Department Abstracts submitted to the 2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)
MIDDLE MIOCENE HIGH-MAGNESIUM ANDESITE AND HIGH-SILICA ADAKITE IN THE JACUMBA VOLCANICS AND ALVERSON FORMATION OF SAN DIEGO AND IMPERIAL COUNTIES, CALIFORNIA
LINKING UNDERGRADUATE RESEARCH WITH COMMUNITY OUTREACH IN ARGENTINA: THE INTERNATIONAL FELLOWS PROGRAM AT THE UNIVERSITY OF WISCONSIN-EAU CLAIRE
In 2013, IFP supported a project entitled Establishing Societal Linkages: Introducing the Relevance of the Geology of the Andes to Argentinian Secondary-School Students. This program expanded on the relationship between faculty and Argentinian professionals established during a semester-long multidisciplinary immersion experience (2011 TIES Argentina) and by a subsequent 2012 NSF Tectonics grant. The IFP had two principal components: 1) educational outreach, involving development of a short course for secondary school students that describes the close linkage between geology and society in Mendoza, Argentina. UWEC students designed a series of ‘hands-on’ laboratory exercises that introduced the students to earthquakes, volcanoes, and plate tectonics, utilizing the Andes as a natural laboratory. UWEC students interacted with approximately 150 students and 9 faculty to present a short course to students grades 9-12 at Colegio ICEI. The educational program was extremely successful, and the Colegio has requested that the presentation become a semi-annual event; 2) student/faculty collaborative research, with the students collaborated in international research, including regional planning sessions, analysis of stratigraphic sections, geologic mapping, and geochronologic and geochemical sampling transects. They participated in analysis of the Las Penas and Uspallata basins, and are currently conducting geochronologic and isotopic geochemistry analyses. The lead author will expand her research interests in Argentina into a M.Sc. project at San Diego State University, starting in 2014. The groundwork set during the IFP has led to three (3) additional UWEC students conducting ongoing collaborative research in Argentina.
CUENCA USPALLATA: AN INTERMONTANE BASIN RECORDS EPISODIC UPLIFT OF THE CORDILLERA FRONTAL AND PRECORDILLERA IN THE LATE MIOCENE
Cuenca Uspallata (Uspallata Basin) straddles the boundary between the Frontal Cordillera and Precordillera, and contains a complex succession of conglomerate, sandstone and mudstone deposited in an arid fluvial system. Basin strata unconformably overlie Permian-Triassic volcanic rocks of the Cordillera Frontal, and basal units contain coarse conglomerates and megaclasts of volcanic debris representing a basin margin facies. Sedimentary provenance, constrained by conglomerate clast composition and detrital zircon data, records an initial influx of detritus from the Cordillera Principal and Cordillera Frontal that interfingers upsection with distinct detritus from the Precordillera. Available U-Pb age constraints from a resedimented tuff and detrital sandstone ages suggest sedimentation was active from at least 9-12 Ma, which is significantly younger (>5 Ma) than synorogenic deposits in the Cacheuta basin to the east. Sedimentologic and geochronological constraints suggest Cuenca Uspallata developed as an intermontane basin trapped between the uplifting Cordillera Frontal and the Precordillera, and was not part of the main foreland succession.
STRATIGRAPHIC ANALYSIS OF THE NEOGENE CACHEUTA BASIN: A RECORD OF OROGENIC EXHUMATION AND BASIN INVERSION IN THE SOUTH CENTRAL ANDES
The Cacheuta basin is subdivided into five distinct formations [Divisadero Largo, Mariño, La Pilona, Tobas Angostura and Río de los Pozos] that reflect episodic sediment flux produced during eastward thrust belt propagation. Stratigraphic analysis, U/Pb geochronology, conglomerate clast counts, and (U-Th)/He analyses constrain basin evolution and subsequent inversion. A ca. 17.9 Ma U/Pb age from a volcanic tuff near the base of the succession constrains initial basin subsidence to >18 Ma, and suggests that previous magnetostratigraphic age constraints require revision. Provenance analysis identifies episodic sediment flux from the Cordillera Costal (JuraCretaceous zircon), Cordillera Principal (distinctive Jurassic conglomerate, Cretaceous fossiliferous limestone, and Tertiary hornblende andesite clasts) and the Cordillera Frontal (PermoTriassic clasts and detrital zircon). Results from the overlapping U-Pb crystallization ages and (U-Th)/He ages on detrital zircon confirm that the Permo-Triassic Choiyoi Group of the Cordillera Frontal has remained at upper crustal levels (<6 km) since emplacement. Evidence of sediment input to the Cacheuta basin from the Precordillera is equivocal at this time.
