Magazine article Oceanus

Shallow Carbonates Drilled by DSDP and ODP

Magazine article Oceanus

Shallow Carbonates Drilled by DSDP and ODP

Article excerpt

Oceanic Benchmarks and Dipsticks for Continental Margins and Volcanic Aseismic Ridges

Shallow carbonates, mostly marine and biogenic in origin, originate within the ocean, instead of being transported there from land as are siliciclastic sediments. Individual carbonate-secreting fauna and flora produce shells, micro- to macroscopic in size, to protect themselves from predators and adverse physical conditions. The bulk of the carbonate production, related to benthic carbonate-secreting flora and fauna living in symbiosis with micro-algae, is limited to the upper 100 meters of the water column where sunlight penetrates. Furthermore, the optimum carbonate production is restricted to relatively warm waters (subtropical and tropical regions) within a narrow range of water depths between low-tide depth and 20 meters. These basic parameters, in addition to the general evolution of oceanic carbonate-secreting biota, have greatly influenced the development of thick carbonate platforms and shelves, usually characterized by successive phases of growth, reduction, recovery, and ultimate demise or "drowning."

Taken together, billions of individual carbonate-secreting fauna and flora produce huge volumes of carbonates, indirectly compensating for the sinking of the substratum and/or the rising of sea level and thus unconsciously attempting to remain within the light. In addition to being relatively accurate indicators or "dipsticks" of sea-level fluctuation, shallow carbonates become also excellent benchmarks for quantifying the magnitude and rate of vertical motion (subsidence and uplift) characteristic of passive continental margins and intraplate volcanic ridges in the context of plate tectonics. Finally, because of the temperature limitation of most carbonate-secreting biota, shallow carbonates are rather precise recorders of latitudinal plate movement (horizontal translation) and climatic and biochemical changes.

During the past 25 years, the ocean drilling programs have recovered numerous shallow carbonate sequences, ranging in age from the late Triassic (230 million years ago) to the Quaternary period (the last 1.6 million years), along continental passive margins and aseismic volcanic ridges in intra-oceanic basins. Several ODP legs have been drilled specifically to address questions about the evolution of shallow carbonate systems. For instance, Leg 101 in the Bahamas, the ODP maiden voyage in spring 1985, was the first drilling leg fully dedicated to a single carbonate system.

Shallow Water Carbonates on Continental Margins

Triassic Development of the Northwest Australian Continental Margin. On the Wombat Plateau, Triassic (230 to 205 million year old) shallow carbonate rocks, so far the oldest sediments recovered by ocean drilling, were first deposited in association with deltaic sediments and then as shelf-lagoon limestone/marlstone and more than 200-meter-thick coral/sponge reef. The reef complex has some close similarities to the spectacular reefs of the Northern Calcareous Alps in the Dolomites that developed at the same time.

Late Jurassic/Early Cretaceous Development of the North Atlantic's Conjugate Continental Margins. In addition to dredging and seismic profiling, drilling on the Blake Plateau, in the Bahamas, and in the southeastern Gulf of Mexico, has helped to constrain the early evolution of the western North Atlantic passive margin. A Mesozoic shallow carbonate shelf or "giga-bank" at some point surrounded the Gulf of Mexico and stretched from the northern part of Cuba and the Bahamas to the Grand Banks off Newfoundland. In the southeastern Gulf of Mexico, the first shallow water limestones encountered by drilling are late Jurassic (about 140 million years old). Drilling in the Bahamas and on the edge of the Blake Plateau showed that the Mesozoic carbonate giga-bank, though segmented by several deep reentrant basins, already existed in the late Jurassic and early Cretaceous periods (approximately 125 million years ago). …

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