Abstract Mesas and buttes of the central Colorado Piedmont are composed of at least twodistinct rock types, which differ in their cohesiveness and resistance to erosion. Thelower parts of the exposed stratigraphic section are poorly cemented, Upper Cretaceousto Middle Eocene sandstones of the Dawson Formation. The caprocks arecomposed of one or more resistant formations of Late Eocene age: the Castle RockConglomerate, Wall Mountain Tuff, and the conglomerate of Larkspur Butte. Theseformations were originally deposited in topographic lows, but due to their resistance,they now cap prominent buttes and mesas of the Colorado Piedmont. Erosion of thecaprock through progressive retreat of the butte scarp produces colluvium that has ahigher resistance to erosion than the poorly cemented underlying sandstone. Once the caprock of a butte has been removed by erosion, the underlying weaklycemented Dawson Formation is readily eroded. Ultimately, the armored lower slopesof the former butte remain as a circular ridge standing as much as 100 m above thesurrounding topography. This process produces a topographic low surrounded byrelict faceted slopes where the flat top of the butte once stood. Prominent alluvial fans are associated with some of these annular features, andthey record the main phases of butte removal and excavation of the central part of thearmored slopes. Multiple generations of alluvial fans contain coarse- and fine-grainedfacies that represent changes in effective stream power and record alternating phasesof aggradation and erosion. The degree of soil development in the fan alluvium andheight of the fan surfaces above streams indicates the oldest preserved gravel fandeposit is of late-middle Pleistocene age. The youngest luminescence (optically stimulatedluminescence) dated alluvial fans were deposited during the late Pleistoceneabout the time of the Pinedale glacial maximum in Colorado, ca. 21,000 yr B.P. Keywords: Colorado Piedmont, talus flatiron, talus flatiron ring, inverted topography.

Abstract Location and Access . Four localities north of Fort Collins, Colorado (Fig. 1) display many of the structural features found throughout the Wyoming province of the Rocky Mountain Foreland. All of the localities can be viewed from public roads that are easily accessible to passenger cars or buses. Significance of Locality . The northeastern flank of the Front Range displays several varieties of Laramide folds as well as critical relationships for demonstrating that the folds in the sedimentary rocks are forced folds resulting from differential uplift of faulted blocks of Precambrian basement. Within this smallarea are several beautifully exposed monoclines; differentially up-lifted blocks (both tilted and untilted) of Precambrian basement;and well-displayed asymmetrical anticlines, synclines, domes, andbasins. The area is also cut by a major lineament that separatestwo major structural styles in the Front Range and Denver Basin(Matthews, 1976). In this small area, one can see most of the crucial relationships which bear on the controversy over the nature and origin of Laramide deformation in the Rocky Mountain Foreland Province. The sites are described in more detail in Matthews and others (1976), Matthews and Sherman (1976), and Matthews and Work (1978).

The structural basement in the Rocky Mountains foreland deforms by faulting and rigid-body rotations. The faults at the interface between the sedimentary and crystalline rocks can be anything from low-angle reverse to normal faults. Despite the range of geometries, the total assemblage of faults and rotations is best explained by a movement system that is dominated by vertical motions along faults, many of which are curved in cross section. The first causes deep within the crust are not sufficiently well documented in the geophysical record to justify a firm interpretation. However, there are certain conditions recorded in the geologic history of the region and in the surface structures that place constraints even on speculations. With these constraints, vertical movements seem more likely to dominate than horizontal movements. The sedimentary layers are deformed primarily by forced folding. Their final geometry is a product of several parameters such as welding and stratigraphic make-up. Measurements on natural folds demand that the section either thinned or detached. Detachment without appreciable thinning further requires that (1) large displacements occur within the sedimentary section and (2) at the termination of these folds, the movement must be in several directions. Geologists’ intuition as to how the layered rocks achieved their shapes is not always correct, but field data combined with experimental and theoretical data provide a basis for understanding these folds. It is concluded that the structural style in the Rocky Mountains foreland is not unique, but rather it is only an excellent example of a more universal class of deformation, namely, forced folding.

