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Diagenetic transformations and silcrete–calcrete intergrade duricrust formation in palaeo‐estuary sediments
Author(s) -
Ringrose S.,
Cassidy L.,
Diskin S.,
Coetzee S.,
Matheson W.,
Mackay A. W.,
Harris C.
Publication year - 2014
Publication title -
earth surface processes and landforms
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.294
H-Index - 127
eISSN - 1096-9837
pISSN - 0197-9337
DOI - 10.1002/esp.3516
Subject(s) - geology , diagenesis , dissolved silica , geochemistry , carbonate , calcite , bioturbation , water table , groundwater , sediment , dissolution , geomorphology , chemistry , geotechnical engineering , organic chemistry
ABSTRACT The Boteti palaeo‐estuary in northern Botswana is located where the endoreic Boteti river, an overflow from the regional Okavango river system, enters the Makgadikgadi pans. The present work considers diagenetic silica and calcium carbonate dominated transformations. The aims are to help identify precursor conditions for the origin of microcrystalline silcrete–calcrete intergrade deposits while developing insight into pene‐contemporaneous silica and calcite matrix formation. General precursor conditions require the presence of cyclical endoreic freshwater inflow into a saline pan. The pan should be deep enough to sustain a permanent watertable under climatic conditions sufficient to cause carbonate fractionation within the groundwater. Freshwater inflow into a saline pan drives the geochemistry of the system (from freshwater to saline, from neutral to high pH). The geochemistry is controlled by the periodicity of inflow relative to salinity levels of phreatic groundwater in the receptor saline pan. The source of most silica and localized CaCO 3 is derived from the dissolution and precipitation of micro‐fossils, while more general CaCO 3 enrichment stems from saline pan based carbonate fractionation. Diagenetic change leads to colloidal then more consolidated bSiO 2 /CaO aggregate formation (amorphous silica) followed by transformations into opaline silica over time. Irregular zones of siliceous sediment forming in otherwise calcareous deposits may relate to the irregular occurrence of biogenic silica in the source sediments, inferring a source for local silica mobilization in intergrade deposits. The distribution of calcareous micro‐fossils may have a similar converse effect. Diagenetic evidence from an intergrade deposit with a low SiO 2 /CaO ratio suggests that transformation occurred more into the pan, while an intergrade deposit with a high SiO 2 /CaO ratio more likely formed closer to a land margin and was frequently inundated by freshwater. Pene‐contemporaneous silcrete–calcrete intergrade formation under the above conditions may take place where dissolved silica crystallizes out in the vicinity of calcite crystals due to local decreases in pH. The continuing consolidation of bSiO 2 /CaO aggregates may be facilitated by the presence of increasing amounts of calcite. It appears that CaCO 3 may act as a catalyst leading to pene‐contemporaneous bSiO 2 /CaO aggregate formation. However the processes involved require further work. Copyright © 2013 John Wiley & Sons, Ltd.