Lawrence E. Mack

Diagenetic modification of Sm-Nd model ages in Tertiary sandstones and shales, Texas Gulf Coast

Sm-Nd model ages of sandstones and shales from the Oligocene Frio Formation of south Texas and the Paleocene-Eocene Wilcox Group of south-central Texas increase with increasing depth of burial. Depleted-mantle model ages (Tdm) of Frio shales increase from ca. 800 Ma at 1 km of burial to ca. 1000 Ma at ∼4km; in Frio sandstones, Tdm increases from ca. 750 Ma at 1 km to over 1200 Ma at 4 km of burial. In Wilcox shales, Tdm increases from ca. 1300 Ma to ca.1500 Ma over a burial depth of 1.5 to 4.5 km; Tdm of Wilcox sandstones increases from ca. 1450 Ma at 2.5 km of burial to as much as ca. 2000 Ma at 4.0 km. The sampling scheme employed minimizes, but does not eliminate, the possibility that these trends in Tdm are due to detrital variability. Close correlation of these trends with diagenetic changes in the rock and evidence of rare earth element mobility preserved in the diagenetic products suggests that diagenesis has modified Tdm. Diagenetic modification of Tdm in Frio sandstone and Wilcox shale is due primarily to loss of radiogenic Nd, whereas increased 147Sm/144Nd produces elevated Tdm in deeper Wilcox sandstones. The Tdm of Frio shales does not change appreciably with depth, owing to the opposite effects on Tdm of decreased 147Sm/144Nd and decreased ϵNd(0). Loss of radiogenic Nd is due primarily to reactions involving unstable volcanic detritus, which is more abundant in the Frio than in the Wilcox. Decreased 147Sm/144Nd in authigenic illite, and consequent increased 147SM/144Nd in pore fluids, results in elevated Tdm in Wilcox and Frio sandstones containing late-diagenetic cement, and in decreased 147Sm/144Nd in deeper Frio shale. The similar Tdm trends observed in both units, which have different detrital characteristics, suggest that modification of Sm-Nd model ages in siliciclastic rocks may be commonplace during burial diagenesis.

Precise timing and rate of massive late Quaternary soil denudation

Strontium isotopes are a unique tool to study soil-erosion dynamics. Changes in Sr isotope ratios ( 87 Sr/ 86 Sr) provide a record of late Quaternary landscape denudation of the Edwards Plateau of central Texas, United States. The use of Sr isotopes as a tracer for soil erosion is based on the observation that, in central Texas, the 87 Sr/ 86 Sr ratio of soil correlates with soil thickness. Plants and animals express the 87 Sr/ 86 Sr ratio of exchange- able Sr in the soil. Therefore, we use changes in Sr isotope ratios through a well-dated stratigraphic sequence of fossil plants and animals in Halls Cave, Kerr County, Texas, as a proxy for temporal changes in soil thickness. By using this record we are able to characterize late Quaternary climate-driven soil-erosion dynamics on the Edwards Pla- teau. We find that continuous erosion removed at least 180 cm of soil at a constant min- imum rate of 11 cm/k.y.; this continuous phase of erosion ended ca. 5 ka. The Sr isotope record of soil erosion is consistent with late Quaternary environmental change in central Texas that has been independently modeled by using local and regional climate records. However, the rate of this climate-driven soil-erosion event was an order of magnitude slower than recent soil erosion caused by human land use. These results link erosion to century- to millennial-scale climate change and are cautionary evidence that even greater landscape degradation may result from coincident climatic variability and anthropogenic influences.

Diagenetic Resetting of Sm-Nd Isotope Systematics in Wilcox Group Sandstones and Shales, San Marcos Arch, South-Central Texas

ABSTRACT Sm-Nd analyses of sandstones and shales from the Wilcox Group (Upper Paleocene-Lower Eocene) suggest that the whole rock Sm-Nd signature is changed by burial diagenesis. The samples are from the San Marcos arch, south-central Texas, at depths ranging from outcrop to 15,000 feet. Sandstones from above the depth of intense feldspar reactions (ca. 10,000 feet) have whole rock Nd model ages of 1400 - 1500 Ma, whereas sandstones buried deeper than ca. 10,000 feet have Nd model ages of 1500 - 2000 Ma. Subsurface shale samples have whole rock model ages that increase from 1300 Ma at 5,000 feet of burial to 1500 Ma at 15,000 feet. Model ages increase most between 5,000 and 10,000 feet, within the zone of intense smectite illitization. The similar depositional age of all samples makes change in provenance an unlikely cause of the observed Nd model age variation. Sm and Nd reservoirs within the shale change considerably with progressive diagenesis. Outcrop samples contain subequal concentrations of Sm and Nd in the acid-soluble and silicate fractions. The acid-soluble reservoir in deeper samples contains progressively less Sm and Nd, and higher Sm/Nd. In all samples, the acid-soluble Nd is more radiogenic than the Nd in the corresponding silicate fraction, and the difference between the two reservoirs increases with depth. Diagenetic enrichment of Sm relative to Nd in the authigenic phases is the most likely explanation for the observed increase in Nd model ages in the sandstones. Decrease in Nd causes the increase in model age in the shales.