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EVOLUTION OF CARBONATE SEQUENCES

SUMMARY
Variations in carbonate depositional settings through time produce corresponding changes in their lateral and vertical facies geometry. High frequency changes in facies are induced by sea level, and climate, whereas long-term low frequency changes are commonly related to paleo-geography and plate configurations or the evolutionary changes in the carbonate producing organisms. The sections below provide a general and classical introduction to the eclectic responses of carbonates to changes in their setting.

CONTROLS ON CARBONATE DEPOSITION

The geometry and facies relationships of carbonate accumulations are closely tied to the paleogeographic setting and changes in that setting through time, particularly water depth variation. Paleolatitude and clastic influx are major influences on the distribution of the carbonates, with facies largely controlled by the proximity of the depositional basin to open marine circulation and the position of the depositional setting within the basin. Carbonate sediments and their associated buildups are largely the by products of organisms whose evolution has a significant influence on carbonate deposition. Wilson (1975) reviews in detail some of the similarities and differences that occur in carbonates of different ages.

As outlined in an earlier section, the position of the depositional setting controls the geometry and continuity of deposits. For instance, deep-water basinal deposits are commonly widespread, thin beds of fine-grained carbonate formed by a combination of pelagic fauna and suspended shelf muds. At the edges of the basin, contributions from the shallow platform are greater. These basin-edge deposits range from turbidite fans and debris flows to reef talus. Porosities may be higher in these deposits than in basinal muds, but continuity may vary greatly. In contrast the margins of the carbonate platform or shelf may consist of linear to mound-like reef or shoal buildups that may have local porosity. To the lee of the margin the "back-reef" lagoon sediments occur in widespread beds of even thickness with discontinuous and mounded patch reefs scattered within their more seaward portions. Toward the updip limit of the platform, the lateral continuity of platform carbonate sands decreases, and supratidal carbonates associated with evaporites and elastics are common.

An example of water depth changes in the Cretaceous.

Changing water depth primarily causes changes in the depositional setting. Worldwide changes in relative sea level have occurred repeatedly and cyclically through geologic time. The rate of relative sea level rise has an obvious effect on the sediment type and nature of deposition, whereas the rate and extent of relative sea level fall has a marked effect on the diagenesis and erosion of carbonate sequences. As indicated elsewhere in this site eustatic sea level changes are believed to have dual origins: either they are glacially induced and have a high frequency or there is a change in the shape of the oceans due to tectonics and low frequencies occur. Despite their small size, the rapidity of high frequency eustatic events makes them the driving mechanism behind much of the cyclic nature of sediments. These sea level changes may be sinusoidal but the corresponding relative changes and movements of the coastline, whether over a narrow or wide shelf, are asymmetric (figure below).

Relative sea level changes may be the products of eustatic fluctuations, but may also be a response to subsidence or uplift of the depositional setting. These later effects may be related to faulting, thermal regime, salt diapirism, or isostacy (See figure below). Local differences occur where the underlying sediment compacts at different rates. Other relative changes are initiated when basins become isolated by the development of sedimentary or tectonic sills in conjunction with eustatic drops in sea level. When this isolation occurs at low latitudes, evaporative drawdown often produces a corresponding drop in sea leve

RESPONSES TO RELATIVE SEA LEVEL CHANGES

The response of the carbonate depositional surface to relative sea level changes include drowning of the surface, catching up with sea level rise, keeping up with the rise, or build up to exposure. Drowning of a reef or platform is caused by the failure of carbonate production to keep pace with a relative rise of sea level so that as the water deepens carbonate accumulation slows and is outpaced by clastic deposition.

The sediment surface leaves the realm of shallow-water carbonate sedimentation altogether and becomes submerged below the euphotic zone (right figure). The onset of drowning is expressed by a change from shallow-water faunas to deeper-water communities in reefs and on lagoonal floors. Buildups truly abandoned by a rising sea are commonly capped by a submarine hardground and enveloped by a shale cap or deepwater limestone. An example of a drowned ramp reservoir is the Devonian Onadaga of New York.

Reservoirs in drowned buildups on rimmed margins include the Devonian Swan Hills, Leduc and Rainbow reefs of Western Canada.The Middle to Upper Triassic of Ragusa Field in Sicily is an example of a drowned isolated platform

The survival of the rim of the platform but not its interior is a complex, but common, response intermediate between complete failure and complete success of a platform's ability to survive. The rate of relative sea level rise is such that only sedimentation on the platform rim (normally a reef) and/or isolated patch reefs on the platform interior keeps pace while the remainder of the platform is drowned, becoming a deep lagoon or shelf sea (right & figures below).

This response, which probably occurs when the rising sea flooded the platforms tops after a period of exposure, is probably the result of small but rapid eustatic rises. The pattern tends to develop in stages: initially the rate of rise exceeds the growth rate of both rim and interior, and the depositional setting shifts to deeper and more open-marine conditions. Carbonate accumulation slows and widespread submarine hardgrounds develop.

