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Post-Paleozoic alluvial gravel transport as evidence of continental tilting in the U.S. Cordillera

Paul L. Heller, Kenneth Dueker and Margaret E. McMillan

Geological Society of America Bulletin 2003;115;1122-1132 doi: 10.1130/B25219.1

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Post-Paleozoic alluvial gravel transport as evidence of continental tilting in the U.S. Cordillera

Paul L. Heller² Kenneth Dueker Margaret E. McMillan³ Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA

ABSTRACT means of relating constructional landforms THREE WIDESPREAD to mantle-driven processes. CONGLOMERATES The western United States contains three thin but remarkably widespread alluvial Keywords: paleohydraulics, tectonics, dy- The three widespread gravel units of the conglomeratic units that record episodes of namic topography, U.S. Cordillera, con- U.S. Cordillera are the Shinarump Conglom- large-scale tilting across the U.S. Cordille- glomerate, paleotopography, syntectonic erate Member of the Chinle Formation (Late ran orogen in post-Paleozoic time. These sedimentation. Triassic) found in the southwestern USA (Fig. units are: (1) the Shinarump Conglomerate 2A); the Lower Cretaceous conglomerates of Late Triassic age exposed in northern INTRODUCTION found over much of the U.S. Rocky Moun- Arizona and adjacent parts of Utah, Neva- tains (Fig. 2B); and gravelly parts of the Ogal- da, and New Mexico; (2) Lower Cretaceous A common characteristic of synorogenic al- lala Group (Miocene±Pliocene) exposed along gravel deposits that overlie the Morrison luvial conglomerates is the relatively limited the western Great Plains (Fig. 2C).1 Formation throughout the Rocky Mountain distance they prograde from their uplifted region; and (3) the gravel-rich parts of the source areas out into the adjacent basinsÐ Shinarump Conglomerate Miocene-Pliocene age Ogallala Group in most are found within a few tens of kilometers western Nebraska and adjacent southeast- of their associated mountain fronts (Fig. 1, A± The Shinarump Conglomerate lies at the ern Wyoming. Paleoslopes of the rivers de- G). The restricted distance of gravel deposi- base of the Chinle Formation, a ¯uvial and positing these units were in the range of tion re¯ects the long-term balance between the lacustrine unit of Late Triassic age (Carnian; 10؊4 to 10؊3, based on paleohydraulic cal- rate of delivery of coarse sediment supply and Lucas, 1993) exposed in the southwestern culations. However, depositional thickness basin subsidence rate that acts to trap the grav- USA (Stewart et al., 1972a). Rivers of the trends of these units are not suf®cient to el. Since on an elastic plate both supply and Chinle Formation ¯owed generally north from have generated such steep paleoslopes. subsidence are proportional to the size of ad- the ``Mogollon highlands,'' an interpreted re- Thus, long wavelength tilting of the Earth's jacent mountain belts, it is no surprise that ba- gional uplands in southern Arizona and adja- surface must have occurred for these grav- sin sedimentation patterns have a similar cent regions (Stewart et al., 1972a; Blakey and els to be transported. Although these units length scale. Gubitosa, 1983), eventually joining major were deposited adjacent to large tectonic In contrast, a few conglomeratic units seen tributaries from farther east. Together they features, including an evolving and migrat- in the U.S. Cordillera are unusually wide- ¯owed northwest to the contemporaneous ing continental arc, and, for the Ogallala spread in distribution (Stokes, 1950; Stewart shoreline (Riggs et al., 1996). While the Shi- Group, the northward-propagating Rio et al., 1972a; Heller and Paola, 1989), being narump Conglomerate is dominantly sand- Grande Rift, the tilting occurred over deposited over many hundreds of kilometers stone, conglomerate is common. Unit thick- wavelengths too broad to be directly gen- downstream from their source areas (Fig. 1), ness varies, but it is typically a few meters erated by these features. These widespread yet being relatively thin over their entire ex- thick. Gravel composition includes abundant gravel units attest to the interplay between tent. These gravels are related to the temporal quartz, quartzite, and chert grains (Blakey and the creation of subduction-related isostatic and spatial pattern of subduction and late Ce- Gubitosa, 1983). The Shinarump Conglomer- and dynamic topography and continental nozoic uplift of the central and southern ate forms an irregular sheet that rests erosion- sedimentation. Hence, paleotopography, as Rocky Mountains. We argue that the occur- ally upon older Triassic and Permian units determined from calculated transport gra- rence of these thin, widespread gravel units (Blakey and Gubitosa, 1983) and is overlain dients of sedimentary deposits, provides a does not re¯ect climatic controls, but must re- by bentonitic shales and mudstones of the cord times of large-wavelength/low-amplitude Chinle Formation (Stewart et al., 1972a). Lo- tilting of the continental interior. Thus, we be- ²E-mail: [email protected]. lieve they are the stratigraphic records of deep ³Present address: University of Arkansas at Little 1Color photos and ®gures to accompany this pa- Rock, Department of Earth Sciences, Little Rock, processes affecting the western U.S. during per can be found at: http://faculty.gg.uwyo.edu/hell- Arkansas 72204, USA. post-Paleozoic time. er/pubs.htm.

GSA Bulletin; September 2003; v. 115; no. 9; p. 1122±1132; 8 ®gures; Data Repository item 2003121.

