Dolomite detrital

To convert calciiun carbonate to dolomite, some of the calcium must have been replaced by magnesiiun, requiring the partial dissolution of the carbonate. This process is promoted by contact with acidic pore water, such as occurs in organic-rich sediments because remineralization produces carbon dioxide. This is probably why dolomites are presently forming in detrital algal mats buried beneath sabkhat. The restricted extent of these modern dolomites reflects a kinetic hindrance to precipitation. Apparently dolomite precipitation in this setting is too slow to form substantial deposits when sea level is rapidly fluctuating. 

Coal contains detrital minerals that were deposited along with the plant material, and authigenic minerals that were formed during coalification. The abundance of mineral matter in coal varies considerably with its source, and is reported to range between 9.05 and 32.26 wt% (Valkovic 1983). Minerals found in coal include (Table 2) aluminosilicates, mainly clay minerals carbonates, such as, calcite, ankerite, siderite, and dolomite sulphides, mainly pyrite (FeS2) chlorides and silicates, principally quartz. Trace elements in coal are commonly associated with one or more of these minerals (see Table 2). 

These clays occur in limestones, dolomites, evaporites, shales, siltstones, and hydrothermal deposits. All the sedimentary material appears to have a diagenetic origin. Although the physical environments vary, the chemical environments should be similar. Saline or even super-saline conditions are implied by the presence of evaporite minerals associated with some of the deposits. In the other deposits it is possible that temporary evaporitic conditions (e.g., tidal flats) existed long enough for brucite to precipitate between the layers of expanded-layer minerals. It appears plausible that the parent material was a montmorillonite-like mineral (probably detrital in most cases). 

Dolomite rhombs have been observed on smear slides of sediments from several DSDP sites, and the occurrence of dolomite in these sediments has been documented quantitatively by Lumsden (1988). Deep-marine dolomite averages about 1 wt % in all sampled DSDP sediments throughout post-Jurassic time. The dolomite is nonstoichiometric, averaging 56 mole % CaC03 (Figure 8.24), and has a crystal size and appearance similar to that of supratidal dolomite. Lumsden (1988) concludes that most is an early chemical precipitate from seawater. He estimated that about 10% is detrital, but he advocates that more criteria are needed to distinguish dolomite formed in cold, deep marine waters from that formed in supratidal deposits before this estimate can be substantiated. 

Figure 8.24. Summary of composition of nonstoichiometric dolomite. A. Data for 246 samples from continental exposures of dolostones and dolomitic limestones. B. Data for 153 samples from continental exposures of dolomites related and not related to evaporite sequences. C. Ranges of Holocene dolomite compositions. D. Data for 35 samples of deep-marine dolomite from DSDP cores. E. Histogram of 245 samples of deep-marine dolomite E evaporite-associated, O organic origin, L samples of Lumsden (1988) above, D detrital, and U uncertain. (After Lumsden, 1988.)...

Figure 4 Ankerite (a) localized as overgrowths on grains of detrital dolomite (d). Similar high degrees of substrate control on nucleation are observed for several of the common authigenic phases in late diagenesis. Weber Formation, Pennsylvanian, western Colorado.

As a result of the anoxic, low sulphate concentrations in the Me zone, carbonates expected to form include siderite and ferroan dolomite/ankerite (Gautier Claypool, 1984). The precipitation of these carbonates occurs in sediments rich in reactive detrital iron (Coleman, 1985), as follows ... 

Our results indicate that kaolinite distribution in the Lunde Formation is not strictly controlled by the Kimmerian uplift and erosion. This is due partly to the formation of kaolinite during eodiagenesis and partly to the strong relationship between kaolinite abundance and detrital composition of the sandstones, particularly the original amounts of feldspars and mud intraclasts. Pervasive kaolinite formation, coupled with dissolution of calcite and dolomite cements, has been substantial in well 34/4-1. In well 34/7-A-3H sandstones, the top of which was buried deeper below the unconformity than that of well... 

Ferroan dolomite precipitation is also prominently localized around partially dissolved detrital K-feldspars (Fig. 7B). Localization is not strictly within the volume formerly occupied by the K-feldspar, but rather is crudely centred on the feldspar, extending also into the surrounding pore space. Ferroan dolomite clearly postdates quartz cementation. In a few samples there is petrographic evidence suggesting that ferroan dolomite pre-dates the formation of the late calcite. 

Most of the dolomites (including the detrital cores) have calcium-enriched compositions (Table 6 Fig. 8). Zoning within the ferroan dolomite is clearly visible in back-scattered electron images, and the higher Fe contents tend to be characteristic of the later zones (Fig. 7A). The degree of Ca enrichment, however, is not strongly controlled by Fe content and more Fe-rich portions of the crystals display a range in Ca content that is nearly as broad as that of less Fe-rich portions. Mn content is correlated positively with Fe enrichment (Fig. 9). 

Fig. 7. Thin-section scale localization of authigenic ferroan dolomite. Back-scattered electron images. Scale bars 100 tm. (A) Ferroan dolomite (f) localized on non-ferroan detrital dolomite (d) that has been fractured, partially dissolved, and partially replaced by the authigenic overgrowth. Note that the outer zones of the ferroan dolomite (f ) are brighter, reflecting their greater Fe content. (B) Ferroan dolomite (ankerite) (f) localized in the vicinity of a partially dissolved and replaced K-feldspar (k). The arrow indicates euhedral termination on a quartz overgrowth.

Fig. 8. Ca content versus Mg content for detrital and authigenic dolomite, plotted for different Fe contents.

Sources of Ca are very uncertain. No detrital carbonate (apart from the trivial volume of detrital dolomite described above) survives in the sandstones (including the early concretions). Carbonate... 

The patterns of diagenetic evolution recognized in this study allow discussion of the conditions for optimum porosity preservation and/or enhancement in the Serraria reservoirs. The best reservoirs of the unit occur in the Caioba area of the distal domain, where porosity was enhanced by dissolution of detrital feldspars and dolomite cement during telogenetic influx of meteoric waters. Similar conditions are expected for other structural blocks of the basin affected by post-rift uplift and erosion, or blocks bounded by major fault systems in which the Serraria Formation was relatively close to the... 

Dolomite, ferroan dolomite and ankerite occur mostly as thin (< 20 pm) overgrowths on detrital dolomite grains (Figs 5A,B,F and 6A) or, rarely, as... 

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