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Remnants of an Early Mississippi River Delta Complex(es):

Sedimentology, Stratigraphy, and Depositional History

Clint H. Edrington and Harry H. Roberts

Coastal Studies Institute, Department of Oceanography and Coastal Sciences, Louisiana State University,

Old Howe-Russell Bldg., Rm. 331, Baton Rouge, Louisiana 70803

EXTENDED ABSTRACT

Early research (e.g., Russell, 1940; Fisk, 1944) recognized the two-phase cyclic depositional habit of the Holocene Mississippi River Delta plain, where fluvial input induces rapid delta building followed by fluvial abandonment and subsequent delta reworking. Upon this recognition, researchers established a geochronological model of five (now six) delta complexes composing the modern delta plain (see Kolb and van Lopik, 1958; McFarlan, 1961). Delta complexes have been subdivided into delta lobes (Frazier, 1967), whereas delta lobes have been further subdivided into higher-order subdeltas and crevasse-splays (Coleman and Gagliano, 1964). These early works have been built upon by numerous others, and together have culminated into the well established two-phase “delta cycle” model, which describes the regressive and transgressive phases of deltaic deposition on all spatial and temporal scales (see Penland et al., 1988; Roberts, 1997). Nevertheless, despite advances in our understanding of the sedimentological and architectural order of the Mississippi River Delta, geochronology on the sub-delta complex scale is still poorly developed.

The Tiger and Trinity shoals area is one such example where precise geochronology is lacking (Fig. 1). Both shoals are shore parallel-to-concave-landward submarine sand bodies located on the western Louisiana Inner Continental Shelf, and are thought to represent the final transgressive stage of separate early Mississippi River Delta lobes. Evidence has been insufficient in determining whether these shoals originated from the same delta complex, or perhaps two separate ones. Penland et al. (1990) associated Trinity Shoal with the Teche Delta Complex. Preliminary interpretations of subbottom profiles from this study, however, suggest that Trinity Shoal, which lies seaward of Tiger Shoal, may have originated from the older Maringouin Delta Complex (Fig. 2). The objective of this study is to evaluate the sedimentology, stratigraphy, and depositional history of Tiger and Trinity shoals in the context of a rising Holocene relative sea level, so as to genetically link these shoals to their appropriate delta complex, i.e., are Tiger and Trinity Shoals linked to the Maringouin Delta Complex or the Teche Delta Complex, or is one shoal assigned to each?

An initial marine geophysical survey collected approximately 1150 km (715 mi) of high resolution subbottom profiles (chirp sonar) spanning both Tiger and Trinity shoals: 42 north-south subbottom profiles, with a spacing interval of approximately 0.9 km (0.6 mi), and 11 east-west subbottom profiles, with a spacing interval of approximately 2.3 km (1.4 mi), were gathered (Fig. 3). Chirp sonar was collected using an Edgetech sub-bottom profiler, a sonar system that uses a linearly swept FM pulse of 2-12 kHz frequency range to penetrate the shallow subsurface (upper ~30 m [100 ft]), with a potential vertical resolution of approximately 10 cm (4 in). Chirp data were brought to Louisiana State University, loaded into a Petrel^TM workstation, and preliminary stratigraphic and geological interpretations were established. Based on these interpretations, vibracoring locations were chosen (see Figure 3) so as to (1) evaluate grain size distribution across the shoals, (2) penetrate various facies for further geological analysis, interpretations, modeling, and dating, and (3) integrate core data with chirp data for extrapolation of stratigraphic and geological interpretations across the study area.

Forty-six vibracores with a maximum length of 4.5 m (15 ft) were extracted. Cores were brought to Louisiana State University where they were logged (P-wave velocity, P-wave amplitude, and gamma attenuation measurements) using a GEOTEK core logger (Fig. 4). Cores were then cut into halves, imaged, and described texturally. Sediments were also sampled at 50 cm (20 in) intervals for grain size analysis: a Gilson Shaker will be used for sand size particles, whereas fines will be evaluated using a Sedigraph Analyzer. Furthermore, organic sediments were sampled for radiocarbon dating, whereas discrete muds were sampled for palynological, dinoflagellate, and foraminifera analysis.

Acquired data reveal distinctions between Tiger and Trinity shoals. First, the base of Tiger Shoal is elevated higher than that of Trinity Shoal (see Figure 2). Second, Tiger Shoal is both thinner and smaller in area than Trinity Shoal (see Figures 2 and 3); hence, it is volumetrically lesser. These preliminary observations suggest that Tiger Shoal was deposited post-Trinity Shoal, under higher sea level conditions, and that its sand source delivered on a much smaller scale.