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Figure 1. Reported distances of gravel deposition (measured normal to orogenic Figure 2. Maps of three gravel units: (A) Late Triassic time showing Shinarump Con- front) with respect to their orogenic source glomerate Member of Chinle Formation (Stewart et al., 1972a; Blakey and Gubitosa, 1983; areas. Examples A±G are from ancient Dubiel and Brown, 1993); (B) Early Cretaceous time showing Cloverly Formation (in foreland basins (Heller and Paola, 1989); Wyoming) and its lithostratigraphic correlatives, including Buckhorn Member of Cedar these are, in order, the Alps, Andes, Ap- Mountain Formation (in Utah), Lytle and Burro Mountain Formations (in Colorado), ennines, Himalayas, Pyrenees, Catalan Lakota Formation (in South Dakota), and lower member of Ephraim Conglomerate (in Coast Range, and Sevier belt. The three un- Idaho and western Wyoming) (Heller and Paola, 1989); and (C) Miocene time showing usually widespread alluvial gravels in the Ogallala Formation of western Great Plains (Swinehart et al., 1985; Gustavson and Wink- U.S. Cordillera are also shown. ler, 1988). Maps include observed (black) and inferred (shaded) original distribution of units, generalized paleo¯ow trends of units (white arrows); line of reconstructed topography (Fig. 7) is shown as white line with round end points, and associated tectonic fea- cal relief of up to 80 m along the basal un- tures are discussed in text (Baldridge et al., 1991; Armstrong and Ward, 1993; Chapin conformity suggests that the conglomerate and Cather, 1994; Erslev, 2001). In Part B, heavy dashed line shows subsequent site of ®lled paleovalleys, especially in its more up- thrusting in Sevier belt. Generalized extension orientation of Rio Grande Rift is shown stream parts (Stewart et al., 1972a; Blakey and with double arrows and its interpreted northward extent (Tweto, 1979; Mears, 1993; Er- Gubitosa, 1983). Valley development dimin- slev, 2001) shown with dashed lines. ishes downstream (northward), indicating that pre-Shinarump valley cutting was not likely due to sea-level change. Toward the north the unit is inferred to record deposition in a broad alluvial plain, and the preservation of mottled facies (Stewart et al., 1972a) is interpreted to represent soil formation in low-lying inter- ¯uves (Hasiotis et al., 2000). Deposition of the Shinarump Conglomerate was penecontemporaneous with the onset of intrusive igneous activity in southeastern Cal- ifornia and southern Arizona (Armstrong and Ward, 1993). The resulting Upper Triassic plu- tons are interpreted to represent the begin- nings of Andean-type arc magmatism in the southwestern USA (Armstrong and Ward, 1993; Dickinson, 2000). The coincidence in timing of Shinarump deposition, at the base of the ash-rich Chinle Formation, and the onset of arc magmatism suggests to us that these events are linked.

Lower Cretaceous Conglomerates of the Figure 3. Example of widespread conglomeratic unit, in this case Lytle Formation (part Rocky Mountains of Lower Cretaceous gravels) north of Fort Collins, Colorado. This unit shows most typical characteristics of gravel units; it is conglomerate and sandstone, which abruptly overlies Discontinuously exposed throughout the much ®ner-grained sedimentary deposits, in this case ¯uvial rocks of Late Jurassic Mor- U.S. Rocky Mountains are conglomeratic ¯u- rison Formation (A). Unit has sharp, disconformable contact with underlying unit (B). In vial rocks of Early Cretaceous age (Figs. 2B most places, unit is a single, sheet-like deposit that pinches and swells (C), but there is and 3). These units are called a variety of evidence of amalgamation of several channel-belt bodies (D), enlarged below with super- names, including the Buckhorn Member of the imposed line drawing. Resistant deposits typically form a cliff-capping unit that can be Cedar Mountain Formation in central Utah, traced over many kilometers (E). Cars at base of cliff show scale. At D (inset), unit consists the Burro Canyon and Lytle formations in of three amalgamated sand/gravel bodies, of which upper two inter®nger with ®ner- Colorado, the Cloverly Formation in Wyo- grained overbank deposits.