The use of reflection seismic data is necessary for the proper interpretation of buried block fault, drape-fold structural features. By the application of certain basic principles, it is possible to make reasonable structural interpretations in areas where seismic information is of poor quality or where complete coverage cannot be acquired. A seismic-model study and a number of seismic sections from various Wyoming basins illustrate the capabilities and limitations of reflection seismic techniques for defining block fault, drape-fold structures.

The vertical basement-block movements and overlying folds in the sedimentary veneer that characterize the deformation style of the Wyoming province are amenable to simple two-dimensional kinematic analyses. These analyses examine the lateral transport needed when no thickness change of the folded sedimentary rocks occurs, and they are based on simple geometric constructions that approximate the folded shape of the uppermost bedding surfaces of the Mesozoic and Paleozoic sections (about 3,900-m total thickness). Lateral-displacement requirements of these rocks are significantly different, and this difference implies that the rocks on the downthrown basement block are put into a state of shear. Additionally, these differences suggest that significant volume problems are created within the fold during the folding process. The first models show that, within the Paleozoic carbonate section (450 m thick), lateral displacements of the top and bottom bedding surfaces differ by nearly 100% at one point during folding but are nearly identical at other times. Refinements of the simple models indicate that some stratigraphic horizons in folds with 1,200 m or less displacement over vertical faults in the basement may not require large lateral movements. If several bedding-plane detachments were present in the Paleozoic carbonate blocks, volume problems within the hinge zones could be significantly reduced. Geometrically scaled laboratory specimens were experimentally deformed, and their final geometry is comparable to that of natural counterparts. Comparison of calculated and observed lateral displacements agree within 7%, based on the simplest analytical model, but only agree within 50% for another model. It is concluded that the kinematic approach to drape folding is useful, because it provides information about the required lateral displacements of large rock masses during folding. It also illuminates other problems that must be investigated before a better understanding of drape-fold structures is possible.

Experimental studies of faulted drape folds differ significantly from traditional tests on short, right-circular cylinders because of the presence of deliberately introduced heterogeneous stress and strain fields. These experiments differ from other experiments on the folding problem (for example, buckling) in that (1) a well-developed analytical basis was not formulated when the study was initiated and (2) the experimental program was designed to gain insight into problems posed from field relations. Experimental data are only one tool available to the geologist, and they are best utilized in conjunction with theoretical and/or scale-model investigations. Coupling of these different approaches aids extrapolation of the results to nature. Specifically, the problem associated with geometric scaling becomes more tractable when analytic studies are used to interpret the experimental results. Differences of deformation mechanisms with scale and rock type require careful consideration, but even here experiments can provide insights into critical field problems. Some of these problems are exemplified by studies of mass transport, corner drape folding, and differences between high-angle and vertical faulting in the forcing member. In all these examples, the most valuable use of the experiments is perhaps to provide insights into which processes are truly critical. Experiments can guide and narrow the field investigation to critical questions, which may in turn be answered even better by further relevant experiments, suggested by the field work.

Folds and faults along the northeastern flank of the Front Range were examined using NASA (U.S. National Aeronautics and Space Administration) high-altitude photographs, published geologic maps, and new field studies. Detailed analysis of each structure reveals that block faulting of brittle basement is the dominant tectonic feature within the area. Twenty-three of the twenty-six folds were created by passive draping of sedimentary rocks over the edges of basement blocks. The other three folds were caused by local compression generated by anomalously tilted blocks.