Future field work will involve collecting a second acoustic survey in the summer of 2009 using a GeoPulse Boomer. The GeoPulse Boomer’s dominant frequency is approximately 1 kHz, which will allow for deeper penetration through the underlying Maringouin Delta Complex and perhaps the Pleistocene Prairie surface, in addition to covering blind spots associated with chirp data. At least 25 additional vibracores will be extracted to augment incomplete core data (short cores) collected in 2008, as well as to cover those areas, deemed necessary after initial analyses, where new data are needed.

ACKNOWLEDGMENTS

This research was conducted under Minerals Management Service funding distributed through the Louisiana Department of Natural Resources and a cooperative agreement (#2512-07-12/435-700668) with Louisiana State University. The authors are grateful for research support and the chance to increase our understanding of the Tiger-Trinity shoals complex. Technical support was provided by the Coastal Studies Institute Field Support Group. Special thanks go to Walker Winans, Floyd De Mers, Chris Cleaver, Darren Depew, and Charlie Sibley. Eddie Weeks and his family are warmly acknowledged for allowing us to use their camp at Cypremort Point as a base of operations for the project. We would also like to thank the Department of Geology and Geophysics at Louisiana State University for access to Petrel^TM software, which was provided through a grant from Schlumberger.

REFERENCES CITED

Coleman, J. M., and S. M. Gagliano, 1964, Cyclic sedimentation in the Mississippi River Delta plain: Gulf Coast Association of Geological Societies Transactions, v. 14, p. 67-80.

Fisk, H. N., 1944, Geological investigation of the alluvial valley of the lower Mississippi River: U.S. Army Corps of Engineers, Mississippi River Commission, Vicksburg, Mississippi, 78 p.

Frazier, D. E., 1967, Recent deltaic deposits of the Mississippi River, their development and chronology: Gulf Coast Association of Geological Societies Transactions, v. 17, p. 287-315.

Kolb, C. R., and J. R. van Lopik, 1958, Geology of the Mississippi deltaic plain, southeastern Louisiana: U.S. Army Corps of Engineers, Waterways Experiment Station, Technical Report 2, 482 p.

McFarlan, E., Jr., 1961, Radiocarbon dating of Late Quaternary deposits, south Louisiana: Geological Society of America Bulletin, v. 72, p. 129-158.

Penland, S., R. Boyd, and J. R. Suter, 1988, Transgressive depositional systems of the Mississippi Delta plain: A model for barrier shoreline and shelf sand development: Journal of Sedimentary Petrology, v. 58, p. 932-949.

Penland, S., D. L. Pope, R. A. McBride, J. R. Suter, and C. G. Groat, 1990, Assessment of sand resources in the Trinity Shoal area, Louisiana Continental Shelf: Louisiana Geological Survey, Cooperative Agreement Submitted to U.S. Minerals Management Service No. 14-12-0001-30387, Baton Rouge, 46 p.

Roberts, H. H., 1997, Dynamic changes of the Holocene Mississippi River Delta plain: The delta cycle: Journal of Coastal Research, v. 13, p. 605-627.

Russell, R. J., 1940, Quaternary history of Louisiana: Geological Society of America Bulletin, v. 51, p. 1199-1234.

Edrington, C. H., and H. H. Roberts, 2009, Remnants of an early Mississippi river delta complex(es): Sedimentology, stratigraphy, and depositional history: Gulf Coast Association of Geological Societies Transactions, v. 59, p. 247-252.

	Figure 1. Location of the Tiger and Trinity Shoals area. 100 km = ~62 mi.
	Figure 2. Subbottom profile lines A-A’, B-B’, and C-C’, which span both Tiger and Trinity shoals. See Figure 3 for locations. Notice that the seaward (southerly) extent of Trinity Shoal is much deeper than that of Tiger Shoal. Vertical exaggeration is 5X. Vertical time scale (i.e., 10 ms) is in one-way-travel time. Subbottom profile lines are not scaled to one another.
	Figure 3. Sea-bottom surface map of the Tiger and Trinity shoals area. Overlying grid represents locations of subbottom proiles gathered, whereas white spheres represent vibracore locations. Subbottom profile lines A-A’, B-B’, and C-C’ shown in Figure 2 are highlighted. 100 km = ~62 mi.
	Figure 4. P-wave travel time log, P-wave signal amplitude log, attenuated gamma count log, and image of core TT-23-08. Notice the change in the attenuated gamma count curve at approximately the sand-sandy mud contact. The red dash line and red arrow indicate this contact. See Figures 2 and 3 for location of core TT-23-08. 150 cm = ~59.1 in.