Geological Society of America Bulletin, September 2003 1123 Downloaded from gsabulletin.gsapubs.org on January 26, 2010 HELLER et al. ming, and the Lakota Formation in the Black metric complexities make broader correlation Hills. All of these units appear to sit discon- very dif®cult (Swinehart et al., 1985). The formably upon the Morrison Formation (Fig. group is composed of predominantly ®ne- 3B), a widespread, mud-rich alluvial unit of grained amalgamated ¯uvial and eolian de- Late Jurassic age (Stokes, 1944; Kowallis and posits with dispersed interbedded conglomer- Heaton, 1987). The duration and signi®cance ates. Deposition began in the middle Miocene of the bounding unconformity has been con- and continued until the earliest Pliocene (19± tested (Furer, 1970; Winslow and Heller, 1987; 5 Ma) based on K/Ar and ®ssion-track dating May et al., 1995) and may represent only a (Izett, 1975; Naeser et al., 1980) and biostrati- lithofacies change in otherwise continuous de- graphic controls (Reeves, 1984; Swinehart et position. The Lower Cretaceous conglomer- al., 1985). The Ogallala Group sits discon- ates have been correlated to each other on the formably upon the Arikaree and White River basis of composition, lithostratigraphy, grain- groups in its northern exposure and uncon- size trends, and general consistency of limited formably on Mesozoic and Paleozoic rocks to age dates (Heller and Paola, 1989). Some the south. The unit marks the regional transi- time-transgressive deposition of the conglom- tion from a period of nearly continuous ag- erate has been suggested based upon strati- gradation to one of net degradation (Diffendal, graphic interpretations (Currie, 1997). Gravel- Figure 4. Flexural analysis of Lower Creta- 1982; Swinehart et al., 1985). clast composition is primarily chert pebbles. ceous gravels. Shown are ®lled sedimentary Fluvial conglomeratic deposits of the Ogal- Lithofacies interpretations suggest a low- basin geometries required to generate back- lala Group are remarkable in that they have sinuosity, braided origin (Heller and Paola, bulge transport slopes of 10؊3, assuming an been transported over 250 km from their 1989; Zaleha et al., 2001). unbroken plate with ¯exural rigidities (D) of source areas. These deposits form irregular The gravel body consists of amalgamated 1023 to 1024 Nm and for various load geome- cut-and-®ll sequences that are most continu- channels that locally include some ®ner- tries (height and length). Width and depth of ous in the Ash Hollow Formation along the grained alluvial deposits (e.g., Fig. 3D; May ®lled foreland basins associated with these Cheyenne Tablelands in eastern Wyoming and et al., 1995). Overall, paleocurrent trends in- back bulge slopes range from 200 km and 2.7 west-central Nebraska. They were deposited by dicate ¯ow toward the northeast (Heller and km deep to 400 km and 5.2 km deep, de- east-northeast-¯owing, low-sinuosity braided Paola, 1989), although some notable complex- pending upon value of D. streams draining the Rocky Mountain front ities are found locally, particularly in the (Goodwin and Diffendal, 1987). To the south, Black Hills region (Zaleha et al., 2001). Age conglomerates are less evident at the surface, control is limited, but available biostratigraph- ``phantom foredeep'' existed (Royse, 1993), but they are found in the subsurface (Gustav- ic control suggests deposition during middle but rather the region was simply dipping gent- son and Winkler, 1988). Gravel compositions Neocomian to Aptian time (Tschudy et al., ly down toward the east. include granite and anorthosite clasts from the 1984; Heller and Paola, 1989). Tectonic activity in the western U.S. during Front and Laramie ranges and volcanic clasts Most accounts interpret the gravels as hav- early Early Cretaceous time (Neocomian) was from ¯ows in north-central Colorado (Black- ing been deposited prior to, and perhaps im- relatively quiescent (Dickinson, 2001). There stone, 1975). Relief on the base of the unit is mediately prior to, the earliest deformation is a notable paucity of sedimentary accumu- up to 100 m, re¯ecting paleovalleys in the up- along the Sevier fold-thrust belt (Heller and lation from that time until the initiation of the stream end and local post-depositional subsi- Paola, 1989; Yingling and Heller, 1992; Cur- Sevier orogenic belt in Aptian/Albian time dence due to salt dissolution in underlying rie, 1998). One interpretation suggests depo- (Heller et al., 1986). The U.S. Cordilleran Mesozoic units (Gustavson and Winkler, sition in an over®lled earlier foreland basin continental arc attained its maximum north- 1988; Oldham, 1996). The cut-and-®ll alter- that formed in front of thrust faults in central south extent in Jurassic time (Armstrong and nations may be the result of climate ¯uctua- Nevada (DeCelles and Currie, 1996; Currie, Ward, 1993). During Late Jurassic time, the tions, autocyclic events, or tectonic uplift in 1998); however, the ages of thrust faults in arc migrated eastward into westernmost Utah the Rocky Mountains during the late that region are now thought to be Early to and had begun to retract westward during Ear- Cenozoic. middle Cretaceous (MacCready et al., 1997), ly Cretaceous time. Apparently little else of Most interpretations of the Ogallala Group too young to generate an early foreland basin. regional tectonic signi®cance took place, at suggest that it records uplift associated with In addition, if the gravels formed in a back- least within or east of the volcanic arc, during the propagating Rio Grande Rift (Trimble, bulge basin setting, as suggested by Currie the Early Cretaceous. Thus, the gravel was de- 1980; Eaton, 1986). Deposition of the Ogal- (1998) and DeCelles and Currie (1996), then posited during continued growth of the con- lala Group occurred at the same time as de- a thick sequence of nonmarine foreland basin tinental arc. formation along the rift that was initiated in ®ll must have existed between the farthest New Mexico between 28 and 27 Ma, reached west exposures of gravel and the inferred Ogallala Group central Colorado by 26±25 Ma (Chapin and thrust belt that no longer exists. Considering Cather, 1994), and is manifested as 10±8 Ma that such a basin must have been very thick, The Ogallala Group is part of a late Tertiary volcanic rocks, post-Miocene normal faulting, in excess of 2 km thick, to develop a back basin-®ll sequence deposited within central and high heat-¯ow on the Colorado±Wyoming bulge slope of 10Ϫ3 that is required by paleo- Rocky Mountain basins and on the western border (Keller and Baldridge, 1999; Mears, hydraulic analysis (Heller and Paola, 1989; Great Plains (Fig. 2C). In western Nebraska, 1998). The link between the two is further sup- Fig. 4), its complete absence is even more dif- geologists recognize several formations within ported by paleohydraulic analysis of Ogallala ®cult to understand. We suggest that no such the Ogallala Group, but lithologic and geo- ¯uvial channels on the Cheyenne Tablelands.

