W. Burleigh Harris

Rb-Sr glauconite isochron of the Eocene Castle Hayne Limestone, North Carolina

The 11-m-thick lectostratotype of the Castle Hayne Limestone in New Hanover County, North Carolina, consists of lower phosphate pebble biomicrudite; middle bryozoan biosparrudite; and upper bryozoan-sponge biomicrudite. The relative age of the Castle Hayne Limestone is equivocal. The planktic foraminiferal fauna and part of the molluscan fauna suggest that the entire formation should be correlated with the Gulf Coast Claibornian Stage (middle Eocene), whereas calcareous nannofossils, bryozoans, barnacles, and some molluscs indicate that the upper bryozoan-sponge biomicrudite is a Gulf Coast Jacksonian Stage (upper Eocene) equivalent. Because of problems correlating the Castle Hayne Limestone to equivalent Gulf Coast stages, the lectostratotype was dated by application of the Rb-Sr glauconite isochron. Five hand-picked glauconite concentrates analyzed for Rb, Sr, and Sr-isotopic composition yielded an isochron age of 34.8 ± 1 m.y. (λ Rb 87 = 1.42 × 10−11 yr−1) with an initial (Sr87/Sr86) ratio of 0.7083 ± 0.0004. The determined initial (Sr87/Sr86) ratio is in good agreement with previous estimates of the Sr-isotopic composition of sea water during the Eocene. Although the age is younger than the value of 37 m.y. earlier proposed for the Eocene/Oligocene boundary, it agrees with fission-track and K-Ar ages of tektites and microtektites, and K-Ar ages of bentonites and glauconites in upper Eocene marine and nonmarine units throughout the world.

Comparison of Rb-Sr and K-Ar dates of middle Eocene bentonite and glauconite, southeastern Atlantic Coastal Plain

Rb-Sr and K-Ar dates for euhedral biotite from high-Mg, Chetotype smectitic bentonite in glaucony-bearing beds of the middle Eocene (Lutetian) Castle Hayne Limestone indicate deposition of volcanic ash in the North Carolina Coastal Plain at 46 Ma. Five evolved (high-potassium) glauconitic micas from 30-35 cm below the bentonite yield a Rb-Sr isochron date of 45.3 ± 0.3 Ma and an initial 87 Sr/ 86 Sr ratio of 0.70764 ± 0.00015; a single split yields a conventional K-Ar date of 46.7 ± 1.8 Ma. Six evolved (high-potassium) glauconitic micas from 25-30 cm above the bentonite yield a Rb-Sr date of 43.1 ± 1.2 Ma and an initial 87 Sr/ 86 Sr ratio of 0.7079 ± 0.00011; a single split yields a conventional K-Ar date of 44.5 ± 1.7 Ma. Agreement of Rb-Sr and K-Ar biotite dates in the bentonite with Rb-Sr and K-Ar glauconitic mica dates in the underlying and overlying carbonate suggests that carefully selected, evolved glauconitic mica that has neither been deeply buried nor experienced cation exchange with ground water can provide reliable isotopic ages and constraints for the standard geologic time scale.

Paleogene stratigraphy and sea-level history of the North Carolina Coastal Plain: global coastal onlap and tectonics

Abstract Coastal onlap of Paleogene depositional sequences, mapped by stage, and compared to curves of global coastal onlap and eustasy facilitate differentiation of the effects of local uplift and subsidence from eustasy on the Onslow and Albemarle Blocks about the Neuse Hinge in North Carolina. Differential uplift and subsidence of these blocks has controlled the stratal geometries and patterns of relative coastal onlap on each block. Potential mechanisms to produce the uplift and subsidence include episodic flexural deformation resulting from sediment loading in the Salisbury Embayment, and horizontal compressional deformation resulting from plate motion. Danian sea-level rise produced the initial Paleogene siliciclastic sequences (TA1.2 and TA1.3) on both blocks. After deposition of the Danian sequences and prior to Thanetian inundation, the Onslow Block was elevated relative to the Albemarle Block. The Ypresian (TA2.5–2.9, one) is generally restricted to the downdip Albemarle Block, has a depositional updip limit, and represents a major basinward shift in local coastal onlap at a time when global coastal onlap and long-term eustatic curves predict a major landward shift. This suggests that both blocks were uplifted during the Ypresian. Lutetian-Bartonian sequences (TA3.3–TA3.5/3.6) represent the most extensive landward shift in coastal onlap during the Paleogene in North Carolina and show erosional updip limits on both blocks. Differences in the degree of overstepping on the two blocks reflects post-middle Eocene uplift, but greater relative uplift of the Albemarle Block. The Priabonian (TA4.1 and either the TA4.2 or TA4.3) has erosional updip limits on both blocks and represents a significant basinward shift in local coastal onlap. This shift occurs during a time when global coastal onlap and long-term eustasy indicate a landward shift as great as that during the Lutetian-Bartonian. Therefore, both blocks were uplifted, but the distribution of the Priabonian on the Albemarle Block indicates greater relative uplift. In North Carolina, the Rupelian (TA4.4) oversteps the Priabonian on both blocks but not as far as predicted by global coastal onlap and eustasy. Based on the offlapping relationship of the erosional updip limit on both blocks, both areas were uplifted prior to Rupelian deposition. Chattian sediments (TB1.1–1.4, lower part) reflect a downward shift in coastal onlap which is consistent with global coastal onlap and long-term eustatic fall.

