ABSTRACT In this study, we determined the timing of burial and subsequent exhumation of Barrovian metamorphic rocks from the Chloride Cliff area of the Funeral Mountains in southeastern California by constraining the ages of different portions of a pressure-temperature ( P-T ) path. Using a split-stream laser-ablation inductively coupled plasma–mass spectrometry (ICP-MS) system, we analyzed 192 domains from 35 grains of monazite within five samples with a spot size of 8 µm to determine U-Pb ages and trace-element abundances from the same samples (same polished sections) that were analyzed to produce the P-T paths. Changes that took place within individual monazite grains reflect localized equilibrium and captured the changes in heavy rare earth element (HREE) abundances in the matrix reservoir that occurred as garnet grew, resorbed, and then regrew, thus constraining ages on different portions of the P-T path. The results show that garnet began growing ca. 168 Ma, began resorbing ca. 160 Ma, began retrograde regrowth ca. 157 Ma, and continued to regrow at least through ca. 143 Ma. The early garnet growth corresponds to a period of pressure increase along the P-T path. The subsequent partial resorption corresponds to the prograde crossing of a garnet-consuming reaction during decompression, and the retrograde garnet regrowth occurred when this same reaction was recrossed in the retrograde sense during further decompression. These results are consistent with previously determined ages, which include a Lu-Hf garnet age of 167.3 ± 0.72 Ma for the early pressure-increase portion of the P-T path, and 40 Ar/ 39 Ar muscovite cooling ages of 153 and 146 Ma in the lower-grade Indian Pass area 10 km southeast of Chloride Cliff. The 40 Ar/ 39 Ar muscovite ages document cooling at the same time as retrograde garnet regrowth was taking place at Chloride Cliff. The oldest monazite age obtained in this study, 176 ± 5 Ma, suggests that southeast-directed thrusting within the Jurassic retroarc was ongoing by this time along the California portion of the western North American plate margin, as a consequence of east-dipping subduction and/or arc collision. The Funeral Mountains were likely located on the east side of the northern Sierra Nevada range in the Jurassic, taking into account dextral strike-slip displacement along the Cretaceous Mojave–Snow Lake fault. The Late Jurassic timing of burial in the Funeral Mountains and its Jurassic location suggest burial was associated with the East Sierran thrust system. The timing of prograde garnet resorption during exhumation (160–157 Ma) corresponds to a change from regional dextral transpression to sinistral transtension along the Jurassic plate margin inferred to have occurred ca. 157 Ma. The recorded exhumation was concurrent with intrusion of the 148 Ma Independence dike swarm in the eastern Sierra Nevada and Mojave regions, which developed within a regime of northeast-southwest extension.

ABSTRACT The vertebrate paleontology, lithostratigraphies, and depositional environments of the Cenozoic continental Titus Canyon and Furnace Creek Formations have been the subjects of several recent investigations. The two units are exposed in the Amargosa Range in northeastern Death Valley National Park, Inyo County, southeastern California, USA. Fossil tracks and trackways are preserved in playa mudflat deposits of the Pliocene Furnace Creek Formation at the Cow Creek tracksite on the western slope of the central Funeral Mountains. The tracksite includes footprints of birds and land mammals, as well as associated sedimentary structures. The lower red beds of the Titus Canyon Formation have produced numerous fossilized bones and teeth at Titus and upper Titanothere Canyons in the southeastern half of the Grapevine Mountains. The fossil remains represent 17 extinct genera and species of land mammals and one genus and species of pond turtle. The taxa constitute the Titus Canyon Fauna. The rodents Quadratomus ? gigans and Dolocylindrodon texanus , the bear dog Daphoenictis n. sp. (small), and the tapir Colodon stovalli are associated elsewhere only in the correlative, late early late Duchesnean Upper Porvenir Local Fauna of Trans-Pecos or Far West Texas. The local fauna occurs in the Blue Cliff Horizon (i.e., above lower marker bed) in the lower part of the Chambers Tuff Formation. The two assemblages share 12 species. The age of the latter unit is constrained by corrected single-crystal laser-fusion 40 Ar/ 39 Ar dates of 37.83 ± 0.09 Ma for the underlying Buckshot Ignimbrite and 37.14 ± 0.08 Ma for the overlying Bracks Rhyolite. However, both determinations should be considered tentative and subject to change with further investigation. The first green conglomerate unit of the Titus Canyon Formation overlies the lower red beds, underlies the Monarch Canyon Tuff Bed, and has produced the first records of land mammal footprints and a land plant (petrified palm wood) from the formation. The Monarch Canyon Tuff Bed and the Unit 38 Tuff Bed, which lies at the mutual tops of the upper “red beds” and the Titus Canyon Formation, are 34.7 ± 0.7 m.y. old and 30.4 ± 0.6 m.y. old, respectively, based on recalculated 40 Ar/ 39 Ar dates. Consequently, the Titus Canyon Formation is latest middle Eocene to earliest Oligocene in age, according to the 2020 Paleogene time scale.

Prepared in conjunction with the 2022 GSA Cordilleran/Rocky Mountain Sections Joint Meeting, this Field Guide showcases trips to geologically interesting areas in Arizona, Nevada, and California. Enjoy a three-day trip to the Buckskin-Rawhide and northern Plomosa Mountains metamorphic core complexes in Arizona. In Nevada, learn about the geology of Frenchman Mountain and Rainbow Gardens and landslide deposits and mechanisms in the eastern Spring Mountains. Or learn about microbialites in Miocene and modern lakes near Las Vegas. When weather permits, unravel the geological history of southern Death Valley, and explore vertebrate paleontology and Cenozoic depositional environments in Death Valley, California.

Abstract Biological soil crusts, composed of soil surfaces stabilized by a consortium of cyanobacteria, algae, fungi, lichens, and/or bryophytes, are common in most deserts and perform functions of primary productivity, nitrogen fixation, nutrient cycling, water redistribution, and soil stabilization. The crusts are highly susceptible to disturbance. The degree of perturbation is governed, at least in part, by the nature, intensity, and spatial and temporal distribution of the disturbance, as well as the soil type and soil moisture content at the time of disturbance. When disturbed, biological soil crusts lose their capacity to perform their ecological functions. Natural recovery of disturbed crusts can range from several years to millennia. Several strategies have been attempted to accelerate recovery of crusts. At present, artificial recovery is not economically feasible on large tracts of disturbed desert landscape. Management options available to the military on arid landscapes include: (1) eliminating or minimizing training in desert ecosystems, (2) avoiding critical seasons, (3) avoiding critical areas, (4) artificially restoring damaged crusts, and (5) considering desert training lands as “sacrifice areas.” Given the need to train in environments representative of the locations of many current and projected world conflicts, the first option is untenable. At this time, the most plausible alternative is to consider desert training lands as “sacrifice areas.” However, it is recommended that attempts be made to avoid critical seasons and areas inasmuch as logistically feasible, and that the military continue to support research into the development of cost-effective technologies for biological soil crust restoration.