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These channels have been post-depositionally neous, and metamorphic pebbles, ranging up tion in the proximal part of basins, degrada- tilted up by an order of magnitude, most likely to cobble size, that were deposited by low- tion in the distal parts of basins (not important by tectonic uplift in the Rocky Mountains sinuosity (braided?) streams (Stokes, 1950; in the case of net deposition), and tilting of (McMillan et al., 2002). Thus, the Ogallala Blakey and Gubitosa, 1983; Swinehart et al., topography by isostatic processes. Group seems to be an indicator of long-wave- 1985; Gustavson and Winkler, 1988; Heller length tectonic activity both during and after and Paola, 1989; Currie, 1998). Gravel bodies Paleoslope Estimates its deposition. are widespread, laterally persistent, albeit not always continuous, and contain coarse-tail up- The key to determining tilt histories is es- Shared Characteristics ward ®ning and/or nested channel cut-and-®ll timating paleoslopes from these widespread structures. Occasionally, the gravel units contain alluvial gravels. Various quantitative paleo- These three ¯uvial conglomerate units share bar accretion surfaces, trough cross-bedding in hydraulic analyses exist (e.g., Cotter, 1971; several key features in common. While ex- the sandier fractions, and clast imbrication. Ethridge and Schumm, 1978; Gardner, 1983); posures today are discontinuous, paleogeo- The units rest on underlying deposits with however, the most appropriate approach to de- graphic reconstructions augmented by well sharp erosional contact (Fig. 3B), which sug- termining paleoslopes for these types of data suggest that these units were far more gests that they are not parts of a continuous coarse-grained deposits is that described by continuous in the past and represent through- coarsening-upward progression but either Paola and Mohrig (1996). This method utiliz- going alluvial systems. The Shinarump Con- ¯owed out upon a preexisting erosion surface es the observed relationship between calculat- glomerate and Ogallala Group are dominantly of low relief or that they ®rst beveled a lowed basal shear stress and characteristic grain sandstone, but gravels are found in locally angle scour surface (Stokes, 1950; Swinehart size of the mobile bed in gravelly braided abundant patches over long down-¯ow dis- et al., 1985). The underlying units are domi- streams. These streams tend to adjust to an tances. The Lower Cretaceous units are more nantly ®ne-grained ¯uvial to nearshore marine equilibrium condition where basal shear stress strongly conglomeratic. We think of these as deposits. Transport directions for units be- is slightly above that needed to move a rep- discontinuous gravel sheets; however, we note neath the Lower Cretaceous gravels (the Mor- resentative grain size. Assuming grain density that: (1) the dispersed presence of low-relief rison Formation) and Ogallala Group (Arika- of 2700 kg mϪ3, slope (S) is related to grain inter¯uves and occasional intercalated over- ree Group) were subparallel to the sheet size (D) and ¯ow depth (H) by: bank deposits demonstrate that they formed as gravels (Peterson, 1984, 1987; Winslow and Ϫ1 an amalgam of individual channel belts (Fig. Heller, 1987; Yingling and Heller, 1992). S ϭ␺DHx 3D), and (2) there are lateral variations in These underlying units are much ®ner grained grain size, particularly for the Shinarump than the gravel sheets, suggesting that they (see Paola and Mohrig (1996) for derivation), Ϫ2 Conglomerate and Ogallala Group, that dem- were transported by rivers of comparatively where ␺ϭ9.4 ϫ 10 when D50 (median di- onstrate that a variety of individual river chan- low slope. In the case of the Shinarump Con- ameter) is used (Paola and Mohrig, 1996), and Ϫ2 th nel systems were active intermittently in space glomerate, the underlying Moenkopi Forma- ␺ϭ2.38 ϫ 10 when D90 (90 percentile and time across the region. Most remarkably, tion was not only a ®ner-grained, hence low- diameter) is used (Heller and Paola, 1989). the units are thin, typically a few meters thick, gradient, coastal plain to deltaic system, but The slope calculation requires a measure- over nearly their entire lateral extent (Fig. 3). paleo¯ow was at acute angles to that of the ment of characteristic ¯ow depth (H) along However, in their upstream areas, they cut and overlying gravel deposit (Stewart et al., reaches in which the measured gravels were then ®ll paleovalleys into underlying deposits. 1972b). Thus, in all three cases, the onset of deposited. We have chosen thickness of the The Shinarump Conglomerate contains many gravel deposition coincided with, or soon fol- ®ning-upward sequence in which gravels were such paleovalleys toward the south that cut lowed, a change to steeper transport gradients. measured as a proxy for ¯ow depth. We re- down into the Moenkopi Formation (Triassic) alize that a preserved ®ning-upward sequence and the DeChelley Sandstone (Permian). METHODOLOGY may vary slightly from true ¯ow depth due to These range up to a few tens of meters deep channel perching during ®lling, which would and up to 5 km wide, but northward the unit Our approach combines two types of infor- lengthen the ®ning-upward sequence, or sub- is more continuous, typically 10 m thick (up mation. The ®rst is a determination of the sequent scouring which might remove the up- to 20 m in places) (Stewart et al., 1972a). The transport slope needed to move gravels out the per part of the sequence. Although these ef- Lower Cretaceous gravels in Utah reach up to observed distances from the source areas. Riv- fects tend to counter each other, we made an 35 m thick along trunk stream paleovalleys er transport can be achieved by either building effort to reduce the second problem by only cut into the underlying Morrison Formation a suf®cient slope or increasing ¯ow depth to choosing sites where sequences include the but are typically more consistent (ϳ10 m) in generate suf®cient shear stresses to carry ®ne-grained sandy to silty capping deposits. thickness (Heller and Paola, 1989; Yingling material. If we can constrain the ¯ow depths Application of this method to modern grav- and Heller, 1992; Currie, 1998). The entire from ®eld observations, we can calculate the elly rivers indicates uncertainties (2␴)ofa Ogallala Group, deposited between 19 and 5 paleoslopes, which then record regional factor of two or less (Paola and Mohrig, Ma (Swinehart et al., 1985), ranges from 30 paleotopography. 1996). The four key assumptions in using this to 240 m thick, however much of this ®ll is The second type of information is unit method are that ¯ows were quasi-steady state, sandstone and the greatest thickness is con- thickness. The preserved thickness of gravel channel banks were noncohesive, bedforms ®ned to paleovalleys (Swinehart et al., 1985; deposits and related units constrains how were absent on the channel bed, and transport Gustavson and Winkler, 1988; Gustavson, much aggradation can account for building the was dominantly as bedload. Thus, application 1996). necessary topography to reach the needed is best applied to deposits that do not show The deposits contain clasts of resistant li- transport slopes. The way these slopes are signs of highly variable, short-lived, rapid de- thologies, such as quartz, quartzite, chert, ig- built comes from a combination of aggrada- position, where there is no evidence of plant

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Figure 6. Interpreted cross section showing Early to middle Cretaceous units of central Utah that formed during initial growth of Sevier orogenic belt (modi®ed from Yingling and Heller, 1992). Note strong asymmetry of basin ®ll and limited transport distance of conglomeratic deposits of ``Cedar Mountain'' and San Pete formations. Pre-foreland deposits include widespread gravel unit (in black)ÐBuckhorn Conglomerate Member of Cedar Mountain Formation of Early Cretaceous age.