The lag phase is followed by catch-up. During the catch-up phase the reef rim and newly established patch reefs in the interior accumulate faster and build to sea level.

A third phase may follow when the interior lagoon fills up and a flat platform top is re-established. Reservoir examples of such rimmed margins are the Lower Cretaceous Sligo and Stuart City trends of the U. S. Gulf Coast. Pennsylvanian production in Aneth Field in the Paradox Basin is from a catch-up ramp. Examples of an interior shelf that has caught up and kept up are the Jurassic Arab "D" and Cretaceous Natih Formations of the Middle East and the Permian Grayburg Formation of the Midland Basin.

In the keep-up response, growth potential of rim and interior matches or exceeds the rate of relative rise (figure right). The platform interior fills to sea level, and in most cases the platform rim progrades seaward, building on excess sediment dumped on the flanks. These rims may consist of reefs or stacked carbonate sand shoals that show little apparent change in water depth during deposition.

The depositional environment over the platform interior varies from supratidal to very-shallow subtidal. During sea level rises, shelf width usually increases and elastics are confined to the landward side of the shelf.

Shoaling upward carbonate sequences usually represent sedimentation, particularly toward the seaward margin of the shelf. Occasionally, during rises, isolated depressions land-ward of the shelf margins produced by wind deflation during a sea level low or during growth of a rimmed margin become evaporative lagoons. Individual shoaling cycles tend to be widespread and where clastic supply is low, are frequently terminated by supratidal evaporite sequences. On narrow shelves with low clastic supply, carbonates may dominate the seaward margin of a clastic-dominated shelf. Where clastic supply is high, carbonates and elastics interfinger rhythmically. When a relative sea level rise slows a carbonate shelf system can be expected to fill up to the supratidal with the excess sediment causing the coast to prograde seaward. Examples of producing shelf margins where carbonate production has kept up with sea level rise are the Permian reservoirs in the Delaware and Midland Basins.

Shoaling upward cycles in carbonates are common in stable platform and shelves ( figure below).

The supratidal evaporites associated with these shoaling upward carbonates are formed only during sea level rises and should not mistakenly be interpreted as forming during sea level falls. The shelf interior has few complete shoaling upward cycles because hiatuses are common and not all sea level rises extend all the way across the shelf interior. In contrast, the shelf margin and basin centers may lack shallow water sediments because the subsidence is so rapid that evidence of the progradation cycles is obscured.

Thus, where subsidence is extremely fast, as on a basin margin immediately following continental breakup, the effects of rapid subsidence may hide cycles. Instead of the asymmetric shoaling upward cycles common to stable shelves, symmetrical shoaling and deepening cycles might be predicted.

While carbonate platforms and reefs have the potential to keep pace with all but the fastest rises in relative sea level, they are very poorly equipped to shift the loci of carbonate production and deposition when there is a relative drop in sea level. The flanks of platforms are usually so steep that reefs or other carbonate fades belts are unable to gradually migrate down slope following the retreating sea. Beach erosion and subsequent terrestrial weathering, quickly remove what little sediment is deposited during this retreat. Consequently, the most common record of sea level drops on carbonate platforms is a subaerial hiatus associated with karst development, cliff erosion, leaching and possibly dolomitization.

During stillstands in the retreat of the sea, the connections of the basin to the open sea may be closed by fringing reefs or structural highs. This tendency toward isolation of the basin and the lack of clastic influx makes carbonate basins particularly prone to evaporite deposition when sea level drops (figure right). Typically a sea level drop terminates carbonate deposition; the exposed shelf is cemented so little detritus is shed and erosion of the elastics trapped on the shelf is minimal.

However, some basins at sea level lows are dominated by shelf derived elastics because their access to the open sea makes them non-evaporative (figure above). Reservoir examples of clastic offlap and smothering of downramp buildups are the Permian Scurry and Jameson Formations of the Midland Basin.

USEFUL REFERENCES

Asquith, G. B., 1979, Subsurface Carbonate Depositional Models — A Concise Review: Petroleum Publishing Co., Tulsa, 121 p. Ali Fazeli = egeology.blogfa.com

Kendall, G. C. St. C. and Schlager, W., 1981, Carbonates and Relative Changes in Sea Level: Mar. Geol, V. 44, p. 181-212. Ali Fazeli = egeology.blogfa.com

Vail, P. R.,et al, 1977, Seismic Stratigraphy and Global Changes of Sea Level: p. 49-212 in C. E. Payton (ed.), Seismic Stratigraphy —Applications to Hydrocarbon Exploration, AAPG Memoir 26, Tulsa. Ali Fazeli = egeology.blogfa.com

Wilson, J. L., 1975, Carbonate Facies in Geologic History: Springer-Verlag, New York, p. 96-347. Ali Fazeli = egeology.blogfa.com