Two areas in the Big Horn Mountains are given as examples of the beauty and simplicity of the drape-folding-over-basement-block model of structural interpretation. The Piney Creek thrust is interpreted as an uplifted fault block with sedimentary strata draped over its edges. Structural cross sections of the Horn area show how sedimentary layers are continuously folded over a faulted basement. In both areas, the role of a ductile stratigraphic unit, the Gros Ventre shale of Cambrian age, is important in partially decoupling the basement from the overlying sedimentary veneer and in providing a slide surface for gravity phenomena. Gravity glide masses probably are more widespread than is generally recognized in the foreland province and should be considered a characteristic element of the foreland tectonic style. The construction of three-dimensional models of the basement surface, based on detailed geologic mapping, and the comparison of observed structures in the sedimentary cover rocks with experimental draping over the model are effective tools for gaining a mechanical understanding of the development of structures in the foreland province.

Excellent exposures in an area of multiple drape folds in the northwest corner of the Big Horn Mountains provide data for the construction of a three-dimensional model of the upper surface of the Mississippian Madison Formation. In the model, the vertical and horizontal scales are the same. Comparison of deformed and undeformed Madison surfaces indicate that displacements required by the deformation vary from place to place on the surface in both magnitude and direction. Calculations made from field measurements show that there is not enough fracturing or thinning in the Madison layer to account for the indicated displacements. Also, reasonable shortening on the basement faults underlying the forced folds cannot account for the calculated displacements in the sheet. Any solution to the mechanics of the drape-folding process must, therefore, meet the described constraints and still account for the indicated nonuniform displacements. Detachment of the sedimentary layers from the basement and intrastratal slip are two processes that meet the constraints and still account for the indicated nonuniform displacements.

Analysis of the geometric relationships at Elk Mountain, Wyoming, indicates that the anticline is a forced fold that formed by the draping of ductile sedimentary strata over an uplifted and rotated basement block that is broken into three secondary blocks. Orientation of structures in the region seem more compatible with a stress field generated by a vertical force rather than an east-west, horizontal compressive force.

The 290-km-long Black Hills uplift extends from the South Dakota-Nebraska border to southeast Montana. A structurally higher east block of northward trend and a northwest-trending west block are separated by the west-facing Fanny Peak monocline. Paleozoic, Mesozoic, and Paleocene sedimentary rocks 2,220 to 3,000 m thick overlaid the anisotropic Precambrian igneous, metasedimentary, and metaigneous basement during Laramide uplift. Draping of the sedimentary section over basement fault blocks produced monoclines with, opposed to, and parallel to regional dip and ramps, terraces, and anticlines as subsidiary structures. Individual monoclines as much as 170 km long consist of linear segments commonly joined in gentle arcs. The dominant fold trend is northward, although one major segment of the Black Hills monocline trends northwest. The folds terminate at intersections with other monoclines, by decreasing stratigraphic offset along strike, or by splaying into several folds of lesser structural relief. Gravity data suggest single basement faults beneath steep, narrow folds and multiple faults beneath wider structures. In a single locality the basement is exposed in fault contact with Paleozoic rocks that show a narrow zone of rotation in both fault blocks. Normal and reverse faults locally extend as high as the massive Mississippian and Pennsylvanian carbonate rocks, but elsewhere these units are folded into five linear segments. Mesozoic siltstone and shale are believed to be more rounded in section view due to flexural flow. In the inclined limb conjugate fault sets and horizontal tension cracks affect the brittle rocks, and ductile layers are thinned by flexural flow. The uplift is separated from the Powder River Basin on the west by the Black Hills and the Fanny Peak monoclines which have structural relief as great as 1,670 m and dips as great as 90°. Gently inclined planar sedimentary units, inferred to parallel the surface of the basement, show changes of a few degrees dip across the folds. Such differences may result from rotation of rigid blocks of the anisotropic basement along curved faults in the style outlined by Stearns (1975). The arcuate eastern boundary with the Interior Lowlands province forms a partial dome. Strain was distributed across at least 32 km, possibly by shear along the Precambrian schistosity. North- and east-trending folds, believed to reflect longitudinal and transverse faults in the arched basement, locally parallel the basement fabric. At this easternmost margin of the Wyoming province, the uplift and adjoining province were both elevated, although unequally, whereas the basin to the west of the uplift was an area of subsidence. This difference in absolute movement patterns is believed responsible for the asymmetry and composite structural style of the Black Hills uplift.

