Abstract Another promotion, this time from the field crew to a division office, marked the end of my doodlebugging days. It was the beginning of an era of respectability as a division interpreter. I had now been on a field crew for five years: 1½ years in field operations and 3½ years in the field office. Was I so stupid that it took me five years to graduate from a field crew? Not really. The field crew interpreter played a very important role in exploration: validating the low velocity and datum corrections, evaluating the quality of the recordings, determining the need to reshoot important shot points, making structural interpretations of the data, positioning additiona1 data points, flanging up a prospect and - not in the least - thoroughly learning field operations as they pertained to interpretation. This complete training served me well in the years ahead. More’s the pity that today’s interpreter does not have similiar field training. The transfer to a division office removed me from the front lines. I would no longer be first in uncovering a prospect. Moves would be fewer, our stays measured in months rather than weeks or days. A new lifestyle. The new job introduced me to a new way of living and working. In the field, you had a job to do right then and you did it. There was no such thing as an eight-hour day, 40-hour week. You worked until you had completed the assignment. You had only one purpose in the field: produce the data to

ABSTRACT Diatom floras in Pungo River Formation sediments of the North Carolina continental margin indicate an age of late early to middle Miocene. Biostratigraphic age assignments of stratigraphic sections approximating third-order coastal onlap events (Frying Pan, Onslow Bay and Bogue Banks sections, from oldest to youngest) are based on zonal indicator species of Abbott’s (1978) Atlantic Miocene Diatom Zones (AMDZ). The Frying Pan Section is assigned to Zone I (Burdigalian), the Onslow Bay Section to Zones II and III (Langhian) and the Bogue Banks Section to Zone VI (Serravallian). Diatom assemblages indicate shallow marine deposition. Influx of oceanic waters varied through time, probably as a response to changes in sea level and intensity of upwelling currents. Predominantly benthic assemblages in the Frying Pan Section give way to greater proportions of planktonic species in overlying sections. Shelf waters cooled and upwelling intensified during deposition of the Bogue Banks Section. Preservation of the diatom assemblages varied as a function of the permeability of enclosing sediments and the composition of associated sediment particles.

ABSTRACT Samples from Onslow Bay vibracores containing siliceous microfossils were obtained for biostratigraphic and paleoenvironmental studies of radiolarians. Three early to middle Miocene radiolarian zones were recognized: Stichocorys wolffii Zone (Frying Pan Section), Calocycletta costata Zone (Onslow Bay Section) and Dorcadospyris alata Zone (Bogue Banks Section) on the basis of 22 age-diagnostic species. An average of 16 taxa (of approximately generic level) per sample were observed in the Frying Pan and Onslow Bay sections, with subequal representation by spumellarians and nassellarians. Seven taxa were predominant (consistently >5% of the assemblage) in the Frying Pan Section, six in the Onslow Bay Section. Radiolarians were too sparse in samples of the Bogue Banks Section for quantitative studies. The assemblages generally consist of shallow-dwelling groups, although a few specimens of deep-dwelling taxa occur in one core from the Bogue Banks Section, perhaps suggesting strong upwelling or intrusions from deep levels of the Gulf Stream. Abundance and diversity of radiolarians are lower in the Onslow Bay sections than in correlative units of the mid-Atlantic Miocene, suggesting that different paleoenvironmental factors prevailed in each region.

