Sand Provenance Within the Mississippi-Gulf of Mexico Drainage System: New Insights From Common Pb Isotopes in Detrital K-Feldspar

Abstract

While the provenance of modern sands delivered to the Gulf of Mexico by the Mississippi is well studied using heavy mineral analysis, scant attention is placed on what insights can be gained from the lighter components of sands. Although proven to detect broad source domains, heavy minerals grains such as zircon can suffer from a number of drawbacks including recycling and non-uniqueness. Conversely, the monocyclic nature of K-feldspar and its modal abundance in sands means K-feldspar grains are more uniquely representative fingerprints of their source rock(s). The association of K-feldspar and quartz particularly in granitoid rocks also mean that it can act as a proxy for the source of at least some of the quartz grains. In addition, only small sample sizes are required as sands and source rocks can be characterised using relatively few grains. A preliminary study shows that the Pb in K-feldspar technique is effective in detecting distinct Pb populations within modern sands of the Mississippi and its tributaries. In order to re-evaluate the provenance of modern river sands on a spatially representative scale, common Pb isotopes in detrital K-feldspars are characterised by LA-MC-ICPMS in over 40 samples collected across the Mississippi drainage basin. Tributaries including the Arkansas, Red, Platte, Missouri, and Ohio Rivers are also sampled in an attempt to evaluate the contribution from each tributary and understand the effect, if any, of dilution and mixing of detrital grain populations. A number of samples are also examined from coastal plain rivers draining into the Gulf of Mexico to determine if diagnostic Pb signatures are present. Understanding the provenance of sands delivered by the Mississippi system has significant implications for the evolution of the drainage basin. By investigating the contributions from different areas within the catchment we can gain a greater understanding of sediment pathways and the processes which govern the supply of sediment to the offshore. In this way, this research has important implications for the distribution and quality of reservoir sands within the Gulf of Mexico. Furthermore, as the Pb in K-feldspar technique has not yet been applied at this spatial resolution to a modern continental-scale drainage basin, this study affords the opportunity to evaluate the Pb in K-feldspar technique and uncover what insights it can offer over, and in combination with, more conventional provenance markers.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015