Evolution of Carbonate Sequences
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 carbonate
s to changes in their setting. Click here to link to a more detailed examination of the details of the sequence stratigraphy of carbonates.
Controls On Carbonate
The geometry and facies relationships of carbonate accumulation
s 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 carbonate
s, 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 carbonate
s 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 carbonate
s associated with evaporites and elastics are common.
Major sea level movements during the Cretaceous
An example of water depth changes in the Cretaceous determined by Maurer et al 2012 that captured eustatic
changes expressed in the Aptian Albian of the Bab basin
of the UAE. Note the glacial eustatic
low that causes a major progradation
al event in the upper Aptian (See block diagram below sea level chart).
Changing water depth primarily causes changes in the depositional setting. Worldwide changes in relative sea level
have occurred repeat
edly and cyclic
ally 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 sequence
s. 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).
General interrelationship of reltative sea level changes, tectonism and sedimentation.
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 level
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. Drowning of carbonate
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.
The above examples represent the evolution of a catch-up ramp. Examples of an interior shelf that has caught up and kept up are the Jurassic Arab "D" and Cretaceous Natih Formation
s 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 sequence
s 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 cycle
s tend to be widespread and where clastic supply is low, are frequently terminated by supratidal evaporite sequence
s. On narrow shelves with low clastic supply, carbonate
s may dominate the seaward margin of a clastic-dominated shelf. Where clastic supply is high, carbonate
s and elastics interfinger rhythm
ically. 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 cycle
s in carbonate
s are common in stable platform and shelves ( figure below).
s of carbonate
The supratidal evaporites associated with these shoaling upward carbonate
s 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 cycle
s because hiatus
es 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 cycle
s is obscured.
Thus, where subsidence is extremely fast, as on a basin
margin immediately following continental breakup, the effects of rapid subsidence may hide cycle
s. Instead of the asymmetric shoaling upward cycle
s common to stable shelves, symmetrical shoaling and deepening cycle
s might be predicted.
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 formation
s of the Midland Basin.
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.
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