THE DEPARTMENT OF ENVIRONMENTAL QUALITY Office of Geology P. 0. Box 20307 Volume 16, Number 1 Jackson, Mississippi 39289-1307 March 1995 POST-OLIGOCENE STRATIGRAPHY AND MAPPING CONSIDERATIONS James H. May and Danny W. Harrelson, Geotechnical Laboratory, U.S. Army Engineer Waterways Experiment Station, Vicksburg; William H. Moore, Consultant, Jackson, Mississippi; David M. Patrick and Christopher P. Cameron, Department of Geology, University of Southern Mississippi, Hattiesburg; Darrel W. Schmitz, Department of Geosciences, Mississippi State University, Mississippi State LNTRODUCTION and Dockery, 1990; Bowen, 1990; Cameron, 1990; May, 1990; Otvos, 1990; Patrick, 1990; and Saucier, 1990). Spe­ Background cifically, we believe that the Neogene formations identified on the state geologic map cannot be mapped in the field using the The nature, distribution, and relationships between the current criteria for their recognition. The following views and post-Oligocene (Neogene) sediments currently named and recommendations are offered in the hope ofstimulating inter­ mapped as Catahoula, Hattiesburg, Pascagoula, and Citronelle est and discussion pertaining to the Neogene of Mississippi. formations in the southern portions ofMississippi have chal­ lenged the authors as well as other stratigraphers, geologic Purpose mappers, and sedimentologists for a number ofyears (Moore, 1965; May, 1980; Adamczak, 1986; Albertson and others, The purposes of this paper are: I) to address the adequacy 1986; Gerald, 1986; Day, 1987). Generally, the challenges of current post-Oligocene (Neogene) stratigraphic terminol­ and difficulties have resulted from several related factors ogy for southern Mississippi, 2) to describe the problems which include: 1) limited outcrops, 2) deep weathering and associated with current usage, 3) to present a hypothesis and alteration, 3) lack of subsurface data including cores and model which more closely describe the nature and distribution geophysical logs, 4) poorly defined stratigraphic boundaries, ofthese sediments and which conform to the rules oftbe North 5) apparent absence of fossils, and 6) an inadequate deposi­ American Stratigraphic Code (AAPG, 1983), and4)to recom­ tional model for these sediments. mend methods for testing this hypothesis. These challenges assume practical significance in under­ standing the occurrence and movement of ground water and CURRENT STRATIGRAPHIC USAGE the siting of solid waste landfills in these materials. They are also important issues facing the state in its efforts to revise the The post-Oligocene (Neogene) of Mississippi as cur­ state geologic map. Many of these issues were addressed at rently defined by the Mississippi Office ofGeology (Dockery, the MISG EO MAP Conference held in Jackson in 1989 (Bograd 1981) utilizes updip and downdip members or formations to help explain complex stratigraphic tem1inology (Figure I). geologic maps and stratigraphic sections. They define updip as to the north, northeast and east and Sim ilarly, Cretaceous age Tuscaloosa gravel deposits in downdip to the south, southwest and west. Usually (but not southern Tishom ingo County are only distinguishable into two always). the overlying and presumably youn ger units are given different age deposits by subtle differences in mineralogy and formation status (i.e., Hattiesburg. Pascagoula, Graham Ferry, size (Russell, 1987). The mapping of simi lar non-marine and Citronelle fom1ati ons). The downdip units occasiona ll y materials in south Mississippi as time equivalent formations have formation status (i.e., Paynes Hammock, Bilox i. Pam lico. has resulted in units being mapped as fonnations that do not and Gu lfport ) but also use lithologic or even paleontologic meet the geologic requirements for mapping, that is, having a modifiers such as the Chickasawhay Limestone, Tatum or top and bottom, and a type section. If no one can identi fy the Hererosregina Limestone. and the Amphisregina Zone. formational contacts in a core or on the outcrop, it is probable Dockery ( 1981) considered the Chickasawhay Limestone, that the laws ofstra tigraphic nomenclature have been violated. Paynes Ham mock , Tatum or HeterosLegina Limestone. and Li and Meylan ( I 994) suggested that tbe Catahoula and the Heterostegina Reef to be age equivalent to the Catahou la Hattiesburg formations are not mappable; however, they did Formation. the Amphistegina Zone to be age equivalent to the map the Citronelle. Ifthey had recognized the subsurface sand Hau icsburg Formation, and the Parn lico, Biloxi, and Gulfport and gravel deposits in the area mapped, they would, most fom1at ions to be age equivalent to the loess deposits in western likely, have had to map the Citronelle differently (Brown, Miss issippi. Saucier and Snead ( 1989) mapped those deposits 1944; May. 1980; Gerald, 1986; Patrick and Zhao, 1989; shown on the I 969 state map as Cit ronelle as the Upland Sturdivant and Patrick, 1990). Complex. A CONCEPTUAL MODEL THE PROBLEM Concurrent with demonstrating the need to re-study and The reason that the mappingofthe post-Ol igocene depos­ re-map the sediments presently shown on the state geologic its ofM ississippi has been so difficult and controversial stems map as Catahoula, Hattiesburg, Pascagoula, and Citronelle, it from the fa il ure to recognize that the Citronelle. Catahoul a, is appropriate to suggest a conceptual model that would be Hauicsburg and Pascagoula formations are a II part ofthe same used to guide any revised mapping. The proposed conceptual massive non-marine ofllapping regressive complex with mul­ model for the post-01 igocene sediments is not a new or radical tiple source areas (Figure 2). The views of Amadeus Grabau model but simply an extension ofthe concept used in mapping made over 70 years ago are applicable here: the pre-Oligocene sediments in Mississippi. .. Thus it is brought about that each successive formation The pre-Oligocene sediment mapping was based on the of the retreated series extends shoreward to a less extent than premise that dip-oriented subsurface beds could be projected the preceding one. As each fomwtion or bed passes shoreward to and mapped on the surface. The mappingofpost-Oligocene imo a coarse clastic it is evident that the shore ends of all the sediments for a variety of reasons did not follow the same formations deposited during the retreat will 'together' consti­ log'ic. Dip-oriented, non-marine clastics were mapped as one tu te a stratum ofs andstone or conglomerate. The sand ends of formation in the subsurface and a different formation at the all the beds will be exposed at or just above sea level and geochemica ll y altered outcrop. Massive subsurface Miocene const itute a continuous sand formation. which, however. is not sand and gravel units, some hundreds of feet thick, were not the same age at any :wo points along a line transversing the recognized at the surface (Brown, 1944; Gerald, 1986; May, direct ion of the shore." (Grabau. 1960 ( 1924)) 1980: Patrick and Zhao. 1989; Sturdivant and Patrick, 1990). The fact that minor transgressive events wh ich occurred Unlike the mapping of the pre-Oligocene beds, multiple clay during the post-Oligocene did not reach in land far enough to and sand zones of different time periods were mapped on the deposit mappable deposits has added to the difficulty in surface as time equivalent. geo log ic in terpretation. i\tternpts to map time-equivalent To correct the errors made in the past, the basic concepts sediments across the ends of the non-time equivalent regres­ used in mapping the pre-Oligocene beds must be applied. The sive deposits without understanding their interrelationship has basic assumptions are as follows: resulted in additional dil emm a. The post-Ol igocene regres­ I) The post-Oligocene sediments are comprised prima­ sive complex has superpos itional fining upward beds of clay. rily of non-marine, time-equivalent, dip-oriented, fining up­ sands and gravels. The correlation problems arise when simi­ ward alluvial beds. lar materials of different ages are mapped as a time equivalent 2) The outcropping sediments have downd ip portions formation (Figure 3 ). For example, if it we re not for diagnostic wh ich are often different in color, induration, and mineralogy foss ils, the outcropping limestones of the Glendon and than the equivalent surface sediments. At least three zones are Chickasawhay formations (Oligocene) in Wayne County, mappable in the subsurface. Mississippi, cou ld easi ly be mistaken for the same unit. Note 3) The differences in the outcrops are caused by differ­ the widespread uncon fom1 ities that are not shown on the latest ences in weathering and geochemical alterations and rework- 2 MISSISSIPPI GEOLOGY, V. 16, No. 1, March 1995 ~ en en en en "tl "tl C) STRATIGRAPHIC COLUMN OF MISSISSIPPI m 0 I David T. Dockery 0 C) -< MISSISSIPPI BUREAU OF GEOLOGY < 1981 en :1 w :1 I w Updlp (North, Northeast, East) z ~ - Oowndlp (South, Southwest, Weal) 0 ~ 1/) Series Group < >- a: 1/) Formation, Member ... w ~ Ill >- Holocene Alluvium; Barrier Islands- Petit Bois, Horn. Ship, and Cat Islands. ... a: n < ::r z Gulfport Fm. a: / \ ... w Loess CD ~ Pleistocene Pratrie or Pamlico Fm Biloxi Fm. CD < ""' ~ c.n => t 0 Pre-Loess Terrace Deposits Citronelle Fm Pliocene Graham Ferry Fm. (.) 0 Pascagoula Fm N 0z Miocene !-iattresburg Fm ~ Amphistegina Zone w (.) >- a: < Tatum or Heterostegina Ls. Het. "Reef" j: Catahoula Fm. ~ ~ a: a.t w a. / Paynes Hammock Fm ~ => / Chickasawhay Ls. Bucatunna Fm ·~ Waynesboro Sand ~ OJigocene Byram Fm t VIcksburg ~ J Glendon L•mestone 0 ~ V•cksburg ..J Marianna Ls. "Reef" M1nt Spr1ng Fm ~ r L Forest Hill Fm f Red Bluff Fm . ~~- Figure I. Post-Oligocene geology of Mississippi (Dockery. 1981 ). (,) ! ! i I f __ J 50 0 SO 100 150 MILES Figure 2. Source areas for Miocene sediments (after lsphording, 1977). ing of underlying formational materials. 2) The terminology and criteria used for the 1969 state 4) The basal Miocene does not maintain a uniform strati­ map do not conform to current and accepted standards of graphic position, but rests on progressively older formations stratigraphic nomenclature and, in many cases, do not fully updip.