded by from the Berriasian through the Barremian, while lacustrine carbonate (Fig. 8). the Peterson, Bechler and Draney Formations accumulated during the Aptian. In this context, the lacustrine-fluvial- Cyclicity, tectonics, climate lacustrine sequence that comprises the Peterson, Gannett Group lithological variation occurs at Bechler and Draney Formations has a depo- scales of both fluvial-to-lacustrine- and micritic sitional duration of about 6 Myr (e.g. Palmer, 0 1996 International Association of Sedimentologists, Sedimentology, 43, 677-689 686 C. N. Drummond et al. 1983). If sedimentation rates in the Peterson and such as the transition from Tertiary Lake Draney lake systems were similar, and if fluvial Bonneville to Quaternary Great Salt Lake deposition of the Bechler sandstone was about an deposition. order of magnitude greater (e.g. Sadler, 1981), In a somewhat similar vein, data on the miner- stratigraphic data on the Gannett Group indicate alogy and composition of Peterson Limestone that basin axis accumulation occurred over about micrites clearly implicate secular variation in the 2.0 Myr for the Peterson Limestone, about 0.7 Myr intensity of lake basin evaporation and precipi- for the Bechler formation and over about 3.3 Myr tation at a comparable scale to that manifest as for the Draney Limestone. Because lithological fluvial-lacustrine cyclicity. Although this sug- and geochemical variations in Peterson Lime- gests significant climate change occurred during stone sections indicate that lacustrine deposition development of individual foreland systems, cor- occurred during half of one long-term arid-to- relation alone does not negate the possibility of humid cycle, formation-scale fluvial-lacustrine some genetic link between intensity of foreland cyclicity therefore has an apparent period of some basin tectonism, elevation of the Sevier orogen 3-4 Myr. and subsidence rate of the adjacent foreland Because abundance of intraformational vari- basin. ation is dependent on the location of measured Subsidence could have played an important sections relative to the basin centre, estimation of role in determining differences between elevation durations of micrite to marly micrite couplets is of lake outlets and the lake basin centre, which in even less certain. However, given the abundance turn could directly influence lake area and poten- of such couplets in lake-centre sections, short- tially regional rates of lakewater cycling. More term cycle period is probably on the order of intense tectonism would serve to mediate cli- several hundred thousand years. matic variation and drive the onset of regional The occurrence of lithological cyclicity occur- aridity. Similarly, uplift alone could have ring with a periodicity of several hundred increased the role of the western Sevier orogen as thousand years is commonly cited as evidence an elevated barrier to the influx of extrabasinal of Milankovitch-band Earth-orbital control on atmospheric moisture. More intense tectonism in regional climate and sediment accumulation (e.g. this case would serve to intensify the orogenic Arthur et al., 1985; Hattin, 1985; Sageman, 1985). rain shadow and bolster the onset of regional We have no compelling evidence within the aridity. Peterson Limestone to indicate that Milankovitch- Based on compositional variation in the band cycles are present. However, the presence of Peterson Limestone, it is apparent that intra- lithological cyclicity alone need not be linearly formational and interformational lithological related to change in extrabasinal variables such as variation is not simply related to foreland tec- climate. Regular alternation of micrite and shaly tonism and changes in amounts of relative relief micrite in the Peterson Limestone, and the abrupt between orogens and adjacent lake basin or fluvial nature of many carbonate to clastic transitions plains; rather, the Gannett Group provides a both within and between formations, possibly striking example of lithological cyclicity within a record attainment of climatic thresholds that foreland basin controlled not solely by episodic served to separate relatively arid from relatively tectonism but rather by either associated or humid conditions. Synoptic-scale computational unrelated changes in regional climate. modelling of lake-atmosphere hydrological feedbacks by Hostetler et al. (1994) indicate that sufficiently large lakes may become hydrologi- CONCLUSIONS cally self-sustaining due to large-scale recycling of meteoric water. As lake area grows in response Combined lithological, mineralogical and iso- to changing climate, patterns of regional rain fall topic data from the lower Cretaceous Peterson may shift from highland only to basin-wide pre- Limestone indicate that cyclical transitions cipitation, providing renewed abundant flux of from carbonate-rich to terrigenous-rich intervals terrigenous material to lake-centre settings. Con- occurred at two scales, and that both relate to versely, decrease in lake area in response to secular variation in rates of regional meteoric regional drying would result in significantly less precipitation relative to rates of evaporation intrabasinal cycling of meteoric water, reduction across the Sevier orogen. Asymmetric patterns in in rates of basin-wide precipitation, and the rapid POand dolomite abundance within individual attainment of extremely evaporitic conditions, limestone beds demonstrate that accumulation of

