Silica-Cemented Sandstones (Groundwater Silcretes) in Outcrops of the Jackson Group, Texas Coastal Plain
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Volcanic-ash-rich sections of the Jackson Group contain sandstone beds cemented to various degrees by several forms of silica and zeolite. Beds are cemented by multiple combinations of opal, opal-CT, unidentified silica mineral (tridymite/cristobalite [?]), length-fast chalcedony, and a Ca zeolite. Where cementation is complete, beds are extremely hard and fracture across detrital grains similar to quartzites. A typical cement sequence in completely cemented beds is banded opal, banded length-slow tridymite(?), and clear length-fast chalcedony. Opal, tridymite (?), and banded chalcedony occur as isopachous grain coats, whereas clear chalcedony is the final pore-fill. Sandstone beds are dominated by detrital grains of volcanic parentage. In the northern part of the study area intergranular volumes (IGV) of sandstones indicate that cementation began after burial of only a few tens of meters, which indicates that the cemented beds are silcretes. Silcretes can form either in a soil zone (pedogenic type) or at a water table. The abundance of opal rather than quartz, pristine character of feldspars, the local presence of authigenic pyrite, and the absence of ferric iron and pre-modern clay skins (cutans) indicate that the silcretes formed in reducing water at a paleo-water table rather than in an oxidized paleosol. Hydrolysis of volcanic ash intercalated with sandstone beds yielded silica and metal cations found in the silica phases and also zeolite. The sequence of silica cements is typical of those precipitated from alkaline water of decreasing silica concentration with time. Opal-CT lepispheres (210 m) reveal blades constructed of 50 nm nannospheres that possibly were precipitated bacterially. The silcretes formed close to the surface, probably during the late Eocene, whereas zeolite cement formed later during deeper burial. Sandstones in the southern part of the study area have IGV values of ~27%, little opal, but abundant zeolite cement. Too little opal cement formed at shallow burial to halt compaction in these incipient silcretes, and zeolite formed over a wide depth range.
