Chronological and geochemical data from scarp and scarp-foot depression sediments (SSDS) have been used for deciphering Saharan paleoclimates and patterns of migration of anatomically modern humans (AMH). U–Th dating of thick accumulations of SSDS indicates prevalent deposition during long-lasting cool glacial periods (marine isotope stage [MIS] 6, 8, 10, and 12) and limited deposition during warm interglacial periods (MIS 5, 7 and 13). In contrast, Quaternary sediments associated with short residence, hydrologic systems in Sinai yielded MIS 2 OSL ages of 27.7 to 10.2 ka. The lack of SSDS of MIS 2 ages and the wide range of warm and cool stages in Eastern Sahara is attributed to: (1) lengthy travel times of groundwater prior to discharge and deposition of SSDS from deep groundwater originating from distant sources, and (2) sampling and dating of SSDS from depressions where natural discharge has been apparently continuous through wet and dry periods. Given that the previously dated depression SSDS could have been deposited during dry periods, and that the ages of SSDS reflect the timing of groundwater discharge rather than the ages of the wet periods during which recharge occurred, we suggest that earlier interpretations of Saharan paleoclimate and AMH migrations that heavily relied on dating of SSDS should be revisited.
Research Article| January 01, 1998 Introduction to special issues : Lithospheric structure and evolution of the Rocky Mountains (Parts I and II) Karl E. Karlstrom Karl E. Karlstrom Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A. Search for other works by this author on: GSW Google Scholar Author and Article Information Karl E. Karlstrom Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A. Publisher: University of Wyoming Received: 20 Dec 1997 Revision Received: 26 Mar 1998 Accepted: 22 Apr 1998 First Online: 03 Mar 2017 Online Issn: 1555-7340 Print Issn: 1555-7332 UW Department of Geology and Geophysics Rocky Mountain Geology (1998) 33 (2): 157–159. https://doi.org/10.2113/33.2.157 Article history Received: 20 Dec 1997 Revision Received: 26 Mar 1998 Accepted: 22 Apr 1998 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Karl E. Karlstrom; Introduction to special issues : Lithospheric structure and evolution of the Rocky Mountains (Parts I and II). Rocky Mountain Geology 1998;; 33 (2): 157–159. doi: https://doi.org/10.2113/33.2.157 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyRocky Mountain Geology Search Advanced Search Two issues (Parts I and II) of Rocky Mountain Geology (RMG) are an attempt to summarize the structure and evolution of the continental lithosphere in a Rocky Mountain transect from Wyoming to New Mexico (Fig. 1). After decades of geologic work, our understanding of the complex history of this region is still incomplete. As in many other regions, first-order questions remain about the deep structure, processes of formation, and evolution of the continental lithosphere. Fundamental new insights are most likely to come through integration of a broad range of data, involving close collaboration among geologists, geochemists, and geophysicists.... First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Cretaceous through Eocene strata of the Four Corners region provide an excellent record of changes in sediment provenance from Sevier thin-skinned thrusting through the formation of Laramide block uplifts and intra-foreland basins. During the ca. 125–50 Ma timespan, the San Juan Basin was flanked by the Sevier thrust belt to the west, the Mogollon highlands rift shoulder to the southwest, and was influenced by (ca. 75–50 Ma) Laramide tectonism, ultimately preserving a >6000 ft (>2000 m) sequence of continental, marginal-marine, and offshore marine sediments. In order to decipher the influences of these tectonic features on sediment delivery to the area, we evaluated 3228 U-Pb laser analyses from 32 detrital-zircon samples from across the entire San Juan Basin, of which 1520 analyses from 16 samples are newly reported herein. The detrital-zircon results indicate four stratigraphic intervals with internally consistent age peaks: (1) Lower Cretaceous Burro Canyon Formation, (2) Turonian (93.9–89.8 Ma) Gallup Sandstone through Campanian (83.6–72.1 Ma) Lewis Shale, (3) Campanian Pictured Cliffs Sandstone through Campanian Fruitland Formation, and (4) Campanian Kirtland Sandstone through Lower Eocene (56.0–47.8 Ma) San Jose Formation. Statistical analysis of the detrital-zircon results, in