[No abstract available] / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

The Late Mesozoic Bisbee basin of southeastern Arizona was an intracratonic back-arc rift basin. Extension was coupled with seafloor spreading in the Gulf of Mexico and back-arc extension behind a magmatic arc along the convergent Pacific continental margin. Tectonostratigraphic evolution of the basin occurred in three phases. Initial mid-Jurassic rifting of the basin, marked by eruption of the Canelo Hills Volcanics, may have been complicated by sinistral strike-slip motion along the Mojave-Sonora megashear. During continued rifting, from latest Jurassic to Early Cretaceous time, the Glance Conglomerate was deposited by alluvial fans and braided streams in grabens, half-grabens, and caldera-related depressions; locally interbedded volcanic rocks represent waning rift-related back-arc magmatism. The upper Bisbee Group was deposited during Early to earliest Late Cretaceous passive thermotectonic subsidence. The Bisbee Group and correlative strata occur in most mountain ranges in southeastern Arizona, and are subdivided into southeastern, northwestern, northern, and western facies. Southeastern facies were deposited in alluvial fan, meandering fluvial, estuarine, marginal marine and subtidal shelf environments as a transgressive-regressive sequence including a marine interval that was continuous with Gulf Coast assemblages during Aptian-Albian marine transgression. Northern facies were deposited in alluvial fan and braided stream environments along the northern rift shoulder of the basin. Southeastern and northern facies sandstones are dominantly quartzose, and were derived mainly from cratonic sources to the north. Subordinate volcaniclastic sandstones in the southeastern facies become more abundant to the west, proximal to eroding Jurassic and Cretaceous volcanic arcs. Basal northwestern facies arkosic strata deposited in alluvial fan, braided stream and lacustrine environments were derived from local basement uplifts, and were ponded in a northwestern depocenter by rift-related topography. A thin estuarine interval within overlying dominantly fluvial facies indicates integration of regional drainage networks by the time of maximum transgression. Transition upward to quartzose sandstone compositions reflects wearing down of local basement uplifts and increasing abundance of craton-derived sediment in the northwestern part of the basin. Western facies alluvial fan, braided stream and lacustrine intramontane deposits are composed of locally-derived arkose and lithic arkose.

No description available.

published_or_final_version / abstract / Earth Sciences / Master / Master of Philosophy

published_or_final_version / Earth Sciences / Doctoral / Doctor of Philosophy

The first part of this dissertation deals with the timing of Mesozoic igneous activity in southern Cordillera of North America and its tectonic implications. A representative section in Santa Rita Mountains is dated using the zircon U-Th-Pb isotopic method. The oldest unit, the lower member of the Mt. Wrightson Formation, is concordantly dated at 210 ± 3 Ma. Initial basaltic andesite to andesite volcanism was followed by deposition of redbeds and associated volcanic rocks that are dated at 200 Ma. Felsic volcanism and eolian sand deposition may have spanned from 190 to 170 Ma. The Piper Gulch Granodiorite, representing the earliest Mesozoic intrusive equivalent, gives concordant dates of 188 ± 2 Ma. A second cycle of andesite and rhyolitic volcanism and sedimentation is dated at 151 ± 5 Ma using the whole-rock Rb-Sr isotopic method. The Hovatter Volcanics in the Little Harquahala Mountains, southwestern Arizona is dated at 165 Ma. Whole-rock Rb-Sr isotopic method on the same rocks gives a coherent reset isochron of 70 ± 3 Ma (Appendix III). A new stratigraphic correlation is proposed based on the dating data. Tectonic models proposed by previous workers to account for what seemed to be the lack of Triassic volcanic rocks are not necessary. This part of the Cordillera was an uplifted arc terrane during the Early Mesozoic (Appendix II) and may have provided volcanic detritus to the Late Triassic Chinle Formation in the Colorado Plateau. The second part of the dissertation deals with magma evolution and crust modification during arc magmatism. Rocks in southeastern Arizona have ƭ(N)(d) values of -3.4 to -6.4, while rocks to the west have ƭ(N)(d) values ranging from -8.5 to -9.2. An ƭ(N)(d) value of +2 for a Jurassic basalt indicates the presence of depleted mantle under the arc. Using lower crust and mantle end-members, 20 to 40% mantle input is estimated. This seems to argue for continuous growth model of the continental crust. Combined REE and isotopic data indicate that assimilation of lower crust by mantle melts followed by fractional crystallization took place. Detailed study indicates that the lower crust along sites of arc magmatism gets progressively hybridized by the mantle, becoming more mantle-like with time.

The Cabullona Basin of northeastern Sonora is a structural depocenter that was formed during Late Cretaceous time. The 2.5 km-thick sedimentary fill of this basin, the Cabullona Group, is composed in ascending order of the Corral de Enmedio formation, Camas Sandstone, Packard Shale, Lomas Coloradas formation, and, laterally equivalent to the last two units, El Cemento conglomerate. Abundant vertebrate and invertebrate as well as pollen identifications from these units indicate the Cabullona Group is of middle Santonian to Maastrichtian age. The Corral de Enmedio formation represents shallow lacustrine deposits. Lithofacies of the Camas and Lomas Coloradas formations indicate they were deposited by braided- and meandering-fluvial systems, whereas the eight members of the Packard Shale represent complex fluvio-deltaic-and-lacustrine systems. The El Cemento conglomerate is a thick clastic wedge of coarse-grained alluvial deposits that was deposited adjacent to the structural margin of the basin. Clast composition and paleocurrent directions of the El Cemento conglomerate indicate it was derived from strata of the nearby uplift of the Sierra Anibacachi-Cerro Cabullona. The low-angle, southwest directed Cabullona thrust fault that separates the uplift from the basin formed the structural margin of the basin. The tectono-sedimentary history of the Cabullona Group, its age and regional tectonic setting support the idea that this basin was formed because of typical Laramide-style deformation and indicates that the Rocky Mountain foreland province of Laramide deformation extended southward to northeastern Sonora.

published_or_final_version / Earth Sciences / Doctoral / Doctor of Philosophy

Poorly preserved fossil wood collected throughout the terrane had been the basis of a Cretaceous age assignment. New age constraints come from paleomagnetic data which fail the fold test at both regional and local levels. These data indicate that the paleomagnetism was produced by a post-folding metamorphic event. The resulting paleomagnetic pole at 58.9°N, 114.6°E falls on the North American apparent polar wander path very near a pole from the Callovian Summerville Formation. These data imply that Callovian is a minimum age for the metamorphism and that the proto1ith is older.

Graduation date: 1994