Preliminary detrital apatite (U-Th)/He analyses suggest an estimated cooling age of approximately 6 Ma, which is interpreted to reflect basin uplift and inversion in the late Miocene.
CHARACTERIZING THE EFFECTS OF ILLUVIATION ON THE PETROLOGY AND CHEMISTRY OF TONALITIC SAPROCK: IMPLICATIONS FOR INTERPRETING COMPOSITIONAL LINEAR TRENDS
The above effects of eluviation contrast markedly with the statistically significant additions of Si, Al, Fe, Mn, Ti, Sc, Cr, Cu, Rb, Y, and Yb mass produced by illuviation. Such additions translate into an overall statistically significant 12-15% increase in bulk mass. The above increases in elemental and bulk mass are a reflection of eluvial processes operating in the now eroded overlying ≥6.7 m section of regolith. Within that overlying section, kaolinite; minute particles of biotite, hornblende, and ilmenite; and ions derived from leaching of these mineral were suspended into downward percolating fluids. The dominance of kaolinite, along with the apparent severity of leaching implied by the downward transfer of significant elemental mass, suggest a climatic regime that is unlike that currently existing in the Peninsular Ranges, or during the Quaternary, ~35,000 years ago, the exhumation age of a nearby corestone. Though poorly constrained, after removal of ~300 km of displacement along the San Andreas fault, the section of regolith studied during this investigation, may reflect Paleocene weathering within a subtropical setting.
On centered p(A)-p(CN)-p(K) ternary diagrams, illuviation resulted in a compositional linear trend anchored by the geometric mean of the corestone samples and the projected composition of kaolinite at the p(A) apex. Notably, this trend is unlike that documented for biotite-controlled and plagioclase-controlled weathering.
LATE CRETACEOUS BASIN EVOLUTION ALONG THE WESTERN MARGIN OF THE INSULAR SUPERTERRANE: THE NANAIMO GROUP, BRITISH COLUMBIA
The tight linkage between orogenic exhumation and basin subsidence was interrupted in early Campanian time, as the arc system was breached by at least one major fluvial system that delivered extraregional sediment from the back arc region into the basin. Significant (locally >50%) quantities of Precambrian detrital zircon (ca. 1300-1800 Ma), apparently derived from the Belt and Windemere Supergroups to the east, mixed with JuraCretaceous grains derived from the CPC. The timing of this older zircon flux corresponds to uplift of Mesoproteroic and Neoproterozoic successions along the Sevier orogenic system to the east. Extraregional sedimentation peaked in late Campanian time, with a distinct increase in quartzite pebbles and cobbles in conglomeratic facies. The extraregional component was substantially diluted by Maastrichtian time by a major pulse of plutonic detritus derived from final unroofing of the Cretaceous arc system.
DEVELOPMENT OF A GEOLOGIC GUIDE FOR FLORISSANT FOSSIL BEDS NATIONAL MONUMENT
The rock units portrayed on the geologic maps are then put into context with an illustration of a stratigraphic column. Each rock layer is further described with photos in outcrop and as a hand sample. Fossils are illustrated for the fossiliferous layers. In depth descriptions of each rock layer include lithology, deposition, paleontology (if applicable), and locations where outcrops can be viewed. The guide ends with a brief text describing the geologic history of the area and a customized time scale, placing the deposition of the Florissant Formation and other surrounding units into context with the entirety of geologic time.