Laramide orogeny

Much of this guide was published originally for the 1976 Annual Meeting of the Geological Society of America:

Matthews, V.; Work, D.F.; LeMasurier, W.; Stearns, D.W., 1976, Mechanism of Laramide deformation along the northeastern flank of the Front Range (Road Log), in Epis, R.C. and Weimer, R.D., ed., Studies in Colorado Field Geology; Colo. Sch. of Mines Prof. Contr. No. 8, p. 386-397.

This version differs in several ways:
1. Color images
2. GPS locations for stops
3. Numerous updates over the four decades in which a wide assortment of groups and associations were taken on this trip by the senior author.  The final trip was led in the summer of 2016 for invited scientists from the USGS, CSM, and CSU.

The Laramide orogeny is a classic yet controversial mountain-building event that resulted , in the southwest United States, in uplifts , sedimentation, and magmatism that can be used to constrain the onset of this event in the region and expand our knowledge of Late Cretaceous to Paleogene tectonism. The McRae Formation marks the onset of depo-sition in the Laramide Love Ranch Basin, which was located to the northeast of the west-northwest–trending coeval Rio Grande uplift in south-central New Mexico, but its age is not well constrained. A previously published late Maastrichtian age for the McRae Formation was based on the presence of dinosaur bones in the upper of two members of the formation. We obtained new U-Pb dates from one dacite clast and three ash-fall tuffs from the lower Jose Creek Member and from one ash-fall tuff from the lower part of the overlying Hall Lake Member of the McRae Formation. The clast yielded a date of 75.0 ± 1.1 Ma, whereas the ages of the tuffs, in ascending stratigraphic order, are 74.9 ± 0.7 Ma, 74.7 ± 0.6 Ma, 75.2 ± 1.3 Ma, and 73.2 ± 0.7 Ma. These dates indicate that the onset of Laramide deposition in the Love Ranch Basin must have occurred earlier, in late Campanian time, similar to deposition in the Laramide Ringbone Basin in southwestern New Mexico. In addition, U-Pb zircon dates of 75.7 ± 1.3 Ma and 75.0 ± 2.8 Ma were obtained on the Twin Peaks stock and on a da-cite sill, respectively, in the Burro Mountains of southwestern New Mexico. These dates are similar to those of other Laramide arc magmatic centers in southern New Mexico, which have a limited range of ages from 75 to 70 Ma, including the Hidalgo Formation in the Little Hatchet Mountains, the Silver City–Pinos Altos region, and the Copper Flat porphyry system. These new and previously published dates indicate that during the onset of Laramide deformation in southwestern and south-central New Mexico, the angle of subduction of the Farallon plate may have been steep enough to allow partial melting of an asthenospheric wedge, resulting in arc magmatism far inboard of the trench.

The Paleogene sedimentary deposits of the Colorado Headwaters Basin provide a detailed proxy record of regional deformation and basin subsidence during the Laramide orogeny in north-central Colorado and southern Wyoming. This fi eld trip presents extensive evidence from sedimentology, stratigraphy, structure, palynology, and isotope geochronology that shows a complex history that is markedly different from other Laramide synorogenic basins in the vicinity.

The only occurrence of monoclines along the eastern flank of the Colorado Front Range is near Llvermore at the northern end of the tten echelonrl fold-and-fault belt (Fig. l). The monocl ines I ie along the Kawuneeche-Livermore Iineament (h/ork and tlatthev,rs, in preparatlon). sedimentary rocks elsewhere along the eastern flank of the Front Range were deformed into either homoclines, anticllnes, or syn-clines during the Laramide orogeny. Erosion has removed the sedlmentary rock cover from the Precambrian basement on the western edge of the area of monoclinal folding. Thus, the area pro-vldes an unusual opportunlty for studying the role of structural basement in the formation of the monocllnes. Our study comblned detalled field work with photo-interpretation of NASA U-2 photography stereo-pa I rs. DESCR I PT ION OF I1ONOCL lNES Northeast of Llvermore, Colorado the Paleozoic and l.lesozoic strata are folded into a series of narrov't linear monoclines (Flgs. 2r3)" The monoctines separate large areas where the strata are planar and lilanuscript received, February 18, 1976; accepted, llarch 3, 1976. fhe Mowttain Geologiet u. 73, no. 2 (Aprti lszol p. 61-66 6]. ABSTMCT: Honoclinal foldlng is the dominant style of deformation in Paleozoic and }lesozolc sedimentary strata northeast of Livermore, Colorado. The markedly llnear monocllnes are narrow and bound large areas of nearly flat-lylng strata. Axes of the monoclines are not parallel, which indlcates that they did not form as a resutt of reglonal horizontal compression. Several mlles to the west, eroslon has stripped the sedimentary cover from the underlylng Precambrian basement. The basement is characterized by brittle deformation in the form of dlfferentlally upllfted fault blocks. The pre-Pennsylvanian erosion surface developed on top of the Precambrlan igneous and metamorphlc rocks Is only sllghtly tilted. Where the faults ln the basement pass under the Paleozoic sedimentary cover to the east, the sedlmentary rocks are folded into unfaulted monoclines. This suggests that the monocllnes in the area are formed by passive draping of sedimentary rocks over dlfferentially uplifted basement blocks. Such an interpretation ls conslstent with the geometrlcal relationships of the monocllnes, with burlal conditlons durlng the Laramlde orogeny, and with experimental results on the behavior of rock materlals.

Debate over the question of whether basement rocks folded or faulted during the Laramide orogeny in the Wyoming province of the Rocky Mountain foreland has raged for many years. Proponentsbl one view or the olher have drawn support for their case from theorctical arguments, from experimental laboratory results, from interpretations of seismic and subsurface borehole data, and from direct observation of surlace outcrops and subsurface miie exposu/es. An anatysis of the validity and applicability ot these various approaches included the generation of synthetic ielsmrc sections and an evaluation of balanced cross sectlons, This analysis indicated that the notion of a folded basement is supported only bl the interpretation of seismic and borehole data whereas the concept of brittle defotmation of the basementls s upportea by ilt of thes; tines of rcasoning, interprctation, and observation. The conclusion is that structural basement did not fotd during the Laramide orogeny.

Seismogenic sedimentary structures—seismites—provide an opportunity to understand the effects of syndepositional tectonics on lacustrine sedimentation. The lowermost Wilkins Peak Member (Eocene Green River Formation, Wyoming, USA) contains laterally extensive intervals displaying folds, load structures, mixed brittle–ductile deformation, microfaults, breccias and sedimentary dykes, which formed intrastratally in a low-energy, sublittoral lacustrine environment. They developed in situ by liquefaction and plastic or brittle behaviour of semi-lithified sediment, or by remobilization and injection of granular material. Their morphological attributes and lateral extent, zonation in deformation intensity, and the depositional setting all imply deformation triggered by earthquake-induced stresses in shallow-buried sediment. Their stratigraphic distribution coincides with a shift in hydrological conditions from a balanced-filled to an underfilled lake type, independent of climate change. This study is the first to ascribe seismites to a pulse of tectonic activity that caused a synchronous change in lacustrine evolution.

The northeastern flank of the Front Range uplift is characterized by three distinct styles of Laramide and post-Laramide deformation: a northern area characterized by differential uplift of basement blocks with associated monoclinal folding in the overlying sedimentary strata; a central area characterized by . .