The origin of strike-slip faults in the Rocky Mountains foreland is somewhat of an enigma; these faults occur in an area where the basement deformation has primarily involved vertical movements. Some of these faults may have originated not from lateral shifts within the crystalline basement, but from an interaction of detached and laterally shifting layers within the sedimentary pile and from the dynamics of drape folding. The folds and related structures of Dinosaur National Monument of Colorado and Utah suggest such an origin for some strike-slip faults. These folds and related features evolved with the uplift and brittle deformation of the Uinta Mountains. Uplift, tilting, and extension along the south flank of this range ruptured the basement core into an array of rigid blocks. During this deformation, the Weber Formation became detached and slid between 1,000 and 1,500 m southward. Where the formation encountered rising steps (fault blocks) in the basement surface that were aligned normal to the sliding direction, it became draped and often thickened. Where steps did not exist, the lateral sliding was accommodated by buckle-type folding. In one region, however, the combination of lateral sliding, drape folding, and the effects of a low, inclined ramp structure caused the Weber Formation to tear in a small strike-slip fault with possibly 1,000 to 1,500 m of offset.

Geologic relationships in the Grand Canyon region demonstrate a spatial, genetic association of monoclines with reactivated, ancient, steeply dipping faults. If one assumes that these relationships hold for monoclines throughout the Colorado Plateau, the monocline pattern, in total, records the upper-crustal expression of many elements of the regional, basement-fault system. The map pattern of monoclines in northern Arizona seems to bear this out, for the curvilinear and branching nature of individual monoclines within an overall angular, orthogonal pattern implicitly favors a relationship of monoclines to basement faults. Analysis of the monocline pattern of the Plateau, in total, demands the additional but less obvious interpretation that many of the basement faults extend well beyond areas where their traces have clear-cut monoclinal expression. Specifically, the monocline (that is, basement-fault) segments can be fit to a systematic, interdependent network of lineaments, regional in extent, inferred to represent traces of basement-fracture zones, the loci of steeply dipping planar elements such as shear zones, faults, jointing, and lithologic contacts in basement rocks. Two sets of major basement fracture zones are recognized in the monocline pattern, and these trend N20°W and N55°E. The fracture zones serve to subdivide the basement of the Plateau into a mosaic of crustal blocks, some of which moved differentially during the time(s) of monoclinal folding. The distribution of uplifts in the Colorado Plateau is seen to be very systematic when viewed in the context of a basement partitioned by basement-fracture zones. Most of the monoclinal folding took place in the Laramide, a time of northeast-southwest compression in the Plateau. As a response to the regional compression, individual basement blocks within the mosaic were uplifted by reverse movements along segments of the high-angle fracture zones. Modeling of the deformation serves to emphasize that a very small amount of horizontal crustal shortening can produce significant structural relief when the shortening is accomplished along relatively widely spaced, steeply dipping faults.

The Palisades Creek branch of the East Kaibab monocline in Grand Canyon National Park, Arizona, can be divided into three structural levels with contrasting styles of deformation: a lower level composed of a vertical fault, the Palisades fault, and a sharp synclinal bend; an intermediate level characterized by a tight, steep monoclinal flexure; and an upper level dominated by an open monoclinal flexure. The three styles of deformation apparently were controlled by a combination of position relative to the fault and of structural behavior of various rock units. The profile of the monocline at depth might have been estimated by study of the profile of the monocline at the surface, but only if data concerning structural units were available. Study of the internal strain and accurate mapping of the gross structure provide fundamental data for a theoretical model. Analysis of small-scale structures, including faults and folds as well as calcite twinning and thicknesses of rock units, indicates the internal strain of the rocks during monoclinal flexuring. Most strain indicators are consistent and imply shortening subparallel to layering at all levels within the monocline. An accurate cross section and local measurements of thicknesses of units indicate that layers in the monocline have changed thickness appreciably. A general model of monocline formation must incorporate effects of layer-parallel shortening as well as differential vertical displacement along the underlying fault. Such a model is presented in Part II (this volume).

