Vanessa Brillo, Jacob Poletti and Johnathan Rice, all UCSB Earth Science graduate students, will each give a talk at Speakers Club on Thursday.  Vanessa's talk is entitled Evidence for fluvial sedimentation in Midcontinent U.S.A. during the Guadalupian (Middle Permian).  Jacob's talk is entitled Titanite as a petrochronometer in rocks with complex zircon U-Pb systematics: insights from the Mountain Pass Intrusive Suite, Mountain Pass, CA.  Johnathan's talk is entitled The Nueces Bay-head Delta Response to the 2.6 ka Climate Event in the Northwestern Gulf of Mexico.

Vanessa Brillo's abstract:

A renewed interest in the Permian Midcontinent of the U.S.A. has led to new depositional models for most of the stratigraphic units of western Oklahoma in which these units are interpreted as lacustrine and eolian in origin. However, with these new interpretations, the question remains as to what is the source of the large volumes of fine-grained mudstones that are common within these rocks. In this study, we measured 43 stratigraphic sections within the Guadalupian Cloud Chief Formation of western Oklahoma. Eight facies were recognized within the measured sections. These facies include gypsum, channelized very fine sandstone, massive mudstones, massive sandstones, variegated mudstones, ripple cross-laminated very fine sandstone, interbedded sandstone and mudstone, and silty claystone. We interpret these facies to represent subenvironments of widespread arid-land fluvial systems. Our interpretations fit well with other recent work calling for arid non-marine conditions within the Midcontinent during the middle Permian. In addition, the presence of extensive fluvial systems provides a convenient explanation for the source of the large volumes of fine-grained sediments comprising the middle Permian rocks of western Oklahoma.

Jacob Poletti's abstract:

Titanite (CaTiSiO₅) is a common accessory mineral in many igneous rocks whose powerful potential as a geochronometer has been recognized for decades due to its propensity to incorporate significant amounts of U, Th, and Pb, and the pertinence of its closure temperature (>700? C) to igneous processes. Recent developments in laser-ablation split-stream inductively coupled mass spectrometry (LASS-ICPMS) enable rapid and simultaneous collection of reliable U-Pb isotopic data and elemental abundances from in-situ titanite. This method permits researchers to exploit the wide range of petrologically relevant trace elements incorporated into titanite (e.g. REEs, HFSEs) by providing: 1) a sufficient quantity of data points to calculate U-Pb isochron ages; 2) complementary elemental abundances and isotopic ratios to link ages to geologically meaningful processes; 3) a tool to assess measured ages in cogenetic phases (e.g. zircon). Here, we highlight the usefulness of titanite LASS-ICPMS petrochronology in rocks with cogenetic zircon that yield complex U-Pb data.

The petrogenesis of the rare-earth carbonatite at Mountain Pass, California has been poorly understood since the deposit’s discovery in the 1950s. The carbonatite is spatially associated with a suite of K-rich silicate rocks, which have been proposed to be genetically related to the carbonatite. However, existing age data suggest that the carbonatite is ~15-25 Ma younger than Mountain Pass silicate rocks. Petrogenesis models have therefore been forced to treat the ore as a separate petrogenetic event with a potentially different genesis mechanism.

New LASS-ICPMS petrochronologic data from 18 samples of K-rich silicate rock from Mountain Pass yield titanite and zircon U-Pb ages from 1428 +/- 9 to 1395 +/- 13 Ma, decreasing but not eliminating the reported age gap between carbonate and silicate magmatism. Zircon in the samples is characterized by highly discordant U-Pb data, abundant inheritance, highly variable trace element content, and complete metamictization in some cases. Titanite in the same samples yields single U-Pb age populations with demonstrably magmatic REE patterns and overall consistent trace element content. In addition, Nd isotopic data from in-situ titanite suggest that the K-rich silicate rocks may not be derived from the same source as carbonatite. In these rocks, titanite yields far more simple and useful petrochronologic data compared to zircon, demonstrating its utility in the study of rocks that would otherwise be difficult to interpret using zircon U-Pb systematics alone. 

Johnathan Rice's abstract:

The purpose of this study is to examine how a bayhead delta responded to Holocene climate changes over the past 5 ky. Twenty-eight vibracores, eight geoprobe cores, and twenty-five kilometers of high-resolution seismic profiles were collected in the Nueces Bayhead Delta on the northwest coast of the Gulf of Mexico in Texas. Sediment cores were described and used in conjunction with the seismic profile to determine lateral facies relationships. The timing of deposition and facies changes were determined by radiocarbon dating.

We identified eight sedimentary facies in the sediment cores which include five distinct deltaic sub-facies, a modern sub-arid facies, a channel facies, and a Pleistocene terrace facies. The sedimentary facies include two distinct mud facies, two distinct sand facies, an oyster reef facies, tan sandy silt facies, a brown fine sand facies, and a black sandy silt facies. Within the 25 kilometers of seismic data, we identified four seismic facies. The cores and seismic data were used to identify seven sand lobes were identified in the seismic profiles and sediment cores. The seven sand lobes are interpreted as mouth-bar deposits and used to determine the approximate location of the seaward edge of the delta through time. Prior to 3 ka, the delta was located 0.5 km seaward of its current location. After 3 ka, the delta backstepped by at least 10 kilometers before prograding to its modern position. The post-3 ka backstepping event correlates with a period of local aridity and was probably caused by a reduction in sediment supply. Additionally, the overall evolution of the bayhead delta is better defined through a sub-delta facies model.