As we leave 2024 behind and enter a new year, I would like to build on last week’s edition and share more about the geology of the Gulf of Mexico. This edition covers some new studies that trace the pathways of sediments entering the Gulf. Best wishes for 2025!
Rasoul Sorkhabi
Editor, Core Elements
Provenance of Modern Sediments
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Several recent studies have focused on the provenance (source area) and transport of sediments on the western GOM.
Beach samples from La Pesca and Tesoro Altamira: Shukla and colleagues from Mexico, published inJournal of Palaeogeography and Applied Geochemistry, collected 47 sand samples from the La Pesca and Tesoro Altamira beaches on the eastern coast of Mexico.
Methods: They conducted grain-size, x-ray diffraction (XRD), x-ray fluorescence (XRF) and scanning electron microscopy (SEM) on the samples. Detrital zircon crystals from four of the samples were also separated for U-Pb geochronology.
Key findings:
Sediments from both beaches are fine-grained and well-sorted sands.
The sediments are mainly composed of silica and aluminosilicate minerals.
The Chemical Index of Weathering gave these samples high values, suggesting intense weathering and warm and humid climatic conditions in the source areas.
Geochemical discrimination data, including major and trace elements, indicate a felsic igneous provenance for both beaches, such as the nearby Mesa Central, Sierra Madre Oriental, and Oaxaquia terrains.
Zircon ages:
U-Pb ages from 400 zircon grains showed similar distributions for sands from both beaches.
U-Pb zircon ages fall into three main clusters: 26 percent were Proterozoic ages (2.5–1.0 billion years old); 24 percent were Jurassic-Cretaceous ages (145–66 Ma); and 24 percent were Eocene-Oligocene ages (56–23 Ma).
From the Rio Grande to the Rio La Antigua: In another study, Lawton and colleagues, published in GSA Bulletin, researchers combine beach samples with fluvial samples collected from riverbanks spanning from the Rio Grande in the north to the Rio La Antigua, south of the Tran-Mexican Volcanic Belt. This covers nearly 650 kilometers of the western GOM littoral basin.
Analytical tools: All 37 samples were studied by point-counting of thin sections for mineral composition. Twenty-six were analyzed by U-Pb dating of detrital zircons.
Key takeaways:
Modern beach sands along the coastline of the western GOM vary systematically in mineral composition from north to south.
The beaches of Tamaulipas contain a high proportion of quartz and Proterozoic zircon ages that cannot be assigned to the predominantly carbonate rocks in the Sierra Madre Oriental. These sediments appear to have come from the southern Rocky Mountains—a transport of some 3600 km from the Proterozoic-age provenance.
Zircon grains of Jurassic-Cretaceous age indicate transport from the nearby Sierra Madre Oriental.
Sediments from the Veracruz beaches and rivers contain abundant mafic minerals and zircon ages of 20–0 Ma indicative of erosion from the Trans-Mexican Volcanic Belt.
Why it matters:
Sediment provenance has a direct impact on the quality of reservoir rock formations.
Comparing studies of modern sediments with those subsurface sedimentary rocks provides evolution of sediment-fill in the GOM.
Go deeper: Read the article by Lawton and colleagues hereand the articles by Shukla and colleagues here and here.
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A study by Morton and colleagues from the United Kingdom, published in Minerals, reveals important information about the sediment provenance of the Wilcox Group in the GOM.
TheWilcox Groupis an important stratigraphic group on the Gulf Coast and GOM, from Mexico to Alabama. It was deposited in the Paleocene and Eocene and is comprised of mudstone and sandstone, which respectively form major petroleum source and reservoir rock in the GOM.
Samples and methods:
Researchers collected samples from four onshore and seven offshore wells from the western and central part of GOM.
The samples were analyzed for their distribution of heavy minerals by crushing, sieving, and heavy mineral separation and counting methods.
Detrital zircon grains were dated by U-Pb geochronology.
Heavy minerals:
Wilcox Group sandstones are limited in diversity, and four mineral species—apatite, garnet, tourmaline and zircon—comprise 92 percent of the heavy minerals.
Considering the present burial depth of 14,000 feet, unstable heavy minerals were decomposed during burial diagenesis.
Garnets in onshore wells are depleted, whereas they are preserved in offshore wells. This is counter-intuitive because the deeper offshore sections should be more subjected to diagenesis and garnet destruction. The researchers tentatively attribute this to a sampling issue: Onshore well samples were cores while offshore well samples were cutting.
Zircon age patterns:
Detrital zircon ages range from 2500–55 Ma with several prominent clusters.
Detrital zircons with Precambrian ages are more abundant in the central and eastern GOM wells, mainly derived from the Precambrian Yavapai-Mazatzal province via the paleo-Colorado/Brazos river systems.
Zircon ages of 300–55 Ma are more abundant in the western GOM wells, largely shed from the Western Cordillera via the paleo-Rio Grande system.
Comparison of data from the Early Eocene and the Late Eocene Yegua Formation sandstones indicate a major shift toward sedimentation sourced from the Western Cordillera and transported by the paleo-Rio Grande.
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