Basins marginal

The modern Black Sea also highlights the impact of siliciclastic dilution on Corg concentrations. At the euxinic sites on the basin margin... 

The basin margins are characterized by calcite and minor dolomite cements, many of which which formed in isotopically light brackish or meteoric water at low temperature. In general, calcites did not form near the sediment-water interface, but during shallow burial. On the east side of the basin these cements are characterized by widely varying 5 Cpdb values (+20 to -30) compared with central basin cements (+5 to -10). Sr isotopic ratios in cements are lower than the marine depositional waters on the east side of the basin, but are higher than expected for depositional waters on the west side. 

This paper includes 58 carbon-oxygen and 25 strontium isotopic analyses from my research group and synthesizes these data with previous published work in the basin. The new analyses extend the coverage of the basin to nine additional areas, including the southern and eastern parts, where no similar data have been published. This paper introduces new comparisons between cementation in the central basin and that in basin margins. 

Fig. 9. Mg-Fe-Mn content of San Joaquin calcite cements. Data from Boles Ramseyer (1987), Fischer Surdam (1988) and Hayes Boles (1993). Note that central basin cements are characterized by relatively high total Fe + Mg + Mn with an (Fe + Mn)/Mg ratio of 3/1 compared with basin margin cements.

Calcite occurs as cement zones 10-150 cm thick. Pore-filling calcite crystals are anhedral blocky spar crystals (0.1-0.5 mm). In general, the petrographic appearance of basin margin calcite crystals is no different from calcite cements occurring in the basin centre. Some samples exhibit moderate to strong luminescence, whereas others are uniformly non-luminescent. 

High cement volumes characterize many calcite cements from the basin margin sediments, and these presumably formed at shallow burial depths before significant compaction (Fig. 4E). Intergranular cement volumes are of the order of 30% to more than 40% in these samples. However, most cements from the basin margins have volumes indicating that they formed after some compaction and, like the central basin, cementation is a process that extends through much of the burial history (al-... 

CpDB= +5.9%o) (Fischer Surdam, 1988) and from the Mount Poso field (5 Opp>B =-2.29%o, 5 CpDB = 4.27%o) (Hayes Boles, 1993), and from the west side in the North Belridge fi eld, where high-volume early marine cements have 5 Op B = -2.69%o and 5 Cpdb = -19.56%o (Taylor Soule, 1993). Overall, however, calcites with S Opug near zero, irrespective of their volume, are relatively rare at the basin margins, indicating that most of these cements did not form in equilibrium with sea water. 

A meteoric origin is suggested for some of basin margin cements that have relatively low oxygen isotopic compositions yet could not have been exposed to high burial temperatures. These cements have relatively high interstitial volumes (> 30%) and are usually calcite, but in some cases are dolomite. Based on their occurrence in shallow... 

Fig. 10. (A) Oxygen and carbon stable isotopic composition of calcite cements from the San Joaquin basin margins. Data sources from Table 1. (B) Oxygen and carbon stable isotopic composition of calcite cements from the San Joaquin basin centre. Data sources from Table 1. Note the restricted range of carbon isotopes compared with basin margin cements (cf. with (A)).

The early marine cements on both basin margins have relatively wide variations in carbon isotopic composition compared with the basin centre (Fig. 10). Carbon isotopes in the basin centre have S CpDB values between +5%o and -10%o, whereas basin margin cements commonly have values between +10%o and -20%o values as low as -30%o and as high as +20%o also occur. Presumably, the overall shallower depositional conditions at the basin margin and/or greater fluid mobility compared with the basin centre have resulted in different reactions, producing dissolved carbon within tens of metres of the sediment-water interface. Negative S Cp B values presumably represent bacterial oxidation or sulphate reduction, and positive values are from bacterial fermentation (Irwin et ai, 1977). 

The meteoric and marine cements of the basin margin are relatively pure, containing less than 5 mol% total Mg + Mn + Fe (Boles Ramseyer, 1987 Fischer Surdam, 1988 Hayes Boles, 1993 Lee Boles, 1996). In contrast, cements in the central basin, particularly those forming at moderate burial depths and temperatures (about 40-70°C), commonly have 5-10 mol% of Mg + Mn + Fe (Fig. 9). 

There are some differences to be noted in the proportion of trace elements in the basin margin cements compared with those of the central basin. High proportions of Mn and/or Fe are found only in meteoric cements (Boles Ramseyer, 1987 Fischer Surdam, 1988). Further studies have shown that meteoric cements may also have Mg as the dominant trace element (Hayes Boles, 1993). This latter work also showed that the trace element distribution of basin margin calcites appears to vary on a reservoir scale. Thus, the proportion of Mg-Mn-Fe is distinctly different in calcite cements of the Vedder sandstone from Mount Poso and Round Mountain fields, even though they are interpreted to have formed at a similar time (i.e. burial temperature). As concluded by Hayes Boles (1993), local reactions in the reservoir are occurring at a faster rate than potential mixing processes between these reservoirs. 

Only a few early calcite cements from the basin margin have a of zero (Fig. lOA), and... 

Table 1. Examples of major carbonate-cemented zones as identified from sonic and bulk density logs and lithological descriptions of cuttings for a variety of structures in dilferent parts of the Eromanga basin (Fig. lA). Note that the carbonate-cemented zones in Jurassic sandstones vary in cumulative thickness, from a few metres (e.g. Strzelecki-10) to about 110 m (Spencer West-1), including over short distances (e.g. Strzelecki). All wells occur near the Cooper basin margin, or along major fault-bounded structural trends where the regional Nappamerri Group seal (Triassic) underlying the Jurassic sandstones is incompetent or missing...

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