Some aspects of carbonate sedimentation and

carbonate sequence stratigraphy

Differences between carbonates and siliciclastics

Siliciclastic and carbonate sedimentary bodies are subdivided by similar surfaces.

Difference: Carbonate accumulation are "in situ production"

Siliciclastics are transported

Carbonate production greatest to air/sea interface

Carbonate facies and fabrics indicators of sea level

Falling & Lowstand system tract

Transgressive system tractTerrigenous clastic Humid lime eco accommodation

Humid lime phys-Accommodation Arid carbonate Evaporites

Highstand system tractTerrigenous clastic Humid lime eco accommodation

Humid lime phys-Accommodation Arid carbonate Evaporites

Subdividing surfaces:Like clastics carbonates subdivided on the basis of bounding and internal

surfaces into sequences, parasequences and/or truncated carbonate cycles. Surfaces: Erosion surfaces (SB) or eroded parasequence boundaries, Some flooding surfaces including transgressive surfaces and/or MFS

Though shallow cycles of carbonate are composed of a relatively conformable succession of genetically related

beds or bedsets these cycles are often truncated and incomplete

Maximum flooding and trangressive surfaces can be missing.

Not, in the strictest sense, a match for the clasticmodels of parasequences

Still cycles can be used like parasequences If they exhibit truncated cycles and miss the sediments

of an initial transgression or maximum flooding event one should consider them as high frequency carbonate

cycles, not parasequences.

Sigmoids

GEOL 325 Lecture 13: Carbonates & Evaporites

Llucmajor reef complex

Kendall Photo

The basic reefal accretional unit is the "sigmoid ".

This is bounded by clear erosion surfaces (the product of sea level lowering and erosion with a matching correlative surface

downdip) but has no obvious marine flooding surfaces .

Updip and landward the sigmoid is represented by a horizontal lagoonal bed that basinward passes in sigmoidal bedded reef-

core lithofacies belt and seaward into clinoform bedded forereefslope beds and sub-horizontal basinal lithofacies.

The coral-morphology zonation within the reef-core facies of the sigmoid migrates seaward, aggrades vertically, or moves landward over the bounding

erosional surfaces.

This enables the sigmoid (like system's tracts) to be tied to specific segments of the sea-level curve.

Time interval between the erosional surface on the underlying sigmoid and the deposition of sediment marking the boundary of the overlying sigmoid: No

sedimentary fill recognized that records these events

Difficult for Parasequence identification

Some carbonate parasequence geometries : Tools for the interpretation of depositional setting

Commonly determined from a combination of

2 and 3 D Seismic data: low frequency resolution well logs: high frequency resolution cores: very high frequency resolution outcrops : Best access to a combination of highfrequency resolution and low frequency resolution

Cycles that deepen upward in the stratigraphic record

Rates of carbonate sediment production respond to photosynthesis and this production rate increases as the water becomes shallower .

Hence carbonate depositional systems commonly fill towards sea level and so "shoal" or "shallow" upward.

However varying nutrient levels can affect the carbonate fill of this basin.

Oligotrophic: Low nutient / low productivity Eutrophic: High nutrient/ high productivity

Relatively shallow

restricted Eutrophic

Deepening upward

pervasive Biotirbation

Indurations Erosion

Firmgroundcementing

Exhumations winnowing

condensation filled burrows

Sea level fall Erosion

Firmground

Cycles that deepen upward in the stratigraphic recordo Rates of carbonate sediment production respond to photosynthesis and this production rate increases as the water becomes shallower .

o Hence carbonate depositional systems commonly fill towards sea level and so "shoal" or "shallow" upward.

o However varying nutrient levels can affect the carbonate fill of this basin.

oCarbonates responded not only to base level change but also the varying nutrient levels in the basin. During sea level "lows" the

shallower and more isolated the basin caused nutrient productivity to rise (eutrophic) while carbonate accumulation slowed.

o As the basin deepened during the onset of the following sea level rise, the rate of nutrient productivity fell (oligotrophic) while the rate of

carbonate production increased. o Eventually during the greatest rate of base level rise the rate of

carbonate production was reduced by the increasing depth of water.