(cj, 1996 International Association of Sedimentologists, Sedimentology, 43, 677-689 Climatic control of fluvial-lacustrine cyclicity 687

lacustrine carbonate occurred more or less Arthur, M.A., Dean, W.E. and Schlager, S.O. (1985) continuously during arid-to-humid transitions, Variations in the global carbon cycle during the but that either those transitions occurred abruptly Cretaceous related to climate, volcanism, and or rates of sediment accumulation slowed dra- changes in atmospheric CO,, In: The Carbon Cycle and Atmospheric CO,: Natural Variations, Archean matically during the onset of more arid con- to Present (Ed. by E. T. Sundquist and W. S. ditions, possibly in response to the presence Broecker), American Geophys. Union Monograph, of hydrological thresholds within the lake- 32, 504-529. atmosphere feedback cycle. Carbonate deposition Barron, E.J., Arthur, M.A. and Kauffman, E.G. (1985) began during intervals of maximum aridity, and Cretaceous rhythmic bedding sequences - a plausible decreased with increased rainfall and decreased link between orbital variations and climate. Earth evaporation until minimum aridity resulted in planet. Sci. Lett., 40, 103-133. Beck, R.H. and Vondra, C.A. (1985) Orogeny and greater fluvial flux of terrigenous sediment. When stratigraphy: numerical models of the Paleozoic in rate of carbonate generation was finally outpaced the eastern interior of North America. Tectonics, 7, by clastic flux, fluvial systems of the Bechler 3 89-4 16. Formation prograded across and along the basin Blair, T.C. and Bilodeau, W.L. (1988) Development of axis. Such long-term trends suggest that fluvial- tectonic cyclothems in rift, pull-apart, and foreland lacustrine cyclicity, at least during this portion of basins: sedimentary response to episodic tectonism. Cordilleran foreland basin deposition, was sig- Geology, 16, 517-520. nificantly influenced by early Cretaceous climatic Brown, R.E. and Wilkinson, B.H. (1981) The Draney Limestone: Early Cretaceous lacustrine carbonate change as well as changing rates of tectonism and deposition in western Wyoming and southeastern uplift of the bounding orogen. Idaho. Contrib. Geol., 20, 23-31. Craig, H. (1957) Isotopic standards for carbon and oxygen and correction factors for mass spectrometric ACKNOWLEDGMENTS analysis of carbon dioxide. Geochim. Cosmochirn. Acta, 12, 133-149. During the course of this study, conversations Ditchfield, P. and Marshall, J.D. (1989) Isotopic vari- with numerous colleagues at the University of ation in rhythmically bedded chalks. Paleotempera- ture variation in the Upper Cretaceous. Geology, 17, Michigan and elsewhere have added greatly to 842-845. our appreciation and understanding of lacustrine Drummond, C.N., Patterson, W.P. and Walker, J.C.G. systems as storehouses of continental climate. (1995) Climatic forcing of carbon-oxygen isotopic Specifically we wish to thank Linda Albertzart, covariance in temperate-region marl lakes. Geology, David Dettman, William Patterson, Brad Opdyke, 23, 1031-1034. James Walker and Kevin Given for helpful discus- Drummond, C.N., Wilkinson, B.H. and Lohmann, K.C sion and encouragement during this investi- (1993) Rock-dominated diagenesis of lacustrine magnesian calcite micrite. Carbonates Evaporites, a, gation. Early drafts of this manuscript benefited 214-223. greatly from substantial reviews by Joseph Eisbacher, G.H., Carrigy, M.A. and Campbell, R.B. Hughes, Tracy Frank, Judy McKenzie, Paul Olsen, (1974)Paleodrainage pattern and late-orogenic basins Mike Talbot and Julian Andrews. Research on of the Canadian Cordillera. In: Tectonics and Sedi- ancient lacustrine carbonates at the University of mentation (Ed. by W. R. Dickinson), Spec. Publ. SOC. Michigan is supported by the National Science econ. Paleont. Miner., 22, 143-166. Foundation, NSF grant EAR-90-19095. Eugster, H.P. and Hardie, L.A. (1978) Saline Lakes. In Lakes: Chemistry, Geology, Physics (Ed. by A. Lerman), pp. 237-393. Springer-Verlag, New York. Eyer, J.A. (1964) Stratigraphy and micropaleontology of REFERENCES the Gannett Group of western Wyoming and southeastern Idaho. PhD thesis, University of Colorado. Armstrong, F.C. (1968) Sevier orogenic belt in Nevada Eyer, J.A. (1969) Gannett Group of western Wyoming and Utah. Bull. geol. SOC.Am., 79, 429-458. and southeastern Idaho. B~i11.Am. Ass. petrol. Geol., Armstrong, F.C. and Oriel, S.S. (1965) Tectonic devel- 53, 1368-1390. opment of Idaho-Wyoming thrust belt. Bull. Am. Fischer, A.G. and Schwarzacher, W. (1984) Cre- Ass. petrol. Geol., 49, 1847-1866. taceous bedding rhythms under orbital control? In: Arthur, M.A. and Dean, W.E. (1991) A holistic geo- Milankovitch and Climate, Part I (Ed. by A. L. Berger chemical approach to cyclomania-examples from et al.), pp. 163-175. Reidel, New York. 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