Computers & Geosciences, Vol. 3, pp. 469-473. Pergamon ~ s s , 1977. Printed in Great Britain

COMPUTER SIMULATION OF BARRIER-ISLAND MIGRATION

DOUGLAS J. GEORGE and BRYCE M. HAND Department of Geology, Syracuse University, Syracuse, NY 13210, U.S.A.

(Received 7 March 1977)

Abstract--Five depositional subenvironments are commonly recognized in barrier island complexes: lower shoreface, middle shoreface, beach-upper shoreface, eolian, and lagoon. Facies belonging to these subenvironments can be characterized in terms of primary structures and sediment attributes. The facies assemblages which occur in the geologic record differ according to strandline still-stand, transgression, or regression and rates of basin subsidence. A FORTRAN program was developed to produce graphic output to aid in understanding facies relationships produced by these controlling parameters.

Key Words: Sedimentnlogy, Simulation, Barrier-island migration.

B ~ R I ~ COASTLINE SUBENVI~ONMENTS A typical barrier island coastline with subenvironments indicated is shown in Figure I (Davies, Ethridge, and Berg, 1971). The profile extends from lower shoreface to lagoon, but could be continued landward to merge with a fluvial coastal plain or other terrestrial deposits. By compiling data from several sources (Dickinson, Ber- ryhill and Holmes, 1972; Bernard, ie Blanc, and Major 1962; Davies, Ethridge, and Berg, 1971), we can charac- terize the subenvironments shown here in terms of primary sedimentary structures. For example, the lower shoreface, that zone seaward of the barrier in water 30 to 40 ft deep, consists chiefly of thin layers of interbedded silty clay and sand (sometimes graded), typically bur- rowed and churned by marine organisms. Some authors have referred to the sediments in this zone as shoreface toe deposits.

The middle shoreface extends from the 30-ft ba- thymetric contour landward to where the water is about 5 ft deep. Bioturbation is extensive in this zone, although some beds typically have lamination and cross lamina- tion well preserved. The less-burrowed beds may consist of coarse shell debris and shelly sand grading upward into finer sand and silty clay. They usually are inter- preted as storm layers that were deposited quickly, then left essentially undisturbed by organisms because of their thickness and coarse particle size.

The beach-upper shoreface zone is usually composed of clean, well-sorted sand. The most diagnostic structure

is delicate lamination or barely discordant cross laminae with gentle inclination matching the slope of the beach at time of deposition. Because physical reworking dominates the burrowing in this zone, laminae generally are preserved intact and represent one of the best available indicators of environment.

The eolian environment is characterized by sand dunes, eolian flats, and deflation basins, with highly variable cross bedding, and cross laminae may be ar- ranged in wedge-shaped sets.

Lagoonal sediments can differ widely, depending on factors such as climate and source areas, but silt is typical. Sands are introduced by tidal currents, storm washovers and streams discharging from the adjacent coastal plain. In addition, there may be variable amounts of shell material, and marsh vegetation may be sufficient to produce beds of peat. In arid climates, evaporites may occur .

Other observations in addition to sedimentary struc- tures may help in differentiating barrier-island suben- vironments. For example, systematic variations among subenvironments have been reported by Davies, Eth- ridge, and Berg (1971) who plotted mean grain size against quartz content. Such plots apply not only to modem barriers, but also to some examples of ancient assemblages, including one which was entirely subsur- face. Because it is possible in many instances to identify barrier subenvironments in ancient rocks, even if the only available information is from wells, it makes sense

~GOON I EOLIAN . BEACH--UPPER

- - _ ~ - ~ - / SHOREFACE

MIDDLE SHOREFACE

/LOWER

(AFTER DAVIES. ETHRIDGE, & BERG. 1971)

Figure 1. Barrier island cross section and subenvironments.

469

470 D. J. GEORGE and B. M. HAND

to see how facies representing various subenvironments can be assembled under different conditions of shoreline advance of retreat and different rates of basin sub- sidence.

rACW.S ~GI~,TIO~ TmP, OUGa TIMe

Three possibilities for accumulating barrier island sands and associated facies are shown in Figure 2. The differences in stratigraphic sequence depend on direction of shoreline migration and its angle of climb through time. In Figure 2A, the barrier has not shifted laterally, so facies boundaries are vertical. Figures 2B and 2C depict transgressive and regressive sequences, respec- tively.

