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PSThe Significance of Temperate-Type (Foramol) Carbonate Systems as Indices of Depositional Dynamics*
By
Fotini Pomoni-Papaioannou1
Search and Discovery Article #50062 (2008)
Posted February 26, 2008
*Adapted from poster presentation at AAPG and AAPG European Region Energy Conference, Athens, Greece, November 18-21, 2007
Editor’s note: Appreciation is expressed to Anne Pinkey for format conversion of the original document.
1Department of Geology and Geoenvironment, Section of Historical Geology & Paleontology, University of Athens, Panepistimiopolls157 84, Athens, Greece
Acceptance of a spectrum of warm- through cold-water shallow-marine carbonate facies remains an important challenge in interpreting the origin and significance of platform carbonates.
In the western Pelagonian domain, Rhodiani area (Northern Greece, Internal Hellenides) (Figure 1), the passage from a Late Jurassic – Early Cretaceous "Bahamian-type" platform, which was frequently exposed under subaerial conditions, to a Late Cretaceous open shelf environment, of ramp-like morphology, resulted in significant carbonate diversification concerning the biota assemblage, mineralogy, microfacies types, and early diagenetic patterns.
The Upper Cretaceous is characterized by calcite-dominated, "foramol" type, grain-supported, rudist-bearing limestones, whose biota assemblage recalls a temperate-type sedimentary setting. Due to intensive bioerosion of rudists and lack of early cementation, a significant amount of bioclasts, transported off-shelf by storm-related sand flows, accumulated in a pelagic realm. An overall destructive early diagenetic regime characterizes the Upper Cretaceous platform carbonates, and major porosity destruction and lithification occurred mainly in response to chemical compaction of calcitic skeletons during moderate to deep burial.
Paleoenvironmental conditions during Early Cretaceous suggest an open shelf domain, with gentle slope margins (ramps), recording a tendency of drowning, due to relative sea level rise coupled with low potential for growth typical of foramol carbonate factories. Complex arrangements of winnowed, remobilized and/or resedimented lithofacies took place during terminal highstands and/or regressive phases of sea level.
Temperate-type carbonate facies also accumulated in the central-west part of Crete (Figure 2), in the Apostoli Basin during the Neogene. The depositional environment corresponds to a shallow ramp; the sediments were deposited in a nearshore environment and under conditions analogous to those prevailing in the circa-littoral bottoms of the Recent counterparts of the Mediterranean Sea.
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Rhodiani Area, West Macedonia, Greece
In the Rhodiani area (Figure 3), Upper Cretaceous diverse carbonate facies overlie sediments that include limestone breccias, conglomerates, and debris-flow units, which in turn overlie Upper Jurassic shallow-water carbonates.
In the Upper Cretaceous interval, fine bioclastic limestones are interbedded with coarse-grained, bioclastic limestones. Sediments are represented by mollusks (rudists)-rich limestones and large benthic foraminifera (Foramol Association). Corals, green algae, and non-skeletal grains are absent. Limestones are rudstones and floatstones, in a matrix consisting of poorly sorted, bioeroded skeletal fragments. The biota were calcite-dominated.
Rapid evolution to open shelf domain, with gentle slope margins (ramps); shallow-water foramol skeletal debris, resulting from bioerosion of a mesotrophic/tendentiously eutrophic association, periodically contributed to hemipelagic deposition (temperate-type setting or tropical areas where water conditions (e.g., cold nutrient-rich upwelled currents) precluded the development of chlorozoan associations. Bioclastic sands, intercalated with the hemipelagic sediments are considered to have been transported periodically off-shelf by means of gravity flows, within a deepening trend (transgressive and highstands of sea level), giving rise to important phenomena of re-sedimentation. Relatively minor aggradation occurred along with a strong tendency toward progradation, with significant migration of the main depocenters. The resulting progradational wedges are composed of uncemented skeletal grainstone sheets and/or channels. This sedimentary setting differs substantially from the tropical carbonate settings, in terms of nature and mineralogy of the components, sedimentary texture. diagenetic potential, and 3-D geometries and is proved to be crucial in oil exploration.
Evidence of early marine cementation is scarcely represented in an overall destructive, early diagenetic regime. Major porosity destruction and lithification occur mainly in response to chemical compaction of calcitic skeletons during moderate to deep burial.
Uppermost Jurassic – Lower Cretaceous These limestones are succeeded by calcareous breccias, whose clasts, derived from the underlying succession, are embedded in a lateritic matrix. Latest Jurassic - Early Cretaceous active tectonic events generated a complex paleotopography over large areas, including also the development of ophiolites and lateritic formation with emersion.
The base of the studied carbonate succession is built-up of Upper Jurassic bioclastic packstones/floatstones-rudstones and coral boundstones, followed by oncoidal floatstones and wackestones with green algae (Chlorozoan Association). Among the components non-skeletal grains as ooids are detectable. The mineralogy of the skeletal and non-skeletal grains was originally aragonite.
Shallow-water carbonate shelf environment with limiting bioconstructed rims or discontinuous patch-reefs (tropical environment).
Early diagenetic processes (desiccation. meteoric diagenesis. subaerial exposures (pedogenic processes).
Rethymnon Region, Eastern Crete, Greece
Two Neogene basins occur in the Rethymnon region (Figure 5): (1) Rethymnon Basin to the north and (2) Apostoli Basin to the south. Elevated pre-Neogene terrane separates these basins. Most of the Neogene sediments filling the Apostoli Basin, in the central-west part of Crete, were deposited in terrestrial to shallow marine environments. Rethymnon Formation consists of alternating bioclastic limestones with marls. Rethymnon limestones correspond to a typical non-tropical carbonate lithofacies. They consist of: (i) Rhodalgal-type lithofacies (Figure 6 A, B) and (ii) an Echinofor-type lithofacies (Figure 6 C, D).
The Apostoli Basin consists of three principal formations:
The Miocene Rethymnon bioclastic carbonates formed on a gentle shallow ramp; they possess many similarities with the recent bioclastic sediments of the circa-littoral bottoms of the Mediterranean Sea.
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