The late Eocene to early Miocene Renova Formation records initial post-Laramide sediment accumulation in the intermontane basin province of southwest Montana. Recent studies that postulate deposition of the Renova Formation were restricted to a broad, low-relief, tectonically quiescent basin on the eastern shoulder of an active rift zone vastly differ from traditional models in which the Renova Formation was deposited in individual intermontane basins separated by basin-bounding uplands. This study utilizes detrital zircon geochronology to resolve the paleogeography of the Renova Formation. Detrital zircon was selected as a detrital tracer that can be used to differentiate between multiple potential sources of similar mineralogy but with distinctly different U-Pb ages. Laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) U-Pb detrital zircon ages were determined for 11 sandstones from the Eocene-Oligocene Renova Formation exposed in the Sage Creek, Beaverhead, Frying Pan, Upper Jefferson, Melrose, and Divide basins. Detrital zircon ages, lithofacies, paleoflow, and petrography indicate that provenance of the Renova Formation includes Paleogene volcanics (Dillon volcanics and Lowland Creek volcanics), Late Cretaceous igneous intrusions (Boulder batholith, Pioneer batholith, McCartney Mountain pluton), Mesozoic strata (Blackleaf Formation, Beaverhead Group), Belt Supergroup strata, and Archean basement. The oldest deposits of the Renova are assigned Bridgerian to Uintan North American Land Mammal (NALM) ages and contain detrital zircons derived from volcanic, sedimentary, and metamorphic rocks constituting the “cover strata” to uplift-cored Late Cretaceous plutonic bodies. Regional unroofing trends are manifested by a decreased percentage of cover strata–sourced zircon and an increased percentage of pluton-sourced zircon as Renova deposits became younger. Zircon derived from Late Cretaceous plutonic bodies indicate that initial unroofing of the McCartney Mountain pluton, Pioneer batholith, and Boulder batholith occurred during Duchesnean time. Facies assemblages, including alluvial fan, trunk fluvial, and paludal-lacustrine lithofacies, are integrated with detrital zircon populations to reveal a complex Paleogene paleotopography in the study area. The “Renova basin” was dissected by paleo-uplands that shed detritus into individual intervening basins. Areas of paleo-relief include ancestral expressions of the Pioneer Range, McCartney Mountain, Boulder batholith–Highland Range, and Tobacco Root Range. First-order alluvial distributary systems fed sediment to two noncontiguous regional-trunk fluvial systems during the Chadronian. A “Western fluvial system” drained the area west of the Boulder batholith, and an “Eastern fluvial system” drained the area east of the Boulder batholith. Chadronian paleodrainages parallel the regional Sevier-Laramide structural grain and may exhibit possible inheritance from Late Cretaceous fluvial systems. Detrital zircons of the Renova Formation can be confidently attributed to local sources exposed in highlands that bound the Divide, Melrose, Beaverhead, Frying Pan, Upper Jefferson, and Sage Creek basins. The data presented in this study do not require an Idaho batholith provenance for the Renova Formation.

Abstract: Integration of biostratigraphic and Sr isotope data constrain the ages of four third-order depositional sequences within the Aurora and Onslow Embayments of the Miocene Pungo River Formation. Within the Aurora Embayment the microsphorite surface capping the Oligocene and marking the base of the Pungo River Formation is –28 Ma. The bulk of Unit A, which constitutes the Aurora Embayment Sequence (AES), is 23.2–21.6 Ma. Unit B, at 19.1–17.0 Ma, is largely contemporaneous with the Frying Pan Sequence (FPS) in Onslow Embayment. Units C and D, at 16.4–14.8 Ma, are largely contemporaneous with the Onslow Bay Sequence (OBS) in Onslow Embayment. The microsphorite surface at the contact of unit D and the Pliocene Yorktown Formation is 12.7 Ma. Within the Onslow Embayment the microsphorite at the unconformity between the Oligocene and the Miocene Pungo River Formation is 21.0–19.5 Ma. The remainder of the Frying Pan Sequence (FPS) is 20.5–16.7 Ma. Most of the Onslow Bay Sequence (OBS1–OBS3) is 15.4–14.8 Ma, with OBS4 being slightly younger. The lower units of the Bogue Banks Sequence (BBS1–BBS5) are 12.5–8.6 Ma. BBS6–BBS8 have not been constrained biostratigraphically; however, Sr ages suggest that the phosphate was reworked from pre–existing units and deposited between 8.6–7.1 Ma. Interpretation of depositional patterns in phosphate–rich sediments of North Carolina has led to the development of a sedimentological model for continental margin phosphogenesis. Sedimentation patterns in an idealized phosphogenic episode reflect various stages of sea–level change. Sea–level lowstand produces an erosional unconformity across previously deposited sediment sequences. Marine transgression is characterized by a fining–upward sequence of fine siliciclastic sediments with intermixed phosphate grain types. During early–stage transgression, a basal microsphorite crust forms and erodes to produce rip–up intraclasts that are then reworked into and dominate the lower portion of the subsequent depositional sequence. Mid–stage transgression is dominated by authigenic phosphate, including skeletal grains and fine sand–sized peloids, which form as disseminated grains below the sediment–water interface in response to the degradation of organic matter in nutrient–rich, suboxic shelf environments where there is little or no sediment winnowing or reworking. During late–stage transgression the phosphogenesis is terminated as oxygen–depleted environments migrate landward across the shelf with the formation of organic–rich dolosilts or foraminifer–rich/diatom–rich muds with little to no phosphate. The sea–level highstand is characterized by carbonate–rich sediments deposited under the influence of normal marine, open–shelf conditions. During the subsequent regression, sediments of these shallower depositional environments are often eroded and reworked to form beds of extensively reworked phosphate–rich sediments. Four episodes of phosphate sedimentation characterize the North Carolina continental margin. Three episodes (I, II, and III) are dominated by primary phosphate formation within the Miocene, whereas one episode (IV) consisting of the Pliocene–Quaternary sediments contains totally reworked phosphates derived from sediments of Episodes II and III. Episode I occurred during the Aquitanian (23.3–21.6 Ma), forms the basal sequence of the Pungo River Formation in the Aurora Embayment, and represents a mid–stage transgression that is only partially preserved. Episode II preserved all sediment facies of an entire sea–level cycle that produced the main Miocene phosphogenic event, which took place during the Burdigalian and Langhian (21.0–14.8 Ma) and extended throughout the Aurora and Onslow Embayments. Episode III was deposited in eastern Onslow Embayment during the uppermost Serravallian and Tortonian (12.7 Ma and likely continuing through 7.1 Ma). Low sea stand during the Messinian led to severe erosion and weathering of Episode III sediments, limiting their distribution and modifying their composition. Episode IV represents many sea–level events that produced Pliocene–Quaternary deposits with local and variable concentrations of reworked phosphate of many types. The four phosphate episodes recognized in North Carolina closely correspond to the Upper Cenozoic phosphorite episodes identified globally by Riggs and Sheldon (1990) .

