Diagenesis and Sequence Stratigraphic Framework—A Case Study in Upper Miocene Carbonates, La Molata, Southeastern Spain
Zhaoqi Li, Robert H. Goldstein, Evan K. Franseen, "Diagenesis and Sequence Stratigraphic Framework—A Case Study in Upper Miocene Carbonates, La Molata, Southeastern Spain", Sequence Stratigraphy: The Future Defined, Bruce Hart, Norman C. Rosen, Dorene West, Anthony D’Agostino, Carlo Messina, Michael Hoffman, Richard Wild
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Upper Miocene carbonates in La Molata, southeast Spain, consist of eight depositional sequences, capped by subaerial exposure surfaces 1 to 7. Known stratigraphy, mineralogy, climate, and duration of exposure provide superb opportunity for studying diagenesis and sequence framework. This integrated study shows that only minor diagenetic alteration has occurred during subaerial exposure (surface 1 to 6) that was short-lived (<533 k.y.) and in an arid climate, or the carbonate sediments were composed primarily of calcite.
Dolomitization occurred during the initial stages of sea-level fall associated with surface 7, by ascending freshwater-mesohaline mixing. This resulted in dissolution to create 10-20% porosity. During this long-lived period of subaerial exposure (greater than 5.3 m.y.) in a wet climate, major amounts of calcite cementation reduced porosity, forming an upper and a lower cemented zone. Cements in the upper zone are nonluminescent, whereas those in the lower zone exhibit luminescent zonation. In the upper zone, isotopic data from calcite cements show two meteoric calcite lines having a mean of d18O at -5.1 ‰ and -5.8 ‰ VPDB. No clear meteoric calcite lines are defined in the lower cemented zone, which has a mean of -6.7‰ VPDB. d13C. Values in both cement zones are predominantly negative, ranging from -10 to +2 ‰ VPDB, suggestive of carbon from soil gas or decayed organics. Tm ice in primary fluid inclusions shows a mode of 0.0 °C in both zones, indicating calcite cementation from fresh water.
These two zones define the position of two different paleo-water tables that formed during a relative fall in sea-level and erosional downcutting during the Plio-Pleistocene. The upper cemented zone pre-dates the lower cemented zone on the basis of known relative sea-level history. Each texture (boundstone, grainstone, packstone, and wackestone) produces a different relationship between percent calcite cement and porosity/permeability. Distribution of cements may be predictable on the basis of known sea-level history, and the effect of the cementation can be incorporated into subsurface geomodels by defining surfaces of rock boundaries that separate cemented zones from uncemented zones and applying texture-specific relationships among cementation, porosity and permeability.