Figure 5. Conceptual model showing meth- sites in southern Nevada, along the east side deposition must be greater upstream for relief odology used in this study. If paleotrans- of the Spring Mountains and at Firehole Val- to develop to the point where slopes could car- port slope (␣) and aggradational thickness ley (locations N-1 and N-3 of Stewart et al. ry the observed gravels far into the basins of deposits (stippled pattern) are known, [1972a]). Slopes for the Lower Cretaceous (Fig. 5, A and B). then tilt of underlying basement (patterned) gravels were determined by Heller and Paola If more sedimentation takes place upstream can be deduced. (A) Required slope is built (1989) using D90 and come from eight sites than is needed to achieve the necessary slopes up atop horizontal basement. Slope is main- between central Utah and southeastern Wyo- to carry the gravel forth, there must be sub- tained during aggradation, leading to bas- ming. Data from the Ogallala Group come sidence in the proximal part of the basin to inward progradation of system. (B) As in from 10 sites across the Cheyenne Tablelands, trap the extra sediment. This situation can be A, except transport slope steepens over which run east from southern Wyoming into observed by hanging the available sedimen- time. This may result in a coarsening-upward western Nebraska (McMillan et al., 2002). tary thickness from the calculated paleoslope sequence. (C) If thickness of unit increases Our results indicate that paleoslopes for and seeing if the basement surface on which upstream beyond that needed to maintain these transport systems span the range of 10Ϫ3 these deposits are laid had a back-tilting ge- transport slope, then back-tilted subsidence to 10Ϫ4, not unlike modern river gradients for ometry (Fig. 5C). This case exists in most must have occurred. This would be true in gravelly braided streams (Church and Rood, synorogenic gravel deposits (e.g. Fig. 6), a typical foreland basin setting (Fig. 6). (D) 1983; Paola and Mohrig, 1996). Estimated un- where ¯exural or structural back tilting ac- If unit does not thicken upstream suf®cient- certainties on these values are about a factor companies source-area uplift. ly to generate needed transport slope, then of 2. The converse is true when the aggradational basement tilting must have occurred to thickness upstream is not suf®cient to generate reach transport slope ␣. Determining Tilt the needed slope to deliver gravels far out into the basin (Fig. 5D). In this case, the alterna- For the streams to reach and maintain the tive way of generating enough topography to roots or ®ne-grained cohesive sediment, and calculated paleoslopes, suf®cient topography reach the needed transport slope is by tilting where channel-®lling gravels are either mas- must have developed prior to or during de- the basement down toward the distal part of sive or at most contain crude, parallel bed- position. One way topography can be devel- the basin. The amount of tilting required is the ding. Field criterion for determining the suit- oped is by higher aggradation rates in the up- difference between the maximum preserved ability of deposits for analysis are more fully stream reaches, such as occurs during alluvial thickness and the required slope. This tilting discussed in Paola and Mohrig (1996). fan growth (Fig. 5A). In this case, early allu- can be preexisting or can occur early during Table DR1 shows our calculations for pa- vial deposition takes place preferentially up- deposition of the gravel deposits. 2 leoslopes at various sites for each unit. Shi- stream and builds out over time; otherwise, A more complex alternative case is where narump Conglomerate was collected at two deposition occurs basinwide but faster up- transport slopes are formed by aggradation in stream so that a suf®ciently steep transport the proximal basin followed by tilting and ero- 2GSA Data Repository item 2003121, data used slope forms. In the former case, sedimentation sion in the same area after deposition of the in paleoslope reconstructions, is available on the Web at http://www.geosociety.org/pubs/ft2003.htm. would downlap basinward over time. In the gravels. Such is the case for the post-orogenic Requests may also be sent to editing@geosociety. latter case, slopes would gradually increase rebound model of Heller et al. (1988) and org. upsection as relief increased. In either case, Flemings and Jordan (1989). However, in this

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phy of the river systems is shown as relief to explain the results, (1) there must have been above the most downstream known extent of several hundred meters of erosion in the prox- the gravel units. Topography is determined by imal basin to remove enough gravel to negate projecting slopes upstream from these down- the requirement of tilting, and there is no ev- stream points. Since we take topographic gra- idence of signi®cant, or any, erosion for these dient to represent depositional slopes existing units (Witkind, 1956; Yingling and Heller, at the end of aggradation, we hang the pre- 1992); and (2) the amount of erosion would served stratigraphic thickness of the gravelly have to be carefully adjusted across the basin units from this datum. In this ®gure, we show so the resultant deposit was of uniform thick- the generalized maximum stratigraphic thickness to yield the present sheet-like thickness ness using results from published measured of the units. sections (Stewart et al., 1972a; Heller and While the thickness of the gravel deposits Paola, 1989; McMillan et al., 2002). Although is insuf®cient to generate enough relief to al- the conglomerate units are locally eroded low such extensive progradation, it could be away at present, we show the deposits as con- that steep slopes were generated by differen- tinuous, which they likely were at the end of tial aggradation and/or erosion of the under- deposition. Generalizing thickness in this way lying sedimentary units. To generate topog- treats the stratigraphy as a continuous sheet raphy in this way, aggradation would have to along the cross sections, while preservation is be concentrated in the proximal part of the patchy in truth. In addition, since we are draw- basin and erosion in the distal part. This is ing an envelope larger than the reported thick- opposite to the observed patterns of erosion nesses, we err on the side of overestimating into underlying deposits. Erosional paleoval- stratigraphic thickness for these units. This is leys are primarily found beneath the upstream a conservative approach (i.e., provides a min- limits of the gravel units and are minor or ab- imum estimate of tilt), since the more aggra- sent farther downstream. dation can account for building transport Underlying units are ¯uvial to ¯uvio-deltaic, slopes, the less tectonic tilting is needed. presumably deposited at lower slopes than the Figure 7. Results for three units: (A) Shi- The results for all three units are the same: overlying gravel. This need not be true for narump Conglomerate, with average slope the preserved stratigraphic thickness of the ®ne-grained ¯uvial deposits, as transport slope of 6 ؋ 10؊4; (B) Lower Cretaceous con- gravelly units is nowhere near enough to de- can be set by sediment ¯ux and not just grain glomerate, with average slope of 1.6 ؋ 10؊3 velop a suf®cient transport slope by aggrada- size. In the absence of an estimate of gradients to west ¯attening to 2 ؋ 10؊4 eastward; (C) tion alone (Fig. 7). This is true even if mini- for underlying deposits, we suspect they were Ogallala Group (Ash Hollow Member) on mum paleoslopes are used (Fig. 7, dashed much lower than the overlying gravels, based Cheyenne Tablelands with average slope of lines). The amount of topography required be- upon: (1) their ®ne-grained nature, which does ؋ 10؊3 to west and1x10؊3 eastward. yond that of the aggradational buildup by the not require steep gradient for transport, (2) the 1.5 Generalized thicknesses of units are hung streams themselves is several hundred meters absence of gravel lenses forming an overall from this surface. Thickness is smoothed vertically over distances of several hundred coarsening-upward sequence, which would from measured sections and isopach maps kilometers (i.e. a slope of 10Ϫ3). To develop show that gradients were building over time reported in Stewart et al. (1972a) for Shi- this relief without evidence of signi®cant dif- to a point where gravel could be continuously narump Conglomerate, Heller and Paola ferential aggradation in the upstream side or transported by the evolving stream system, (1989) for Lower Cretaceous gravels, and degradation in the downstream parts of the and (3) the presence of disconformity and par- Nebraska Geological Survey (1969) for system requires tilting of the substratum. aconformity below the gravel units, indicating Ogallala Group. Two transport slopes are that gravel deposition commenced after a dis- shown. Best-®t slope (solid line) and mini- DISCUSSION crete, indeterminate period of erosion. mum transport slope (dashed line; a factor Thus, we interpret these results to indicate of 2 less than best-®t slope). Lines of cross Our results suggest that topography must that suf®cient slopes existed to transport the sections onto which data are projected are have formed prior to, or during, deposition of widespread gravels and that the most plausible shown in Figure 2. the gravel units for the slopes to have been explanation is tilting of the Earth's surface over built steeply enough to deliver gravel far into long (hundreds of kilometers) wavelengths. the basins. Whatever mechanism generates case, a signi®cant unconformity would form such slopes, it is apparently not a typical oc- Role of Climate atop the gravel units in the proximal part of currence as evidenced by the rarity of such the basin (Heller et al., 1988; Flemings and widespread gravelly deposits. Given the thin, abrupt nature of these de- Jordan, 1989), which does not seem to be the The mis®t between required slopes and posits, it might seem like rapid climate change case in our examples. available sediment thickness is so large that could play a role in their origin. The primary second-order effectsÐsuch as differential in¯uence of climate on the origin of alluvial RESULTS compaction, modest erosion of the proximal ba- stratigraphy is through four mechanisms: (1) sin deposits or how the sections are hungÐare an increase in precipitation, and therefore wa- Figure 7 shows the results of our analysis. not contenders. If signi®cant post-depositional ter ¯ux in river systems; (2) changing the For each unit studied, the calculated topogra- erosion of the proximal basin occurred, then grain-size distribution derived from the source

