Rocky Mountain Geology 36.1

Neoproterozoic kimberlite emplacement in the Front Range, Colorado

A. P. LESTER, E. E. LARSON, G. L. FARMER, C. R. STERN AND J. A. FUNK

A paucity of isotopic age determinations for the more than 100 kimberlitic diatremes, dikes, and sills within the Colorado-Wyoming kimberlite province has fostered the prevailing notion that emplacement of these bodies occurred during a discrete interval of ultramafic magmatism spanning Early to Late Devonian time. However, new geochronologic data for two kimberlite bodies (Chicken Park and Green Mountain kimberlites) support a Neoproterozoic emplacement age for these bodies.

⁴⁰Ar/³⁹Ar analysis of fine-grained matrix phlogopite from the Chicken Park dike indicates emplacement between 620 and 640 Ma. ⁴⁰Ar/³⁹Ar dating of the Green Mountain diatreme, utilizing both phlogopite megacrysts and mineral separates from xenoliths, is somewhat equivocal because of excess Ar, but suggests emplacement between 500 and 800 Ma. A Sm-Nd isochron, based on megacrystic phases from the Green Mountain diatreme, yields an age of 572 ±49 Ma.

Neoproterozoic emplacement indicates that the Colorado-Wyoming kimberlite province apparently has undergone multiple intervals of kimberlitic magmatism, similar to other kimberlite provinces worldwide. The episodes of kimberlite emplacement coincide with periods of relative orogenic and magmatic quiescence in Colorado and Wyoming.

Keywords: kimberlites, geochronology, Front Range, Colorado

Lithospheric buckling of the Laramide foreland during late Cretaceous and Paleogene, western United States

B. Tikoff and J. Maxson

Foreland deformation, in the form of arches or uplifts, occurred throughout the eastern Rocky Mountains region during the Laramide orogeny (75–50 Ma). Arches with the same structural style developed in the mid-continent region, east of the Rocky Mountains, although these were subsequently buried by Tertiary sedimentary rocks. We attribute deformation in the Rocky Mountain foreland and continental interior to folding of the entire lithosphere (lithospheric buckling) as a result of horizontal endload on the western edge of North America. The observed wavelength of arches in the western United States is ca. 190 km, a spacing consistent with a lithospheric buckling interpretation. This buckling model provides a mechanical explanation for the distinction between "thin-skinned" Sevier-style and "thick-skinned" Laramide-style deformation, which depends respectively upon the decoupling or coupling of lithospheric layers. Additionally, the buckling model explains concurrent tectonism east of the Rocky Mountains during Late Cretaceous time. Dextral shearing within the block uplifts indicates that the deformation was broadly transpressional during the Laramide orogeny.

Keywords: Rocky Mountain foreland, Laramide orogeny, lithospheric buckling

Reworked Cretaceous elasmobranch teeth and provenance of the Paleocene Hanna Formation (Hanna Basin, Wyoming)

J. H. Burris

The Hanna Formation, exposed in the northeastern Hanna Basin, Wyoming, represents deposition from late early Paleocene into earliest Eocene time in alluvial, floodplain, and lacustrine environments. A 600-m-thick section that yields abundant vertebrate fossils begins 975 m above the local base of the formation. This section has been dated as latest Torrejonian through middle Tiffanian using mammalian index fossils. The terrestrial mammals are accompanied by numerous elasmobranch teeth, representing species thought extinct since the end of Cretaceous time. They are species known from two locally widespread marine Cretaceous units, the Wall Creek Member of the Frontier Formation and the Steele Shale. These units are broadly exposed on flanks of the Sweetwater arch, which define the northern margin of the Hanna Basin. The elasmobranch teeth from the Hanna Formation range in size from a few millimeters to over three centimeters and display transport-induced abrasion not seen on their in situ Cretaceous counterparts. Enameloid of cutting edges, crown points, and cusplets is rounded or sometimes broken, and bony bases commonly are etched or dissolved away. These teeth were reworked and transported from the Wall Creek Member and Steele Shale during uplift and erosion of the Sweetwater arch in middle Paleocene time. Lithic clasts from Paleozoic and Mesozoic strata derived from that arch also occur in association with the elasmobranch teeth in the Hanna Formation. The Cretaceous clasts and teeth indicate a local, northerly source for part of the Hanna Formation in the northeastern Hanna Basin.

Keywords: Chondrichthyes, Cretaceous, Elasmobranchii, Frontier Formation, Hanna Basin, Hanna Formation, Paleocene, provenance, reworking, Steele Shale, Sweetwater arch

Age and style of deformation and stratal thinning at the transition, Wasatch Plateau to Great Basin, central Utah

R. E. ANDERSON, S. F. DIEHL AND T. P. BARNHARD

The Salina area of central Utah contains an unusually complete Tertiary stratigraphic record, affording unique opportunities to study details of the Tertiary geologic history. Stratigraphic and structural studies reveal Neogene ages for tectonically significant parts of all major structures such as the 80-km-long Sanpete-Sevier Valley anticline (SSVA) and its flanking structures, the Wasatch monocline and Sevier Valley syncline. Newly identified critical components of the Neogene history in the SSVA include: (1) a protracted Eocene through early Miocene period of quasi-continuous sedimentation and relative structural quiescence that buried an early phase of the anticline; (2) east-trending folds in rocks of the anticlinal core; (3) widespread plutons in the southern part of the core; (4) strike-slip faults; (5) major thinning of strata (vertical structural convergence) between uplifted Jurassic rocks of the anticlinal core and downdropped superjacent Tertiary rocks; and (6) a widespread zone of altered rock at the contact between core rocks and overlying Tertiary rocks. We interpret the thinning and widespread alteration (5 and 6) to be linked through protracted processes of extensional faulting (possibly including detachment faulting), fluid flow, massive dissolution, and dissolution collapse at the contact between the Jurassic core rocks and overlying Tertiary rocks.

We suggest that the SSVA, one of several diapir-like elongate uplifts in central Utah, is somewhat analogous to flank ridges that develop at landslide margins by outward and upward flow of mechanically weak clay-rich rock or gouge. By analogy, the SSVA results from outward and upward movement of the relatively incompetent Arapien Shale from beneath the adjacent Sevier Valley.

If the structures formed during these newly recognized components of Neogene history are palinspastically restored, much of the evidence for contractional deformation at shallow structural levels is eliminated; this affects resource assessments based on models of that deformation. Also, the association of extensional faulting with non-tectonic dissolution collapse and with lateral motion of mechanically weak rocks implies shallow penetration and non-seismogenic to weak seismogenic behavior of some normal fault systems in central Utah.

Keywords: Arapien Shale, Green River Formation, Neogene, structure, fault, deformation, dissolution, Sanpete-Sevier Valley, Sevier, Wasatch Plateau, Basin and Range province, Utah