| OCR Text |
Show 61 limestone matrix that in diameter in the area occurs (Figure 21). in irregular, discontinuous This facies is restricted in around Arches National Park overlain and was up to 2 observed m only (Kane Springs, Arches, Yellowcat Trail, and chert facies it may be underlain or blocky concretions occurrence Dewey Bridge sections; Figure 9). The concretionary within siltstone facies sediments, or by typically occurs the siltstone facies and by the mudstone facies. Chert concretions may have a variety of origins related to: (1) precipitation within alkaline lakes; (2) pedogenic silcrete formation; (3) early diagenetic groundwater silcrete (4) diagenetic silica precipitation and/or replacement. formation; Direct or inorganic precipitation of silica has been described from the modem der Borch, ephemeral carbonate Coorong lakes in Australia (Peterson and and from Pleistocene associated with the modem saline, alkaline Lake deposits von 1965) Magadi in Kenya (Eugster, 1969; Schubel and Simonson, 1990). In both cases, the chert forms as lacustrine associated with alkaline (high pH) and precipitates conditions. However, silica precipitation deposition of lacustrine dolomites, and chert concretions that occur Continental cherts most (Thiry, 1997). Pedogenic alternating wet and lakes is associated with direct cherts typically occur in occurrences are similar to the thin « 50 irregular, in the Tidwell Member. commonly occurs as pedogenic or groundwater silcretes silcretes form in near-surface sediments in climates with dry conditions. and nodular horizons related to soil This structural Coorong Magadi modem em) bedded layers. Neither of these blocky in the probable evaporative development was These silcretes typically show laminated, columnar, profiles (Simon-Coincon et aI., 1996; Thiry, 1997). not observed in the Tidwell Member chert concretions. |