Geological Society of America Bulletin, September 2003 1127 Downloaded from gsabulletin.gsapubs.org on January 26, 2010 HELLER et al. area by altering the mechanisms of weather- constructing the history of slope changes in tively coarse grained. Based on detrital fossils ing; (3) changing vegetation patterns, which underlying, ®ner-grained rocks, which was not preserved within clasts of the Lower Creta- can affect bank strength and hydrograph done in this study and would require a differ- ceous gravel (Winslow and Heller, 1987; Hell- shape; and (4) by forcing increased regional ent methodology. er and Paola, 1989), the source area includes erosion that leads to isostatic rebound and Fourth, while climate-induced erosion and upper Paleozoic rocks of Nevada. These areas tilting. resultant isostatic rebound provides a possible have a suitable stratigraphy for unroo®ng to Changing water ¯ux in rivers affects river link between climate and tilting (e.g., Molnar play a major role in the evolution of the Late capacity and competence, which can generate and England, 1990), in the case of the wide- Jurassic through Early Cretaceous stratigraphy at least transient gravel transport events (Paola spread gravels, the area of deposition was over in the Rocky Mountains. Triassic rocks of Ne- et al., 1992). However, increased water ¯ux wavelengths far larger than ¯exural response vada contain many marine mud-rich units, in- does not play a role in the origin of these distances of earth lithosphere (Watts, 1992; cluding the Auld Lang Syne Group, often re- widespread gravels. Flow depth is a key ob- McKenzie and Fairhead, 1997; Stewart and ferred to informally as the ``mud-pile.'' These servation made in reconstructing paleoslope; Watts, 1997). Thus, for this mechanism to deposits sit atop Pennsylvanian-Permian age therefore, any increase in water depth has al- play a role would require broadly distributed, shelf carbonates that include secondary chert ready been accounted for in the paleoslope de- but asymmetric, erosion along most of the nodules. Erosion of this source area could lead termination. In addition, in truly braided river length of the depositional basin. Tilting at dis- to the dominantly mud-rich ¯uvial rocks of systems, changes in discharge are compensat- tances far from the eroded source areas re- the Morrison Formation. As deeper levels ed for by changing the number of active chan- quires an unusual distribution of erosion were unroofed in the source area, the Paleo- nels and, therefore, the width of the active across the basin such that some areas are erod- zoic chert-bearing carbonates could be tapped, braid plain. Such lateral variations are unim- ed deeply enough to generate signi®cant iso- providing clasts found in the Lower Creta- portant in generating the shear stress neces- static rebound, while immediately adjacent ar- ceous gravels. If this is the case, it may well sary to move gravel at any single point. If the eas continue to transport and deposit sands be that tilting began in Late Jurassic time in sheet-like distribution of the units required and gravels that record this tilt. Such a sce- this area and helped generate slopes in the multiple channel threads active at the same nario is not only unlikely and oxymoronic for Morrison Formation. This possibility could be time, then high water discharge would be re- aggradational settings, but this hypothesis has tested once a methodology for reconstructing quired. However, it is not clear whether these been tested and negated for the Ogallala slopes in ®ner-grained ¯uvial systems is units formed by multiple channels active at Group (McMillan et al., 2002). established. the same time or by the amalgamation of de- Finally, if these deposits were related to cli- posits formed by fewer channels active over a mate, it would be curious for such events to Tectonic Timing of Continental Tilting sustained period. be found only at these times and in these lo- The second possible in¯uence of climate is cations and not more broadly in North Amer- In search of a causal mechanism for large- on the grain-size distribution delivered to riv- ica or the world. It seems too geographically scale continental tilting, we note that each of ers from hill slopes. A change in weathering limited. the gravel sheets is associated with changes in process could change the relative abundance mantle density structure associated either with of gravel derived from source areas. A signif- Role of Source Area Lithology subduction or late Cenozoic arrival of warmer- icant increase in the input material grain size than-normal mantle beneath the southern would change the sorting of resultant deposits. If source areas contained a change in li- Rocky Mountains. The Shinarump Conglom- While this in¯uences the availability of grav- thology either in depth or over an area, then erate was deposited during Late Triassic time el, it does not mitigate the need for tilt. a change in abundance of gravel production (Stewart et al., 1972a; Blakey and Gubitosa, The third possible in¯uence of climate is on could re¯ect, respectively, simple unroo®ng or 1983), coincident with a transition from a pas- channel pattern. If channel braiding is primar- integration of streams into larger catchments. sive (and/or transform) margin to initiation of ily a result of reduction of bank cohesiveness Either way, lithologies capable of generating an Andean-type convergent margin in the (Parker, 1978), then climate can come into more resistant gravel-sized clasts could be southwestern USA (Walker, 1988; Dickinson, play by both decreasing the abundance of tapped over time. This is similar to the second 2000). Continental arc plutonism began after clay-sized material in the water load and by possible climatic in¯uence discussed above, deposition of the Moenkopi Formation and altering the type and abundance of vegetation and, as with that case, it has no bearing on the was commensurate with deposition of the along the channel banks. The cohesiveness of need to generate a tilt to allow these gravels Chinle Formation, of which the Shinarump is clay exerts primary in¯uence on bank stability to reach points farther out across the deposi- the basal member (Stewart et al., 1972a, b; and, therefore, channel form (Schumm and tional basin. However, it does provide an in- Armstrong and Ward, 1993; Dickinson, 2000). Thorne, 1989). Climate can also in¯uence triguing possibility that tilting began earlier Lower Cretaceous gravels were deposited vegetation types along the ¯ood plain, which than we assume and that the occurrence of sometime during Neocomian-Aptian time, can in¯uence bank stability through rooting gravel re¯ects unroo®ng sometime after tilting close to, but arguably immediately prior to, behavior. None of this negates the necessity to had started. We suspect that such may be the earliest thrusting in the Sevier belt (Fig. 2B) generate suf®cient slopes to transport the ma- case for the Lower Cretaceous gravels in the (Heller et al., 1986; Yingling and Heller, 1992; terials. The only way for this scenario to work Rocky Mountains. The underlying unit, the Currie, 1998). The onset of deformation in the is if steep slopes formed earlier than the gravel Morrison Formation of Late Jurassic age, is Sevier belt post-dated deposition of the grav- deposits and the transition into gravels record- ®ne-grained overall, but it does contain gravel els, as determined from source area prove- ed a grain-size change from unroofed sources, along its most upstream, western, exposures nance of gravel composition (Heller and Pao- not the onset of steeper gradients. However, (Peterson, 1980). In particular, the Salt Wash la, 1989; Yingling and Heller, 1992), lack of evaluating this possibility would require re- Member of the Morrison Formation is rela- contemporaneous foreland basin subsidence