Actual examples of these three possibilities are shown in Figure 3. Figure 3A is a cross section of Padre Island, modified after Fisk (1959). Shoreline position has remained relatively fixed through time, so that facies build vertically. Laterally adjacent facies interfinger with one another while thickening in place.

A Holocene barrier on the coast of Georgia (Fig. 3B) provides an example of a transgressive shoreline in which the angle of climb in the landward direction is steep enough to preserve a moderate thickness of barrier sand overlying lagoonal sediments.

Marine regression, with sealevel relatively stable and little subsidence is represented by Galveston Island (Fig. 3C). Facies succeed one another vertically, and within the barrier sand prism subfacies boundaries are essen- tially horizontal. In time, the lagoon should fill up and cease to exist as an active depositional environment.

A. BARRIER STILLSTAND

~trctnd I;ne h • t2-~l SL

°" """ :" : ' f:;;:; 4' ~: :.~'; '.~' ~'::-':-'==:-':::-

B. TRANSGRESSING BARRIER

strand line strand t|ne t 2 ~1.,~1' trongre~ion !l/~ t I SL

C. PROGRADING BARRIER

strand llne strand llne t l i regr eSl fall t2

" % ~ f SL

TERRESTRIAL [ ] -DEPOSITS ~ BARRIER SANDS

LAGOONAL ~ . SHOREFACE TOE "{~POSITS ~ DEPOSITS

(AFTER SLOSS. 1962)

Figure 2. Facies relationships within barriers (schematic).

A, PADRE ISLAND (AFTER FISK, 1

B. HOLOCENE BARRIER (AFTEB HOYT, 19e7)

- ~ ~ . SL

12 f t ~ - - ~ - - - ~ - ~ . . . . . . . .

C. GALVESTON ISLAND (AFTER BERNARD. LEBLANC, & MAJOR, 1962)

[ ] LAGOONAL ~ SHOREFACE TOE M.BARRIER - DEPOSITS ~-DEFOSIT5 ~ SANDS

Figure 3. Observed facies relationships within three modern barrier systems.

However, with a small amount of sealevel rise (or basin subsidence), the lagoon should be self-renewing and its deposits could well succeed the eolian facies.

A slightly different situation (Fig. 4), involving erosion of previously deposited sediments is the Charlestown Island barrier at West Bay, Rhode Island. Eolian and washover sands overlie lagoonal sediments. Most of the shoreface, however, is erosional rather than being a locus of net sediment accumulation. The system is migrating at a low angle of climb, similar to Galveston Island, but in a landward direction. As the Charlestown barrier continues to migrate, lower shoreface sediments might well be the first deposits on top of an erosional unconformity. Thick, permeable barrier sands are not likely to be preserved.

The examples so far described have been modern barrier systems, but the stratigraphic record is replete with examples where ancient nonmarine sediments grade upward into a marine section. The marine sequence may begin with shallow-water facies and grade into sedi-

TRANSGRESSING RHODE ISLAND BARRIER

(AFTER DILLON, 19701

Figure 4. Facies relationships resulting from low-angle landward migration of Rhode Island barrier.

Computer simulation of barrier-island migration 471

A

B

C

D

E

Figure 5. Computer simulation of barrier migration, showing time lines (bedding planes).

472 D. J. GEOROE and B. M. H^ND

ments deposited offshore. Alternatively, the marine sec- tion may follow an unconformity with offshore (deep water) sediments directly on the unconformity and the nearshore facies missing. The preserved sequence clearly depends in large part on the angle of climb and its direction.

Well known examples of regressive-transgressive cycles occur in the Upper Cretaceous Mesaverde Group (Hollenshead and Pritchard, 1961). Sands of the middle and upper shoreface, laid down during shoreline migra- tion now serve in many instances as reservoirs for petroleum. The thickest sand accumulations (best poten- tial reservoirs) occur where the shoreline remained fairly fixed geographically while subsidence and sedimentation continued. Where shoreline migration was rapid, the sands are markedly thinner. The importance of direction of migration as a control in determining sand thickness is well illustrated by this example. During low-angle transgression, the barrier sand facies was thinned by marine erosion, whereas regressive sequences preserve the full thickness of sand. Preservation of these sands occurs because the trailing edge of the barrier is noneroding.

Therefore the presence or absence of erosion is a key factor in determining the thickness of sand deposited by various barrier subenvirouments. Erosion normally oc- curs only when the barrier system migrates landward and the angle of climb determines which units will be thinned and by how much. These variables of migration direction and angle of climb can be controlled during computer simulation of barrier migration to provide examples of the diverse facies assemblages that may occur in nature.

