300-4 Deep-Water Sand Composition Insights from a Preliminary Global Modern–Pleistocene IODP Petrographic Database

Session: Reconstructing Earth Surface Processes in Orogenic Systems (Posters)

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Petrographers have commonly point-counted and compiled sandstone petrographic datasets using two established schemes: the “Indiana” (QFR) and the Gazzi-Dickinson (QFL) method. The Indiana method contains a diverse array of modal categories that enable precise interpretation of not only provenance history, but transportational, depositional, and diagenetic history of the sample. However, previous studies suggest that petrographic data are skewed by grain size when evaluated by Indiana methods, and thus Gazzi-Dickinson methods were introduced to limit grain size effects and better quantify sand modal composition. The Indiana method may overestimate polymineralic (lithic or rock fragment) grains; whereas the Gazzi-Dickinson method aims to minimize this grain size effect by counting any sand-sized grain (>63µm), even within a fragment. Thus the Gazzi-Dickinson approach allows for comparison between unsorted sand or sandstone. Relative to more proximal sources, the fine-grained, well-sorted, and relatively homogeneous bulk grain size of deep-water sand brings into question whether both of these methods will result in the same petrographic data. This study tests the hypothesis that the Gazzi-Dickinson and “Indiana” methodologies can be used interchangeably for relatively fine-grained deep-water samples. 

We present preliminary results from a subset of 10 point-counted samples drawn from an expected dataset of 90 Pleistocene–modern deep-water thin sections collected from International Ocean Discovery Program sample repositories. A Zeiss Imager M2m microscope was used for thin section imaging, and digitally point-counted via the novel software PetroImage. This point counting method is used in tandem with traditional microscopy and allows users to easily share and store data, as well as re-evaluate individual point counts, thereby enhancing dataset reproducibility for future studies. Preliminary point counts using the Gazzi-Dickinson and Indiana methods show average percent differences of 10.4% for quartz, 24.3% for feldspar, and 23.8% for polymineralic phases. The overall composition results of the preliminary 10 samples show a connection with local bedrock geology, and an enrichment in quartz at low latitudes, reflecting an expected provenance trend. Our preliminary results demonstrate that grain size effects are still present between the two petrographic methods in fine-grained siliciclastic deposits. Continued efforts will provide a global comparison of deep-water sand composition and petrographic methods, and improve predictions of sand source area, basin fill composition, and transport histories in modern and ancient deep-water depositional systems.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11280

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