1128 Geological Society of America Bulletin, September 2003 Downloaded from gsabulletin.gsapubs.org on January 26, 2010 TILTING OF THE U.S. CORDILLERA

(Heller et al., 1986), and the known chronol- raphy is transient because it results from the changes in extensional-compressional re- ogy of tectonic activity in adjacent parts of stresses created by the evolving pattern of gimes. Of the cases studied, the Shinarump Nevada (Dickinson, 2001). Also, there is no mantle convection. Dynamic topography is Conglomerate is the least compelling for dy- geologically reasonable rendition of thrust belt calculated by solving the equations for mantle namic topography because the wavelength of loading that can result in basinward tilts of convection with a speci®ed distribution of deposition is smaller and the variability of pa- suf®cient steepness or width to generate the density and viscosity throughout the Earth's leocurrents is greater than in the other cases. observed distribution of Lower Cretaceous mantle. The effect of the 660 km phase tran- However, the correlation of timing of deposi- gravels (Fig. 4; c.f. Heller et al., 1986; De- sition on mass transport across this boundary tion of the Shinarump following the onset of Celles and Giles, 1996; Currie, 1998). More can also signi®cantly modulate the time evo- subduction is consistent with isostatic adjust- likely, basin partitioning by thrust-belt defor- lution of dynamic topography, especially on ments associated with arc creation playing a mation led to local trapping of gravel deposits, short time scales (Pysklywec and Mitrovica, major role in regional tilting. More data on the such as seen in the early foreland basin ®ll of 2000). timing of Late Triassic volcanism and tecto- Utah (Fig. 6). Deposition of the Lower Cre- While the magnitude of dynamic topogra- nism in the southwestern USA and/or a better taceous gravels instead took place following a phy is model-dependent, it is now widely ac- estimate of plate kinematics at the time are period of continued eastward expansion of the cepted that most lateral variations in mantle needed to constrain dynamical models. Mesozoic continental arc in the western USA density structure can be reconstructed by in- (Armstrong and Ward, 1993; Dickinson, tegrating the last 180 m.y. of plate subduction Cretaceous Conglomerate 2001). (Richards and Engebretson, 1992). The re- As the western U.S. subduction zone The Ogallala Group was deposited eastward maining dynamic topography probably results lengthened over the next few tens of millions across the western Great Plains following in- from warm upwellings from the deep mantle of years, so too did the associated continental creased magmatic activity in Colorado and (Lithgow-Bertelloni and Silver, 1998). In the arc grow along the North American Cordillera New Mexico (Chapin and Cather, 1994), up- case of the western USA, the subduction of (Armstrong and Ward, 1993). By Cretaceous lift, and northward-propagation of the Rio 7000 km of oceanic plate over the last 180 time, the North American continent had drift- Grand Rift. In New Mexico and southern Col- Ma is well documented (Engebretson et al., ed over the large mass of negatively buoyant orado, rifting began in Oligocene time (Chap- 1992) and apparently has exerted a ®rst-order subducted slab that had accumulated since in and Cather, 1994). Farther north, the sur- control on the dynamic topography of the re- Late Triassic time. The sinking of this mass face manifestation of rifting is discontinuous, gion as documented by the sedimentary record creates lithospheric subsidence with relief of and available age data suggest a more recent (Mitrovica et al., 1989; Coakley and Gurnis, hundreds of meters over wavelengths of hun- initiation (Leat et al., 1991; Mears, 1993; Er- 1995; Burgess et al., 1997; Pysklywec and dreds of kilometers (Mitrovica et al., 1989; slev, 2001). Young uplift of the western Great Mitrovica, 2000). More controversial are the Gurnis, 1991, 1992). These tilts, approaching Plains, inferred to relate to propagation of the roles of tectonic uplift (due to increased man- 10Ϫ3, are suf®cient to promote the observed rift (Angevine and Flanagan, 1987), is coin- tle buoyancy) versus climate change in the widespread distribution of Lower Cretaceous cident with deposition of the Ogallala Group southern Rocky Mountains as the cause of de- gravels (Fig. 8B). With continued westward (Miocene) and younger ¯uvial units in that position of the post-Laramide Ogallala Group migration of the continent over the moment of area (McMillan et al., 2002). Symmetric de- (McMillan et al., 2002). accumulated slab mass in the middle, mantle position to the west of the rift was neither con- Our observations suggest that 400±800 m migrates beneath the central North American tinuous nor widespread, presumably due to in- of differential uplift occurred over areas 400± continent by Late Cretaceous time. This, co- herited topography from earlier orogeny that 800 km wide on time scales of 1±10 m.y. incident with a time of global sea level high- forced deposition into more isolated basins. These tilts are consistent with the prediction stand (Haq et al., 1987), produces the subsi- of dynamic topography from time-varying dence needed to create the Western Interior Tilt Origins subduction models (Mitrovica et al., 1989; Seaway (Fig. 8C) (Gurnis, 1992; Mitrovica et Lithgow-Bertelloni and Silver, 1998; Burgess al., 1989). It is notable that topography in- In each case, gravel sheet deposition mostly and Moresi, 1999; Pysklywec and Mitrovica, ferred for the Western Interior Seaway (a few took place inboard of the locus of tectonic ac- 2000). While it is true that the dynamic to- hundred meters of relief over a few hundred tivity and was not directly affected by asso- pography is ®ltered by the lithosphere's elastic kilometers in length [Martinson et al., 1998]) ciated structural deformation; therefore, crustal- response at wavelengths shorter than 500± is the same scale as is needed immediately scale structure played no role in regional 1000 km, we do not consider its effect im- preceding deposition of the Lower Cretaceous tilting. Tilting at wavelengths greater than a portant to our ®rst-order conclusions. gravels somewhat farther west. While we sus- few hundred kilometers can be accomplished pect that other widespread ¯uvial units depos- in a couple of ways. Isostatic topography re- Shinarump Conglomerate ited between Triassic and Cretaceous time, sults primarily from lithospheric density var- While little is known about plate kinematics speci®cally the Kayenta and Morrison for- iations. A de®ning characteristic of isostatic during Late Triassic time, we assume the Shi- mations, may be related to the same phenom- topography is that it generally changes slowly narump Conglomerate is related to the pene- ena (e.g., Lawton, 1994), paleoslope recon- in response to erosion and major orogenic/ contemporaneous onset of subduction along structions of these ®ner-grained units are not magmatic episodes. Dynamic topography, on the southwestern U.S. continental margin (Fig. yet available. the other hand, is the de¯ection of the Earth's 8A). Certainly substantial variations in iso- surface that balances the stresses applied at the static and dynamic topography are predicted Ogallala Group base of the elastic lithosphere by mantle con- during the early evolution of any subduction Finally, we relate continental tilting asso- vection (Burgess and Moresi, 1999). In con- zone as a result of initial slab emplacement, ciated with the Ogallala Group with the oc- trast to isostatic topography, dynamic topog- initiation of arc magmatism, and attendant currence of the Neogene uplift of the southern