Rb-Sr glauconite isochron, Maestrichtian unit of Peedee Formation (Upper Cretaceous), North Carolina

Five hand-picked, authigenic glauconite concentrations that were analyzed for Rb, Sr, and Sr isotopic composition yielded an isochron age of 68.1 ± 1 m.y. (λ Rb 87 = 1.39 × 10 −11 yr −1 ) for Maestrichtian strata in North Carolina. The initial (Sr 87 /Sr 86 ) 0 ratio calculated from the isochron is 0.7070 ± 0.0004 and is in good agreement with previous estimates of the strontium isotopic composition of sea water during Late Cretaceous time. The excellent fit of the isochron age into current age boundaries of the Maestrichtian suggests that the Rb-Sr isochron method could be more widely applied to glauconite.

The Cretaceous-Tertiary boundary on the cape fear arch, North Carolina, U.S.A.

Abstract Petrologic and faunal study of a 72.5 m continuous corehole drilled in southeastern North Carolina has provided an opportunity to study a relatively uninterrupted vertical sequence across the Cretaceous-Tertiary boundary. The following stratigraphic sequence occurs; upper middle Maastrichtian Peedee Formation, −65.8 m to −51.8 m below mean sea-level (BMSL), upper middle Maastrichtian Rocky Point Member of the Peedee Formation, −51.8 m to −27.4 m BMSL, and middle to upper (?) Eocene Castle Hayne Limestone, −27.4 m to −15.2 m BMSL (base of casing). The Peedee Formation consists of moderately indurated, very fine to fine, sandy foraminiferal biomicrite and sandy biomicrite. Silt-size zoned dolomite rhombohedra form up to 30% of the upper Peedee Formation and are most abundant where bioturbation is common. A diverse and well-preserved foraminiferal fauna indicates a middle to outer continental shelf environment. The Rocky Point Member conformably overlies typical Peedee Formation lithology and consists of well-indurated sandy, fossiliferous biomicrite that grades upward into sandy, pelecypod biomicrosparite, and finally pelecypod biomicrudite. The Peedee Formation and the Rocky Point Member represent an overall shallowing-upward sequence with the upper surface forming the Cretaceous-Tertiary boundary. The Castle Hayne Limestone disconformably overlies the Rocky Point Member and consists of lithoclast-bearing, bryozoan-molluscan biomicrudite grading upward into bryozoan biomicrudite. The Castle Hayne Limestone was deposited in an open, normal salinity environment between 30 m and 100 m in water depth.

RbSr glauconite ages, Sabinian, Claibornian and Jacksonian units, southeastern Atlantic Coastal Plain, U.S.A

Abstract Hand-picked, disordered to ordered authigenic glauconite from the Nanjemoy Formation (Virginia), the Santee Limestone (restricted) and Cross Formation (South Carolina), and the Castle Hayne Limestone (North Carolina) yield RbSr isochron ages (λ87Rb = 1.42 × 10−11 yr−1) for Eocene units in the southeastern Atlantic Coastal Plain. The units, zonal assignment of their contained nannofossil floras and radiometric age, are: 3 . Unit Nannofossil zone Age (m.y.) Castle Hayne Limestone NP 19–20 34.9 ± 1.1 Cross Formation NP 18, NP 19, NP 20 34.1 ± 1.5 Santee Limestone (upper part) NP 16–17 36.7 ± 0.6 Nanjemoy Formation NP 13 46.7 ± 3.0 These ages, and their nannofossil assemblages, provide means of correlating southeastern Atlantic Coastal Plain units with Gulf Coastal Plain and European stratoypes. Correlation to Gulf Coast stratotypes, based on nannofossil zones, indicates that the Nanjemoy Formation (in part) is equivalent to the Hatchetigbee Formation; the upper part of the Santee Limestone (restricted) is equivalent to the upper Lisbon Formation, the Gosport Sand, and the Moodys Branch Formation (upper Claibornian-lower Jacksonian); and the Cross Formation and the Castle Hayne Limestone are equivalent to the Yazoo Group (Jacksonian). Previously reported KAr glauconite ages for the Gulf Coast units are different from the RbSr ages determined in this study. As the RbSr age determinations show internal consistency when compared to their contained nannofossil zones, whereas the KAr ages do not, it is suggested that the previously reported KAr ages for the Gulf Coast Claibornian and Jacksonian are in error. Correlation of the southeastern Atlantic Coastal Plain units to European stratoypes based on nannofossil zones indicates that the Nanjemoy Formation (in part) is equivalent to the type Lutetian; the upper part of the Santee Limestone (restricted) is equivalent to the Upper Bracklesham and Barton Beds; and the Cross Formation and Castle Hayne Limestone are equivalent to the Lower and Middle Headon Beds. Previously determined KAr and RbSr radiometric ages for the type Lutetian, the Bracklesham and Barton Beds are identical to the ages determined in this study. As a result, it is suggested that Rb-Sr glauconite isochron ages can be used for regional and inter-regional correlation and that the Eocene/Oligocene boundary is younger than 34 m.y.

