Saddle (Baroque) Dolomite in Carbonates and Sandstones: A Reappraisal of a Burial-Diagenetic Concept
- Sadoon Morad
Published Online: 17 APR 2009
Copyright © 1998 The International Association of Sedimentologists
Carbonate Cementation in Sandstones: Distribution Patterns and Geochemical Evolution
How to Cite
Spötl, C. and Pitman, J. K. (1998) Saddle (Baroque) Dolomite in Carbonates and Sandstones: A Reappraisal of a Burial-Diagenetic Concept, in Carbonate Cementation in Sandstones: Distribution Patterns and Geochemical Evolution (ed S. Morad), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304893.ch19
- Published Online: 17 APR 2009
- Published Print: 29 MAY 1998
Print ISBN: 9780632047772
Online ISBN: 9781444304893
- saddle (baroque) dolomite in carbonates and sandstones –burial diagenetic concept;
- palaeoaquifiers and Mississippi-Valley-type (MVT) ore deposits;
- saddle dolomite –distinctive characteristics;
- compositional zoning in saddle dolomite crystals;
- stable isotopic compositions of saddle dolomite;
- quantitative constraints on saddle dolomite Formation
Saddle (baroque) dolomite, defined as coarse-crystalline dolospar with regularly to irregularly curved crystal boundaries and sweeping extinction, has been described from numerous diagenetically altered carbonates and sandstones in hydrocarbon reservoirs, palaeoaquifers and Mississippi-Valley-type (MVT) ore deposits. This chapter reviews petrographic, geochemical and fluid inclusion data on saddle dolomite from carbonate rocks and sandstones published since 1980, in order to reassess the original interpretation of this type of dolomite as a potential high-temperature diagenetic geothermometer.
The compilation shows that saddle dolomite from various sedimentary basins has the following characteristics: (i) variable Fe + Mn and Ca enrichment of saddle dolomite hosted in sandstones relative to saddle dolomite hosted in carbonate rocks; (ii) carbon isotopic compositions ranging from slightly positive (in carbonate rocks) to moderately negative values (in sandstones); (iii) moderately negative oxygen isotope values in sandstones and carbonate rocks; and (iv) strontium isotope ratios commonly more radiogenic than Phanerozoic seawater (> 0.708). Fluid inclusions in saddle dolomite homogenize at temperatures ≥ 60–80°C, with a maximum between 90 and 160°C. The salinity of the palaeofluids is uniformly greater than that of seawater (∼18–25 wt% NaCl eq.). Low eutectic temperatures suggest a complex aqueous solution dominated by NaCl + CaCl2 ± MgCl2 ± KCl.
Geochemical and fluid inclusion data, in conjunction with mineral paragenetic information, demonstrate that saddle dolomite is a reliable indicator of rock–brine interactions at temperatures that coincide largely with the liquid hydrocarbon ‘window’ and extend well into the dry gas zone.