conjunction with paleocurrent data, reveals three distinct changes in sediment provenance. The first transition, between the Burro Canyon Formation and the Gallup Sandstone, reflects a change from predominantly reworked sediment from the Sevier thrust front, including uplifted Paleozoic sediments and Mesozoic eolian sandstones, to a mixed signature indicating both Sevier and Mogollon derivation. Deposition of the Pictured Cliffs Sandstone at ca. 75 Ma marks the beginning of the second transition and is indicated by the spate of near-depositional-age zircons, likely derived from the Laramide porphyry copper province of southern Arizona and southwestern New Mexico. Paleoflow indicators suggest the third change in provenance was complete by 65 Ma as recorded by the deposition of the Paleocene Ojo Alamo Sandstone. However, our new U-Pb detrital-zircon results indicate this transition initiated ∼8 m.y. earlier during deposition of the Campanian Kirtland Formation beginning ca. 73 Ma. This final change in provenance is interpreted to reflect the unroofing of surrounding Laramide basement blocks and a switch to local derivation. At this time, sediment entering the San Juan Basin was largely being generated from the nearby San Juan Mountains to the north-northwest, including uplift associated with early phases of Colorado mineral belt magmatism. Thus, the detrital-zircon spectra in the San Juan Basin document the transition from initial reworking of the Paleozoic and Mesozoic cratonal blanket to unroofing of distant basement-cored uplifts and Laramide plutonic rocks, then to more local Laramide uplifts.
Detrital sanidine dating coupled with magnetostratigraphy indicates that the Colorado River was first integrated from the Colorado Plateau to the proto-Gulf of California at least half a million years later than previously argued. In Cottonwood Valley, 40Ar/39Ar dating of a 5.37 Ma ash in pre-Colorado River axial-basin deposits plus magnetostratigraphic analyses indicate that the overlying Bouse Formation, which records arrival of the Colorado River, was deposited after the beginning of the Thvera subchron, which started at 5.24 Ma. Detrital sanidine in the Bullhead Alluvium, the first coarse-grained aggradational package of the Colorado River, indicates a maximum depositional age of 4.6 Ma for that unit in the same area. At Split Mountain Gorge, new detrital sanidine dating coupled with previously published magnetostratigraphy and detrital zircon dating of Imperial Group sediments indicate that the first Colorado River sediment arrived at the proto-Gulf of California between 4.8 and 4.63 Ma (during the C3n.2r subchron), not at 5.3 Ma as has been previously proposed. The new geochronology supports models for rapid downward integration of this continental-scale river system extending its reach from Cottonwood Valley after 5.24 Ma to the opening Gulf of California between 4.8 and 4.6 Ma. This is consistent with the previously dated 5.0-4.9 Ma Lawlor tuff interbedded in the Bouse Formation at the highest levels in the Blythe basin, which records the last filling of that basin prior to integration of the river system to the proto-Gulf of California. Additionally, the data suggest there was little or no hiatus between integration of the Colorado River, incision into the siliciclastic Bouse Formation, and initial deposition of the Bullhead Alluvium, which seems to be a response to rapid profile changes caused by integration.
Research Article| July 01, 1986 Early Proterozoic geology of Arizona Karl E. Karlstrom; Karl E. Karlstrom 1Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011 Search for other works by this author on: GSW Google Scholar Clay M. Conway Clay M. Conway 2U.S. Geological Survey, 2255 N. Gemini Drive, Flagstaff, Arizona 86001 Search for other works by this author on: GSW Google Scholar Author and Article Information Karl E. Karlstrom 1Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011 Clay M. Conway 2U.S. Geological Survey, 2255 N. Gemini Drive, Flagstaff, Arizona 86001 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1986) 14 (7): 625. https://doi.org/10.1130/0091-7613(1986)14<625:EPGOA>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Karl E. Karlstrom, Clay M. Conway; Early Proterozoic geology of Arizona. Geology 1986;; 14 (7): 625. doi: https://doi.org/10.1130/0091-7613(1986)14<625:EPGOA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract No Abstract Available. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