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%.

A New Species of Turritelline Gastropod from a Turritelline-Dominated Limestone in the Paleocene of North Carolina

Turritelline gastropods (family Turritellidae, subfamily Turritellinae; sensu Marwick, 1957) are common components of many Cretaceous to Recent benthic marine assemblages worldwide. They are frequently the dominant or even the sole macrofossil in such assemblages (Allmon, 1988), termed “turritelline-dominated assemblages” (TDAs; Allmon and Knight, 1993). They are defined as macrofaunal assemblages in which turritelline gastropods: 1) comprise either at least 20% of the total actual or estimated biomass or at least 20% of the macroscopic individuals in the assemblage, and 2) are at least twice as abundant as any other macroscopic species in the assemblage (Allmon, 2007). TDAs have been widely reported from siliciclastic and carbonate sediments of the U.S. Gulf and Atlantic Coastal Plains, but turritelline-dominated limestones (sometimes referred to as “turritella limestone” or “turritella rock”) appear to be limited to the Cretaceous and Paleogene (Allmon and Knight, 1993; Allmon, 2007; Allmon and Cohen, 2007). The assemblage considered here was recognized in cores drilled offshore of southeasternmost North Carolina in 1990–1993 by the U.S. Army Corps of Engineers. It is dominated by a previously undescribed turritelline species, and represents the first reported occurrence of a TDA in a limestone in the Paleocene of the Atlantic Coastal Plain. The Bald Head Shoals Formation (BHS) is located on the axis of the Cape Fear Arch in Long Bay, seaward of the mouth of the Cape Fear River, along a 5.8 km northeast-southwest transect of the ship channel that connects to the Cape Fear River (Fig. 1). The formation is known from 15 cores drilled along the length of the ship channel; only three of these cores, however, penetrate the entire unit. Thickness of the formation ranges from 1.1 m in core BS93-05 to 3.6 m in BS92-21 (southernmost) to 6.9 m in BS92-12 (northernmost). The BHS disconformably overlies the …

Paleocene faulting within the Beaufort Group, Atlantic Coastal Plain, North Carolina

Postrift faulting in the Atlantic Coastal Plain of the eastern United States is a focus of several recent studies, particularly because of seismic activity associated with areas such as Charleston, South Carolina (earthquake of 1886). Understanding the ancient behavior of fault systems can contribute to increased awareness of earthquake potential where traditional mechanisms of earthquake activity are poorly understood. In the coastal plain of eastern North Carolina, the Graingers wrench zone, a more recently active northeast- southwest–trending fault system, overprints a Paleocene east-west fault trend and preserves evidence of episodic activity during Mesozoic and Cenozoic time. The western border of the Graingers basin is coincident with a northeast-southwest–oriented feature delineated on aeromagnetic and gravity maps. This northeast-southwest–trending western border fault records movement that initially occurred during the Paleozoic, was reactivated during the Triassic–Jurassic and the Cretaceous (Cenomanian and Maastrichtian), and has been active into the Holocene. Paleocene deformation records a shift in motion from the northeast-southwest trend to the east-west. This shift in direction controls the present-day distribution of early to late Paleocene strata of the Beaufort Group. Stratigraphic relationships between the Paleocene and underlying Cretaceous units suggest that the fault zone propagated to the southwest during the late Danian–Selandian and into the Selandian–Thanetian. Post-Paleocene activity of the northeast-southwest–trending faults exhibited a down-to-the-east displacement pattern and appears to have a minor component of strike slip, which may offset the older east-west–trending structures. Preservation of topographic features, such as a fault-line scarp with as much as 13 m of relief, triangular facets, and extensive ravinement normal to the scarp orientation, suggest Holocene movement on the faults. Historical records indicate seismicity in the area during the latter part of the nineteenth century. The combined northeast-southwest and east-west fabric has geometry similar to those of fault zones in Georgia and seismically active areas of South Carolina. The faults that affect the area were probably formed by interaction of the Neuse hinge and the Roanoke Island–Goldsboro fault, which border the Graingers basin to the south and north, respectively. Differential movement on these larger structures may be the result of compression caused by changes in direction of motion of the North American plate.

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------------