Abstracts of Selected Citations in the 2000's

EXTREME CARBONATE DEPOSITS: CHALKS ON LAND AND IN THE OCEANS [abs.]

Scholle, P. A.

Geological Society of America, Abstracts with Programs, v. 32, no. 7, p. A313, 2000.

Abstract: Chalks represent several sedimentary extremes based on their geographic and temporal distribution, grain size, composition, sedimentation rates and effect on ocean chemistry. Chalks are limestones predominantly composed of calcareous nannofossils (mainly coccolithophores) and subordinate planktic foraminifers. Thus, they are restricted to the Jurassic-Recent interval and, because of the nature of the organisms, are extremely fine-grained (0.1-1 mm) and slowly deposited. Chalks contain mainly low-Mg calcite, a very stable form of calcium carbonate that makes chalks resistant to near-surface diagenesis.

Given the absence of volumetrically important pre-Jurassic calcareous plankton the rapid rise of such planktics in the Jurassic-Early Cretaceous was a paleontologic and sedimentologic revolution. Earlier carbonate sedimentation was largely confined to shelves, with starved sedimentation in deeper waters protected from terrigenous influx. In the post-Jurassic era, the locus of carbonate sedimentation shifted, with 60% or more of all carbonate sediment being deposited as chalks in deep-water areas. Dissolution in very deep waters (currently below 4-5 km), and dilution by terrigenous influx from continents, limits distribution of deep-sea chalks. However, the continued input and dissolution of pelagic carbonate in undersaturated ocean waters led to a depression of the carbonate compensation depth and altered the geochemistry of much of the world ocean.

Distribution of shelf-sea (non-oceanic) chalks is largely a function of eustatic sea-level stands. The largest perturbation occurred in Late Cretaceous when sea levels were ~200-300 m higher than today. Reduced land masses (and thus availability of terrigenous sediment), coupled with an abundance of broad and deep shelf seas, led to chalk deposition over nearly 30% of Europe, large parts of North America (especially the Western Interior seaway and Gulf and Atlantic coastal plains), and many other regions. This certainly marked an extreme in sedimentation. Coupled with Cretaceous anoxic events, it also marked an opportunity for remarkable global petroleum generation and entrapment.

SEDIMENTOLOGY AND PETROLEUM POTENTIAL OF CRETACEOUS LIMESTONES IN THE PREAPULIAN ZONE, IONIAN ISLANDS, WESTERN GREECE [abs.]

Scholle, P. A. and Patsoules, M. G.

American Association of Petroleum Geologists 2001 Annual Convention Program, v. 10, A180, 2001.

Cretaceous (Aptian to Maastrichtian) rudist reefs and associated platform and slope facies were examined on the islands of Zakinthos, Kefallonia, Lefkada, and Paxos. These modern islands correspond roughly to the geometry of the Cretaceous platforms. Each island exposes one or more thick carbonate platform complexes (hundreds of meters to >1000 m in thickness) or has indicators of platform proximity in areas not presently exposed on the islands. The Cretaceous of Zakinthos includes only platform interior and platform margin facies; Kefallonia has platform interior, platform margin, slope, and proximal basin facies; and Lefkada and Paxos encompass only slope deposits.

Field and petrographic examination demonstrated extensive synsedimentary submarine cementation and porosity occlusion primarily in reef and immediate back-reef facies. Extensive subaerial exposure, leaching, and secondary porosity development followed during development of a widespread latest Cretaceous to Early Eocene unconformity, and probably also during later tectonic uplift. Leaching produced micromoldic or chalky porosity in the fine-grained shelf-interior strata. In reefal strata, porosity consists mainly of poorly-connected, primary intragranular voids plus some moldic pores created by removal of coarse, originally aragonitic debris. The best porosity (locally >20%) is developed in fore-reef slope to toe-of-slope environments, with extensive molds connected by primary intragranular pores.

Known oil seeps in this region, coupled with excellent potential Cretaceous reservoirs, make this largely undrilled region an interesting exploration target, comparable perhaps to the Cretaceous of Mexico. Deeper burial and overlying seals are required; these conditions may exist beneath major thrust sheets east of the Preaupulian Zone.