The Eocene Cocoa Sand Member of Yazoo Formation is fine grained, moderately to well sort-

ed, poorly cemented, quartz arenite. Surface exposures are poor, but it has been mapped from

west Choctaw County, Alabama to eastern Jasper County, Mississippi. In the subsurface, the Co-

coa Sand Member is identified by obvious protrusion both in Spontaneous and Resistivity Logs.

Northeast to southwest cross-sections (perpendicular to the paleo-shoreline) and northwest to

southeast cross-sections (parallel to the paleo-shoreline) are being developed, along with iso-

pach maps, to determine the sequence stratigraphic setting and a depositional model of the Co-

coa Sand Member. Previous work has interpreted the Cocoa Sand Member as a shelf margin

sand deposited as part of a lowstand systems tract or as a transgressive sand. The presence of

rip-up clasts at the base of the Cocoa Sand Member supports the presence of a transgressive

surface at the contact with the North Twistwood Creek. Grain size analysis indicates that the

sand coarsens upward and there is evidence in core that the upper contact of the Cocoa Sand

with the Pachuta Marl is sharp, representing an upper erosion surface. Based on an isopach

map of the Cocoa sand member in Wayne County, Mississippi, in general, the sand thickens to-

wards the south-east and is thickest near the Alabama border. Continuation of the isopach map

into Alabama is pending acquisition of additional logs from the geological survey of Alabama.

However, initial review of the isopach map indicates the possible development of sand ridges.

Further study is required to confirm a depositional model for the Eocene Cocoa Sand Member of

the Yazoo Formation.

Subsurface Stratigraphy of the Eocene Cocoa Sand Member in Mississippi and Alabama

ZHANG1, Xiaodong; GRISGBY1, Jeffery D.; FLUGEMAN1, Richard H.

1:Department of Geological Science, Ball State University, Muncie, Indiana, 47306 Email: xzhang@bsu.edu

RESULTS STRATIGRAPHY ABSTRACT

DISCUSSION

ISOPACH MAP

STUDY AREA RESULTS

Plaleogene of the Eastern Gulf Coastal Plain

This project concentrates on subsurface well log analysis from southwest Alabama into northwest Mississippi. Well logs are derived from Mis-

sissippi Department of Environmental Quality and Alabama Oil and Gas Board. Detailed isopach maps and cross sections have been constructed

to further describe the stratigraphy of Cocoa Sand Member of Yazoo Formation. According to the cross sections either perpendicular (NE-SW) or

parallel (SE-NW) to the paleo shoreline, we can see the distribution and structure of the Cocoa Sand Member clearly. It thickens toward southeast

part of Wayne County in Mississippi down to northwest part of Washington County in Alabama. On the basis of the isopach map, there are some

kind of sand ridges eroded by the wave processes, which is consistent with the cross sections. It is evident that when the thickness of Cocoa

Sand Member becomes thinner, the thickness of North Twistwood Creek Member correspondingly gets thicker. Rip clasts are found on the bottom

of the Cocoa Sand member, which supports the contact between Cocoa Sand Member and North Twistwood Member is a erosive surface. We in-

terpret the Cocoa Sand Member of the Yazoo Formation is deposited as part of the transgressive system tract with its source being the erosion

and reworking of the North Twistwood Creek Member during transgression. During transgression, the North Twistwood Creek Member was bro-

ken down, sediments reworked and re-deposited with rising sea level.

Reference

Baum, G.R. and Vail, P.R., 1988, Sequence stratigraphic concepts applied to Paleogene outcrops, Gulf and Atlantic Basins, in Wilgus, C.K., Hastings, B.S., Ross, C.A., Posamantier, H., Von Wagner, J. and Kendall, C.G.S.C., eds., Sea Level Changes;

an Integrated Approach: SEPM Special Publication, v. 42, p. 309-327.

Berggren, W.A. and Pearson, P.N., 2005, A revised tropical and subtropical Paleogene planktonic foraminiferal zonation: Journal of Foraminiferal Research, v. 35, p. 279-298.

Dockery, D.T., III, Thompson, D.E., and Ingram, S.L., 1994, The Mobil-Mississippi Office of Geology Core-Hole Project: Mississippi Geology, v. 15, p. 8-15.

Echols, R.J. et al., 2003, Sequence Stratigraphy of the Eocene/Oligocene Boundary Interval: Southern Mississippi: in Prothero, D.R., Ivany, L.C., Nesbitt, E.A., eds., From Greenhouse to Icehouse: The Marine Eocene-Oligocene Transition:

Columbia University Press, p. 189-222.

Mancini, E.A., 2000, Sequence stratigraphy and chronostratigraphy of upper Eocene and lower Oligocene strata, eastern Gulf Coastal Plain: Transactions, Gulf Coast Association of Geological Societies, v. 50, p. 379-388.

Miller, K.G. et al., 2008, Eocene-Oligocene global climate and sea-level changes: St. Stephen’s Quarry, Alabama; Geological Society of America Bulletin, v. 120, p. 35-53.

The Yazoo Formation is part of the Jackson Group, which extends in outcrop from southwest Alabama, northwest to Ya-

zoo County, Mississippi. The Cocoa Sand Member is mainly distributed in Wayne County, Mississippi. Southeast to

northwest cross section line and northeast to southwest cross section line are highlighted with red colors. The red points

on the map are the location of the wells

Based on well log data, there is a clear trend thickening toward Missis-

sippi east border with Alabama. The distribution of Cocoa Sand mem-

ber is easily to define with the isopach map. It covers most Wayne

county, part of southern Clark county, and a little bit of southeast Jas-

per County in Mississippi. The black dots are the well locations.

BIOZONES

The Cocoa Sand Member of the Yazoo Clay correlated to Biozone E14 as defined

by Berggren and Pearson (2005). The benthic foraminiferal assemblage in the Co-

coa Sand member of the Yazoo Clay has a low diversity. A higher diversity benthic

foraminiferal assemblage at the base of the Cocoa Sand identified by Echols et al.

(2003) is considered to be associated with rip-up clasts of re-worked North Twist-

wood Member.

Sand

Calcareous Shale

Shale

Shaly sand

Shale SE-NW Cross Section in Wayne County, Mississippi

N

Sand

Shale

Shaly Shale

Shale

Calcareous Shale NE-SW Cross Section in Wayne County, Mississippi

N