SIMULATION OF BARRI]~-ISLAND MIGRATION

The purpose of the FORTRAN program described here is to make predictions of the exact relationships that determine facies thickness associated with barrier-island migration. The program calls for a barrier-island profile, the positions of facies boundaries on the profile, the angle of migration, and the overall distance of migration. After the appropriate calculations are made, graphic

output is provided via a CALCOMP plotter. The output can display the resulting barrier deposits in terms of either time lines or facies boundaries.

Figure 5 displays time line results of program runs where migration was changed progressively in five steps from seaward (with no basin subsidence) to landward (no basin subsidence). The time lines represent bedding planes. Figures 5A and 5B are prograding barriers whereas Figures 5C-E are transgressive. The pro- grading profiles display an increase in width (lateral ac- cretion) with no associated erosion. Figure 5C depicts a transgressive barrier at an angle steep enough to prevent erosion. However, Figures 5D and 5E show the profiles migrating landward at angles that are sufficiently low to induce erosion on the seaward side.

Examples of runs producing results with facies boun- daries are shown in Figure 6. The examples display nine runs beginning with a low angle of climb in the seaward direction. The angle then is steepened and reversed, and decreased until a low-angle landward direction is at- tained. Figures 6A-C depict prograding sequences, whereas Figures 6D-I are transgressive migrations. Loss of facies is evident as angle of climb becomes sufficiently small in the landward direction. In Figure 5I, only two of the five barrier-island facies are preserved.

CONCLUSIONS Preservation of barrier-island facies depends on the

direction of migration (seaward, stillstand, landward) and the angle of stratigraphic climb. Because regressive and s~stand sequences are nonerosive, they tend to preserve all barrier facies. However, landward migration (transgression) at low angles induces shoreface erosion and may cause the reduction or elimination of facies. The resulting facies succession is therefore dependent on the direction of migration and its angle of climb.

Barrier island profiles differ and therefore can display different degrees of facies preservation with similar migrations. The computer allows speed and flexibility in altering barrier profiles, facies boundaries, migration

A

/

I

Figure 6. Computer simulation of harrier migration, showing facies boundaries.

Computer simulation of barrier-island migration 473

angles, and migration distances. As a result the program can be a useful aid in predicting the relationships that can determine facies sequences resulting from barrier- island migrations.

~'ucENCES Bernard, H. A., LeBlanc, R. J., and Major, C. F., 1962~ Recent

and Pleistocene geology of southwest Texas; in Geology of the Gulf Coast and central Texas, and guidebook excursions: Rainwater, E. H., and Zingula, R. P., ed., Houston Geol. Soc., Houston, Texas, p. 175-224.

Davies, D. K., Ethridge, F. G., and Berg, R. R., 1971, Recognition of barrier environments: Am. Assoc. Petroleum Geologists Bull., v. 55, no. 4, p. 550-565.

Dickinson, K. A., Berryhill, Jr., H. L., and Holmes, C. W., 1972, Criteria for recognizing ancient barrier coastlines, in Recog-

nition of ancient sedimentary environments: Rigby, I. K., and Hamblin, W. E., ed., Soc. Econ Paleontologists and Mineralo- gists, Tulsa, Oklatioma, p. 192-214.

Dillon, W. P., 1970, Submergence effects on a Rhode Island barrier and lagoon and inferences on migration of barriers: Jour. Geology, v. 78, no. 1~ p. 94-106.

Fisk, G. H., 1959, Padre Island and Laguna Madre Flats, coastal south Texas: 2rid Coastal Studies Inst., Louisiana State Univ., Baton Rouge, Louisana, p. 103-152.

Hollenshead, C. T., and Pritchard, R. L., 1961, Geometry of producing Mesaverde sandstones, San Juan Basin, in Geometry of sandstone bodies: Peterson, J. A., and Osmond, J. C., ed., Am. Assoc. Petroleum Geologists, Tulsa, Oklahoma, p. 98-118.

Hoyt, J. H., 1967, Barrier-island formation: Geol. Soc. America Bull., v. 78, no. 9, p. 1125--1136.

Sloss, L. L., 1962, Stratigraphic models in exploration: Am. Assoc. Petroleum Geologists Bull., v. 46, no. 7, p. 1050-1057.