Petrogenesis of sediments in the absence of chemical weathering: effects of abrasion and sorting on bulk composition and mineralogy

Abstract

Comminution in the glaciers that debouch into Guys Bight Basin, followed by selective sorting in the fluvial system, has had little effect on the bulk composition, or on the mineralogy, of the basin sands and muds. Most striking are the feldspar contents, and the feldspar‐quartz ratios in sands and muds, both of which remain similar to those of average bedrock. The feldspar contents of sands and muds range from 48 to 52% feldspar whereas average bedrock contains 51·7% feldspar. Feldspar‐quartz ratios average 1·58:1 in bedrock and 1·54:1, 1·66:1 and 1·69:1 in the medium to coarse sands, fine sands and muds, respectively, indicating minimal feldspar enrichment in the fine‐grained sediments. In the absence of appreciable chemical weathering, extreme abrasion followed by hydraulic sorting has not produced mature sediments such as quartz arenites and clay‐mineral‐rich muds. There is, however, some chemical differentiation. Preferential accumulation of mafic minerals in fine sands and muds is reflected in bulk compositions by higher abundances of MgO, FeO and TiO2, and in the mineralogy by enrichment of biotite in the fine grades. Bulk compositional studies focused solely on muds and mudstones will result in an overestimate of the mafic contribution from source rocks. This work shows that bulk compositional studies of sediments and sedimentary rocks should include all available granulometric grades.

Geochemistry of surface sediments from the fjords of Northern Chilean Patagonia (44–47°S): Spatial variability and implications for paleoclimate reconstructions

“…Low chemical weathering is also confirmed by (1) the clay mineralogy, which is dominated by minerals characteristic of physical weathering (illite and chlorite); and by (2) the low values of the Chemical Index of Alteration (CIA; Nesbitt and Young, 1982) Pankhurst et al, 1999). Low chemical weathering in Northern Patagonia is due to the combined effect of (1) the characteristic cold climate of the region (Nesbitt and Young, 1996); (2) the regional lithology, i.e. granitoids is one of the less reactive lithologies (White and Blum, 1995) (Fig.…”

“…dense, minerals are independent variables, although both seem to co-vary with TI. The main factors that control the mineralogy and geochemical composition of siliclastic sediments are (1) the nature of the provenance, (2) the intensity of physical and chemical weathering and (3) the processes that occur during sediment transport (Nesbitt and Young, 1996). Here, we assess the importance of these three factors for the fjords of Northern Chilean Patagonia.…”

“…One of the most important studies in this respect is by Nesbitt and Young (1996), who studied the composition of sediments deposited in the fluvio-glacial system of Guys Bight, Baffin Island. These authors found that the primary mafic minerals enriched in fine sands and muds (i.e., silt + clay), while the coarse sands contain a greater amount of Si.…”

Geochemistry of sandstones and shales from the Ecca Group, Karoo Supergroup, in the Eastern Cape Province of South Africa: Implications for provenance, weathering and tectonic setting

“…Nonetheless, their chemical and mineralogical composition can be influenced by factors like source rock characteristics, weathering, sorting processes during transportation, sedimentation and diagenetic processes to an extent [1][2][3]. Trace elements such as La, Y, Sc, Cr, Th, Zr, Hf, Nb and rare earth elements (REE) are thought to be useful indicators of provenance, geological processes and tectonic setting due to their relatively low mobility and insolubility during sedimentary processes [4][5][6][7][8].…”

Grain-Size Control on Petrographic Composition of Sediments: Compositional Regression and Rounded Zeros

“…So far, most studies that aimed to shed light on the quantitative relations between grain size and sediment composition have a low grain size resolution (e.g., in sand-silt-clay, Lim et al 2006), placed their attention on the sand domain only (e.g., Grantham and Velbel 1988;Nesbitt and Young 1996;Arribas and Tortosa 2003;Solano-Acosta and Dutta 2005;Kiminami and Fujii 2007), and/or use debatable statistical techniques that ignore the compositional nature of the data (e.g., Zhang et al 2002;Chandrajith et al 2001). Whitmore et al (2004) presented both chemical and petrographic data with a higher grain-size resolution (−1 to 4 phi-grade, in 1 phi step) from modern sediments from rivers of Papua New Guinea; however, the fine-grained fractions (silt and clay) are not differentiated.…”