Dolocrete

Figure 2.10 Geometries of groundwater calcretes and dolocretes. Linear, ribbon-like valley calcretes are known from the late Cenozoic of central and Western Australia, and the late Triassic of the Paris Basin (see text). Lacustrine groundwater calcretes or dolocretes forming halo-like masses are known from the Danian of Provence (see text). Alluvial fans are associated with sheet-like bodies, and are known from the Plio-Pleistocene of Oman and the Palomas Basin (New Mexico), and the Paleocene of the Lisbon and Sado basins (see text).

Colson, J. Cojan, I. (1996) Groundwater dolocretes ina lake-marginal environment an alternative model for dolocrete formation in continental settings (Danian of the Provence Basin, France). Sedimentology 43, 175-188. 

Spotl, C. Wright, V.P. (1992) Groundwater dolocretes from the Upper Triassic of the Paris Basin, France a case study of an arid, continental diagenetic facies. Sedimentology 39, 1119-1136. 

Geometries of groundwater calcretes and dolocretes. Schematic diagram showing the laterite-ferricrete genetic relationship, and the natural continuum between the autochthonous (i.e. in situ weathering profiles) and allochthonous end-members. 

Sources of Ca and Mg for calcrete and dolocrete are uncertain, but are often believed to be windblown dust. Calcium and Mg may also be derived from pyroclastic material (Bestland Retallack, 1993) and oceanic aerosols (Quade et al., 1995). These sources are also relevant to phreatic carbonates. In some cases the groundwater may bring ions... 

El-Sayed, M.I., Fairchild, I.J. Spiro, B. (1991) Kuwaiti dolocrete petrology, geochemistry and groundwater origin. Sediment. Geol. 73, 59-75. 

Carbonate cements are often among the dominant components of diagenesis and hence are of decisive importance in determining the reservoir quality of sandstone sequences. Despite this, the timing, the geochemical conditions of precipitation and dissolution, as well as the source and fate of these cements are not fully understood. In continental and near-shore sediments, cements commonly precipitate as calcretes and dolocretes in the vadose and phreatic zones, and attain a variety of mineral-ogical, textural and distribution patterns as well as elemental and isotopic compositions. These cements form lenses and layers of densely cemented... 

In this paper the terms dolocrete and calcrete are used to indicate sediments extensively cemented by displacive dolomite and calcite under conditions ranging from the soil horizon to shallow phreatic. Conversely, we use the expression carbonate cement in sandstones where is no evidence of displacive, near-surface precipitation. Kaolin is used as a... 

Fig. 8F). Some dolocretes contain crusts of microcrystalline dolomite with collomorphic textures composed of alternating bands of dolomite and calcite (Fig. 8E). 

The EMP analyses revealed that dolomite and ankerite vary considerably in chemical composition, particularly in terms of Fe content (FeCOj = 0.3-36 mol%) (Table 1 Fig. 9). The microcrystalline dolomite rims and crusts in dolocrete, as well as the small rhombs replacing clays and occurring within mica in the sandstones, are relatively Fe poor to moderately ferroan (0.3-7.8% FeCOs) (Table 1 Fig. 9). However, both the small rhombs and the large blocky crystals in sandstones are zoned, with Fe increasing outwards to an ankeritic composition (Fig. lOE). Furthermore, the dolomites are slightly to moderately calcian ( 53-62 mol% CaCOj) (Table 1) and have relatively low Mn content (0.2-1.9 mol%). 

Barite occurs as scarce, large crystals (up to 2 mm) filling vugs and cracks and engulfing as well as replacing kaolinite and carbonate cements in dolocretes and calcretes (Fig. 14F). In the sandstones, barite occurs as a few poikilotopic and small crystals which cover, and thus postdate, chlorite rims around framework grains. 

Barite occurs in trace amounts as patchy poikilo-topic, grain-replacive cement associated with partially dissolved carbonate cement. Anhydrite cement forms < 1 vol% in the fine- to medium-grained sandstones that contain micritic carbonate intraclasts (calcretes/dolocretes), occurring in the upper part of... 

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