unfilled accommodation space - a fundamental problem in cyclostratigraphy

UNFILLED ACCOMMODATION SPACE -A FUNDAMENTAL PROBLEM IN

CYCLOSTRATIGRAPHYGREGOR P. EBERLI(1) and G. MICHAEL GRAMMER(2)

(1)Comparative Sedimentology Laboratory, University of MIami

(2)Texaco EPTD, Houston

ASSUMPTIONS IN CYCLOSTRATIGY: • Shallow-water carbonates are precise

accommodation gauges• Sediment fills all the available accommodation space

created by relative sea level changes

These assumptions are made in both modeling and display of cyclic deposition

Simulation models input parameters:• High-frequency sea level changes

• Subsidence

• Water-depth dependent sedimentation rate for each facies

• Initial lag time (lag depth)

• Filling of accommodation space

Fisher plot parameters:• linear subsidence

• equal duration of sedimentary cycle

• completely filled accommodation space

In the shallow-water carbonate environment most accommodation

space remains unfilled.• Filling of accommodation space is dependent on

a) the energy level at each location b) facies and sedimentation rate

• Problem = Assessment of unfilled accommodation space

THE MODERN ACCOMMODATION SPACE -

FLORIDA

Florida Bay MapDisplaying the modern,partly filled accommodation space.

The Pleistocene reef tract form the Florida Keys.

Holocene accommodation space in

Florida Bay • Filling of Holocene

accommodation space in Florida Bay produces a shallowing-upward depositional unit on islands. Deposition in nearby "lakes" conists of a thin veneer of mud or grains.

Irregular Pleistocene surface in Florida

• The irregular Pleistocene surface in Florida documents unfilled Pleistocene accommodation space. Holocene deposition remains restricted to this unfilled space. Such a situation can lead to mis-correlation of depositional cycles in

the rock record.

Aerial view of northern Florida Bay with mud banks.

• Holocene islands and Pleistocene reef crests are above the modern

sea level.

THE ACCOMMODATION SPACE - BAHAMAS

• 1) Andros Island and Joulters Cay

• 2) Schooners Cay

• 3) Platform interior

Unfilled accommodation space in the Holocene

1) Ooid Shoals (Joulters Cay) and mud flats (Andros Island). In both

locations accommodation space is overfilled next to unfilled space.

Unfilled accommodation space in the Holocene

2) Ooid Shoals (Schooner Cays, Bahamas)

Unfilled accommodation space in the Pleistocene

• On the interior of modern Great Bahama Bank water depth is presently about 7 m, and the Holocene sediment thickness is only about 2.5 m. During the last interglacial sea level is estimated to be about 6 m higher than at present. During this interval, a mean cycle thickness of 5.5 m and up to 15.5 m of unfilled accommodation can be calculated.

FILLING OF ACCOMMODATION SPACE -

A QUESTION OF LOCALITIES AND FACIES

Clifton Pier & Unda• Location of the two

sites in the Bahamas Clifton Pier is a high energy environment Unda is located in a platform interior setting

Variable accumulation rates • Variable accumulation rates of different facies result in

variable amount of filling of accommodation space. Most facies never fill the available accommodation space

Stratigraphic column and line drawing • Stratigraphic column and line drawing of the high-energy

deposits at Clifton Pier (Nassau, Bahamas). Note that within the Brunhes only 2 - 3 cycles can be distinguished.

Schematic stratigraphic column

• Schematic stratigraphic column of core UNDA, 16 km awayfrom the platform margin. Here 8 cycles are recognized above the Brunhes/Matuyama boundary.

Aerial photograph • Aerial photograph of rhomboid shoals in the lagoon of Belize, illustrating the

facies dependence for the filling of accommodation space. The fast growth rate of the reefs enable them to track sea level and keep the lagdepth to a minimum. Lagoonal sediments are not able to keep up withthe sea level rise.

• The cores of these patch reefs consist of Pleistocene reefs, documenting that the lagoon never completely filled during the last interglacial. Most likely it will not occur in the Holocene either.

RECOGNITION OF UNFILLED

ACCOMMODATION SPACE IN ANCIENT DEPOSITS:

EXAMPLE PARADOX BASIN

Location map of Paradox Basin

Pennsylvanian strat of the Paradox

basin

• Pennsylvanian strat of the Paradox basin displays cyclic deposition.Exposure horizons on top of nearly each cycle suggest a complete filling of the accommodation space.

Depositional cycles

• Stratigraphic column displaying the cycles and the exposure horizons.

Depositional cycles

• Stratigraphic architecture with bed thickness variations documents that accommodation space was not completely filled. Buildups and prograding shoals require unfilled space to develop.

Thinning of cycles

• Thinning of cycles from the buildup to the shelf area document

unfilled accommodation space

Outcrop Photograph

• Outcrop photograph of buildup and cartoon explaining the origin of the irregular top of the buildup. Buildups grow into the available accommodation space, while the intermound facies lag behind.When relative sea level sinks both the buildups and the shelf area become exposed. The next cycle of deposition first fills the remaining topography, thus helping to maintain nearly horizontal bedding planes.

• I) Accomodation Space - varying fill rates

Paradox

• II) Sedimentary RecordParadox

• ¥ Shallow-water carbonates often do not fill all available accommodation

space¥ Filling is dependent on the energy level on each locality, which is partly reflected in facies¥ Carbonates have a base level¥ Reefs and shoals are good indicators of unfilled accommodation space

• => Bed thickness is of limited value for assessing accommodation space=> Modeling of carbonates should include unfilled accommodation

CONCLUSIONS and IMPLICATIONS