Equatorial carbonates: an earth systems approach

Abstract

The hypothesis here is that an earth systems ‘processes to products’ approach can be used to better develop predictive models for the recognition and assessment of under‐evaluated equatorial carbonate systems. Warm temperatures, together with common clastic, fresh water and nutrient influx, as well as basinal settings in the equatorial tropics, all have a major impact on carbonate deposition and diagenesis. Specific features of equatorial carbonate systems resulting from the combination of processes acting in the region include: common occurrence of photoautotrophs and heterotrophs, aragonitic and/or calcitic dominant mineralogies, lack of coated grains or aggregates, common associations with clastics, lack of associations with evaporites, and diversity of platform types, including oligophotic ones. Additional diagenetic characteristics include: common micritization and bioerosion, paucity of marine cements, extensive vadose dissolution and concomitant phreatic cementation. There is also significant replacement of aragonite by calcite in regions of meteoric groundwater flow, common burial compaction and leaching, as well as localized massive dolomitization via sea water or continental derived groundwater flow. Although equatorial carbonates fall into the warm‐water Photozoan Association, many of the features described above are at odds with models derived from their warm‐water, arid‐zone counterparts. Instead, a range of the equatorial carbonate features show some similarities with those formed in cool waters, and there have been difficulties separating carbonates from these two very different climatic regimes. Recommendations for the recognition of Phanerozoic regional equatorial carbonate development are: (i) a diversity of calcitic and/or aragonitic photoautotrophs; plus (ii) common elements of the Heterozoan Association; plus (iii) independent (for example, isotopic) evidence for warm temperatures (>22°C). Additional indicators towards a humid equatorial setting are: (iv) situation in appropriate palaeolatitudes; (v) lack of association with sedimentary evaporites, coated grain or aggregates; and (vi) geochemical evidence for reduced marine salinity and/or nutrient upwelling. The aim is that this work will lead to greater awareness and understanding of equatorial carbonate systems, and contribute to the development of globally predictive models to better understand past and likely future environmental change.

Diagenesis and fracturing of a large-scale, syntectonic carbonate platform

“…The diagenesis of the Tonasa Limestone and other similar platforms differs markedly, however, from many Neogene systems in the region that may comprise reservoirs in the subsurface Fulthorpe and Schlanger, 1989;Grotsch and Mercadier, 1999;Vahrenkamp et al, 2004). These Neogene reservoirs with porosities of up to 10-40% commonly have a layered development due to repeated subaerial exposure and leaching in the vadose zone, pervasive phreatic cementation, with early fabrics overprinted but commonly not masked by later diagenesis Dunn et al 1992;Zampetti et al 2003;Vahrenkamp et al, 2004;Wilson, 2012).…”

“…The climatic, oceanographic and basinal context unique to any platform will also influence platform alteration and these factors may be indirectly affected by tectonics (Moore, 2001;Wilson, 2012). The major question addressed here is: to what extent does bioclasts that were reworked downslope as well as a range of lithic clasts derived from the Tonasa and underlying formations (Wilson and Bosence, 1996;Wilson, 1999;Wilson et al, 2000).…”

Unravelling the depositional origins and diagenetic alteration of carbonate breccias

“…Differential marine cementation is a feature of platform margin slope breccias, both preand early post-reworking, attributes most reasonably related to primary facies variability and flushing by seawater together with its degassing in upper slope to platform margin settings. This feature is seen, albeit rarely, not only within other Cenozoic regional examples (Tonasa: Arosi and Wilson, 2015;Kedango: Madden and Wilson, 2013;Berai: Tanos et al, 2013) but also more commonly other modern and ancient slope breccia examples ( Wilson, 2012;Arosi and Wilson, 2015). In comparison, meteoric dissolution features are common for subaerial talus slope breccias and karstic/cavern collapse breccias and may be accompanied by geopetal matrix infiltration and meniscus or cirucm-clast cements (Table 3 and references there in).…”

“…Although secondary mouldic and vuggy porosity have been partially infilled by blocky to drusy and fibrous calcite and microdolomite, the effects of ?multiple stages of, and in particular late stage burial, dissolution have resulted in high porosity with good connectivity Tanos et al, 2013). These late leaching fluids are thought to be basinal derived, perhaps associated with the early stages of hydrocarbons generation during burial, and are noted elsewhere along the margins of the Berai Platform Saller and Vijaya, 2002;Wilson, 2012;Subekti et al, 2015). While the throughput of post-brecciation burial fluids may have been enhanced in the Berai breccias, post-brecciation diagenetic effects are uncommon for the Batu Gading, Kedango and Tonasa breccias; limited to compaction effects and some late stage fracture filling cements.…”

Fundamental controls on fluid flow in carbonates: current workflows to emerging technologies

“…In recent years, there have been several attempts to develop a broader perception of the systems that drive diagenesis and to explore common diagenetic themes within given geographical regions (van Buchem et al 2010;Machel 2010;Coimbra & Olóriz 2012;Wilson et al 2013). Wilson (2012) promoted an Earth Systems approach, identifying common environmental, sedimentological and diagenetic factors as a means of identifying equatorial carbonate systems. In this volume, Li et al (2014) provide a preliminary demonstration of the value of comparative studies based on their analysis of the La Molata outcrop, southern Spain.…”