Gerald R. Baum

Eocene biostratigraphy of South Carolina and its relationship to Gulf Coastal Plain zonations and global changes of coastal onlap

An assimilation of mega-invertebrate assemblages in South Carolina with Gulf Coast biozones allows for the subdivision of the middle and late Eocene into four regional zonations that can be compared with proposed global sequences controlled by eustatic changes of sea level. The middle Eocene can be divided, in ascending stratigraphic order, into the Cubitostrea perplicata zone , C. lisbonensis zone , and C. sellaeformis zone. The late Eocene is represented by the Crassatella alta–Chlamys cawcawensis zone. These zones are separated by Type 1 or Type 2 unconformities that bracket global sequence boundaries (unconformity types after Vail and Todd, 1981). The following sequences are proposed for the middle and upper Eocene: TE2.1, Cubitostrea perplicata zone, followed by a Type 2 unconformity; TE2.2, C. lisbonensis zone, followed by a Type 2 unconformity; TE2.3, C. sellaeformis zone, followed by a Type 1 unconformity; and TE3, Crassatella alta–Chlamys cawcawensis zone, followed by a Type 2 unconformity. Although not recognized in the downdip areas of South Carolina, the TE3 sequence can be further subdivided into a TE3.1 sequence and TE3.2 sequence, separated by a Type 2 unconformity. This regional break lies at the Gosport Sand-Lower Moodys Branch Formation/Upper Moodys Branch Formation contact in the Gulf Coastal Plain and at the base of the Tivola Tonque in Georgia.

Foraminifera and Rb-Sr glauconite ages of a Paleocene Beaufort Formation outcrop in North Carolina

A 3.3-m section of the Paleocene Beaufort Formation, which crops out along Mosley Creek at the Lenoir-Craven county line, North Carolina, consists of alternating, unconsolidated, sandy, foraminiferal-glauconitic sediments and thinner, slightly glauconitic, foraminiferal biomicrosparites. A single Beaufort Formation sample collected from unconsolidated sediments about 1.5 m below the Paleocene-Eocene boundary was analyzed for foraminifera and radiometrically dated by glauconites. The occurrence of Globorotalia aequa Cushman and Renz, G. pseudomenardii Bolli, G. pseudobulloides (Plummer), and Globigerina triloculinoides Plummer indicates that the sample is Thanetian in age and is part of the P4 planktonic foraminiferal zone. The zone, identified by the occurrence of Globorotalia pseudomenardii , has a suggested absolute age range of 56 to 58 m.y. Three hand-picked, mammillated to lobate, glauconite concentrations were separated from the same sample and analyzed for Rb, Sr, and Sr-isotopic composition. Model ages of 64.5, 55.7, and 57.8 m.y. (λRb 87 = 1.39 × 10 −11 yr −1 ) using an initial Sr 87 /Sr 86 ratio of 0.7078 were determined. Although the older age (64.5) is anomalous, the average model age of 56.8 m.y. for the two younger determinations is in excellent agreement with recent time-scale estimates for the late Paleocene. The results suggest that hand-picked glauconites with a well-documented diagenetic history can yield Rb-Sr radiometric ages accurate to within 1% to 2%.

ABSTRACT: Eustatic Controls on Incised Valley Shape and Geometry: La Pascua Formation, Eastern Venezuela

Ten Fourth-order sequences in the Lower Oligocene La Pascua Formation were identified and mapped using over 700 wells in a 3500 km2 study area in eastern Venezuela. One third-order sequence (OR-2) and elements of another were identified on the basis of aggregate stacking patterns. Net sand maps for incised valleys for each sequence showed variations in geometry, width and thickness dimensions, and were related to their position on the constructed relative sea level curve. Unique incised valley geometries were common to each third-order sequence-set. Vshaped, coalesced and elongate-straight valleys were most often found in the lowstand sequence-set where they were widest (7.3 kilometers average.), and had highest width-tothickness ratios of between 220-277. Elongate-slightly-sinuous and elongate-sinuous geometries were most commonly found in transgressive sequence-sets. These contained narrowest valleys (2.2 kilometers average) for the entire sequence, with width-to-thickness ratios of between 76-150. The fourth-order sequence associated with maximum retrogradation showed no evidence of erosion or

Abstract: Subsurface Sequence Stratigraphic Analysis of the Eocene-Lower Oligocene of Alabama

Abstract South Alabama contains world-class outcrops of Eocene and Oligocene strata that have been the object of intense scrutiny over the past 50 years. Despite such thorough examination, these outcrops have failed to assuage controversy as to the physical nature of the Eocene/Oligocene (E/O) boundary in this region. Some workers have made this faunal boundary coincident with a sequence boundary, while others place it within a stratigraphically condensed section during a relative sea level highstand. This problem is exacerbated by the fact that because outcrops in Alabama are limited in areal extent, large-scale stratal patterns such as onlap and downlap have not been used to place constraints on this matter. A grid of wireline logs (SP and resistivity) has been assembled and digitized in the area of interest to examine the nature of the E/O boundary on a seismic scale. The log data are calibrated to lithology and biostratigraphy using two stratigraphic test cores. In these cores the Eocene/Oligocene boundary, defined using multiple biostratigraphic events, occurs in a relatively fine-grained interval bounded by unconformities. These unconformities are interpreted as sequence boundaries amalgamated with transgressive surfaces of erosion. To address the physical nature of the E/O boundary on a regional scale, transgressive-regressive (T-R) cycles reflected in the wireline logs are currently being analyzed and correlated. Mapping T-R cycles into the subsurface of Alabama provides information that was unavailable to earlier field-based workers. In addition, such mapping may clear up other areas of contention, such as the number of cycles in the middle Eocene Lisbon Formation. End_of_Record - Last_Page 711-------

Sodium Distribution in Eocene Dolomites from Castle Hayne Limestone, North Carolina: ABSTRACT

An 11-m section of the bryozoan biomicridite facies of the Castle Hayne Limestone in the Martin Marietta quarry, New Hanover County, North Carolina is locally dolomitized. About 6.5 m below the overlying unconformity, a 1.0-m zone consists entirely of sucrosic dolomite. The percentage of dolomite decreases fairly uniformly above and below this zone, and 3.6 m below the upper unconformity, the unit is undolomitized. The dolomite is nonferroan and occurs as fine anhedral to subhedral crystals. Above and below the zone of maximum dolomitization, the dolomite selectively replaces the micrite matrix. Where dolomitization increases toward a maximum, calcite allochems are replaced. Acid-soluble sodium ranges from a low of 252 ppm in calcite to a high of 1,500 ppm in dolomite. Microprobe analysis revealed that sodium is concentrated in heulandite-group zeolite. The interlocking nature of the dolomite and zeolite crystals, the euhedral morphology of the zeolite, and the strong positive correlation between percentage of dolomite to sodium concentration suggest that both mineral phases are authigenic and formed penecontemporaneously from an open-system, stratified fluid (Dorag). Unless the sodium distribution can be documented, these data suggest that whole-rock sodium concentrations in ancient dolomites may not be an accurate indicator for hyposaline versus hypersaline dolomitization. Dolomitization in proximity to the overlying subaerial unconformity has greatly enhanced postdepositional permeability in the micrite facies of the Castle Hayne Limestone. Dorag dolomitization caused by a lowering of eustatic sea level in conjunction with favorable hydrologic and lithologic conditions can have a profound effect on reservoir properties and permeability distribution in ancient carbonates. End_of_Article - Last_Page 263------------