Geological Society of America Bulletin, September 2003 1129 Downloaded from gsabulletin.gsapubs.org on January 26, 2010 HELLER et al. and central Rockies (Fig. 8D) (Eaton, 1987; McMillan et al., 2002). This uplifted region has been correlated with low-velocity upper mantle currently imaged beneath this region that is interpreted to be mantle at or near the dry peridotite solidus (Goes and van der Lee, 2002). The North American upper mantle density model of Goes and van der Lee (2002) predicts ϳ1 km of isostatic elevation difference between the southern Rocky Mountains and Kansas. This effective tilt on the order of 10Ϫ3 is suf®cient to enable transport of the gravel deposits of the Ogallala Group. We would suggest that a peak in volcanism in Colorado and New Mexico ca. 35 Ma (Mc- Millan Nancy et al., 2000), coincident with the onset of extension along the Rio Grande Rift (Chapin and Cather, 1994), marks the onset of a lithospheric warming event associated with regional uplift. At longer wavelengths, the dynamical rebound of the western USA is predicted as the North American continent drifted well west of the Farallon slab, which now re- sides in the middle mantle off the east coast of North America (Grand et al., 1997). Thus, a combination of dynamic rebound and the regional lithospheric warming event are respon- sible for the origin of the Ogallala Group, as opposed to subsidence, which led to the formation of the other two gravel sheets. The Figure 8. Interpreted relationship of mantle features to regional tilt and deposition of preservation potential of the Ogalla Group is widespread units. (A) Subsidence associated with initial underthrusting of subducted slab therefore low; continued uplift and tilting of is correlated with tilt and deposition of Shinarump Conglomerate in Late Triassic time. the deposits as the rift continues to grow leads Penetration of slab may have caused subsidence either by inducing asthenospheric coun- to erosion and reworking of the previously de- ter¯ow above subducted slab or by disrupting vertical heat loss from mantle. (B) Contin- posited materials (Heller et al., 1988). ued subduction, growth, and migration of continental arc is related to northeast-migrating tilt and depocenter recorded by Lower Cretaceous gravel units in Rocky Mountains. (C) CONCLUSIONS Continued eastward migration of tilt and Western Interior Seaway depocenter of Late Cretaceous age is tied to gradual eastward migration of low-angle subducted slab. SLÐ By combining paleohydraulic requirements sea level during Cretaceous eustatic highstand. (D) Tilting associated with progradation of gravel transport with preserved isopach ge- of Ogallala Group is interpreted to relate to regional uplift and active extension associated ometries, we analyzed the tilt history of the with evolving Rio Grande Rift in Miocene time. western USA at various times in its evolution. The presence of widespread gravels, such as those described here, indicates large-scale de- ¯ections of continental crust in areas with lit- slab subduction and mantle convection on sur- REFERENCES CITED tle preexisting topography. Such regional con- face processes. As such, paleotopography, as tinental tilts result from dynamic topography determined from calculated transport gradients Angevine, C.L., and Flanagan, K.M., 1987, Buoyant sub- created by mantle convection. The rarity of surface loading of the lithosphere in the Great Plains of sedimentary deposits, provides a means of foreland basin: Nature, v. 327, p. 137±139. these widespread units suggests that either pe- relating constructional landforms to mantle- Armstrong, R.L., and Ward, P.L., 1993, Late Triassic to riods of regional continental tilting are unusu- Earliest Eocene magmatism in the North American driven geodynamics. Cordillera: Implications for the Western Interior Basin, al or that preservation potential of these de- in Caldwell, W.G.E., and Kauffman, E.G., eds., Evo- posits is low. However, it is possible that lution of the Western Interior Basin: Geological As- ACKNOWLEDGMENTS similar thin, widespread conglomeratic units sociation of Canada Special Paper 39, p. 49±72. 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