Monoclinal flexures, which are isolated asymmetric flexures, range in scale from a few millimetres in kink bands to hundreds of metres in monoclines on the Colorado Plateau. A general model of monoclinal flexuring of multilayers is proposed here; the multilayers include layers with various rheologies, densities, thicknesses, and strengths of contacts between layers. The multilayers are subjected to displacements at their base, stresses at their edges, and a free surface at their tops. We study in detail three modes of this general model, assuming linear, incompressible elastic or viscous multilayers: Drape folding, in which a monoclinal flexure develops over a vertical fault; buckling, in which an initial monoclinal flexure is amplified by layer-parallel compression; and kinking, in which monoclinal kink bands develop unstably by compression inclined to the layering. Selected solutions are presented for the first two modes, and previous research is summarized for the kinking mode. According to analyses of the three special cases of the general model, the profile of the monoclinal flexure, the displacement field, and the strain distribution within the flexure are useful criteria for distinguishing among the three modes of monoclinal flexuring. The Palisades monocline, described in detail in Part I (this volume), is interpreted to be a result of a combination of drape folding over a fault in Precambrian basement rocks and buckling, which together appear to account for most of the field observations. The Yampa monocline in Dinosaur National Park changes form along its length, but each form can be compared with characteristics of a combination of modes, including faulting at depth and layer-parallel compression. In some places it closely resembles a large kink band.

This paper presents conceptual models of basement configurations for several structural types in the Rocky Mountains foreland—upthrusts, rotated basement blocks, and plateau uplifts. These hypotheses of structural development are supported by calculated stress states and fracture patterns derived from them. The goodness-of-fit between field observations and the final geometries of the models leads to the suggestion of possible loading conditions for the initiation of Laramide structural development of uplifts in the foreland. These suggested loading conditions for individual features lead, in turn, to a hypothesized stress state consisting of a regional component of crustal horizontal compression superposed on the controlling stress fields caused by more local load variations at depth beneath the brittle upper crust.

The inexact geometric fits of the conceptual models of Couples and Stearns (this volume) are due largely to idealizations required by the theoretical analysis. The primary reasons for the mismatch are the assumptions of continuity and isotropy. Real rocks are not adequately described by these properties. Additional considerations of the state of stress—that is, stability index and principal-stress reorientation—are interesting in themselves but do not seem significantly to affect the construction of models using the theoretical solutions. On the basis of information currently available, the mechanical models of Couples and Stearns appear reasonably sound.

In terms of plate tectonics, most orogenic belts are arc, collision, or transform orogens marked by regional batholiths, overthrust nappes, and en echelon fold trains, respectively. None of these models fits the crustal buckling of the classic Laramide orogeny, marked in the central Rocky Mountains by fault-bounded, basement-cored uplifts separated by intervening sediment-filled basins. Reported patterns of current seismicity, volcanism, and deformation in the modern central Andes document two modes of subduction; one involves plate descent at an abnormally shallow angle and may simulate Laramide conditions. In the more familiar mode, a plate descending steeply into the asthenosphere beneath the continental margin generates standard arc morphology with an active volcanic chain; crustal seismicity outside the subduction zone near the trench is confined mainly to a back-arc fold-thrust belt. In the unfamiliar mode, the descending slab of lithosphere slides along under the overriding plate of lithosphere, with which contact is maintained; crustal earthquakes are widespread across the dormant arc massif, within which local block uplifts bounded by reverse faults are prominent, and magmatism is meanwhile suppressed because the asthenosphere is never penetrated by the descending slab. The largely amagmatic Laramide style of deformation can be ascribed to the dynamic effects of an overlapped plate scraping beneath the Cordillera. That inference is strongly supported by the close correlation, in both space and time, between a prominent magmatic null or gap in the western Cordillera and the classic Laramide orogeny in the eastern Cordillera.