ABSTRACT The Miocene Pungo River Formation in Onslow Bay represents three episodes of deposition, each with an estimated duration of approximately 1 million years. The Frying Pan Section is middle Burdigalian in age (18.4 to 17.4 Ma), the Onslow Bay Section spans the Langhian (15.9 to 14.9 Ma), and the Bogue Banks Section is middle Serravallian in age (13.0 to 12.0 Ma). Each section approximates a third-order seismic sequence within second-order supercycle TB2 of Haq and others (1987). Pungo River Formation sediments were deposited in a middle sublittoral to upper bathyal bathymetric setting. The Frying Pan Section in southern Onslow Bay contains phosphorites which accumulated in nutrient-enriched, oxygen-poor waters introduced by marine up-welling. To the north, clastic sediments accumulated in more oxygen-enriched bottom waters. The Onslow Bay Section comprises prograding clinoforms of siliciclastic sediments that accumulated in high-energy, well oxygenated bottom conditions. Phosphorites in the Bogue Banks Section of central Onslow Bay coincide with a minor upwelling event, whereas mixed phosphorite-siliciclastic sediments in northern Onslow Bay correspond with the introduction of colder, well oxygenated waters, possibly from a northerly source.

Abstract Modern multi-trace geometric attributes produce three-dimensional volumes that can facilitate the recognition of karst geomorphology by avoiding the need to pre-interpret irregular horizons and by enhancing subseismic lateral variations in reflectivity. These geometric attributes include the well-established coherence technology, coupled with recent developments in spectrally limited estimates of volumetric curvature. Coherence measures lateral changes in waveform, and as such, is often sensitive to joints, small faults, sinkholes and collapse features. The many components of reflector curvature, including the most negative, most positive, Gaussian curvature and related shape indices (e.g. valleys, saddles, domes), are complimentary to coherence measures. Short wavelength estimates of curvature will illuminate small-scale lineaments while longer wavelength estimates of curvature illuminate more subtle flexures and compaction features. We show the results of applying a variety of multi-trace geometric attributes to a three-dimensional seismic volume from the Fort Worth Basin, where a collapse system extends vertically some 800m from the Ordovician Ellenburger carbonates through the dominantly siliciclastic Mississippian–Pennsylvanian interval. The collapse features in our data set appear as rounded, sinkhole-like appearances on time and horizon slices in the Pennsylvanian Marble Falls Limestones and the Ellenburger horizon displays features that can be interpreted as cockpit karst, dolines and frying pan valleys. Although a variety of palaeocave breccia facies in core and image logs indicate that the Ellenburger surface has been karsted, these breccias are not confined to the mega collapse features visible in seismic. The large (up to 700 m diameter) collapse chimneys can be shown in multi-spectral curvature attributes to have elongate rhombohedral shapes associated with intersections of Pennsylvanian age, field-scale to basin-scale, basement lineaments and faults. Isochores indicate greatest tectonic growth on faults from Mississippian until early Pennsylvanian, coincident with thickest fill of collapse features. Thus we interpret the origin of the chimneys to be primarily tectonic. The multi-trace geometric attributes permit better imaging of the three-dimensional shapes of the collapse features, provide better constraints on timing of their formation, allow us to begin to separate karst processes from tectonic processes and provide a means of predicting most likely locations of fluid movement along faults.

ABSTRACT Planktonic foraminifera and calcareous nannofossils recovered from Pungo River Formation sediments in Onslow Bay permit correlation of three stratigraphic sections, each of which approximates a third-order coastal onlap event, with standard ages: Frying Pan Section = middle Burdigalian, Onslow Bay Section = Langhian, and Bogue Banks Section = upper Serravallian. Integrated foraminiferal and nannofossil data also provide biostratigraphic constraints for some fourth-order seismic sequences: FPS-1 and FPS-2 = lower Zone N6, FPS-6 = upper N6 to lower N7, OBS-2 and OBS-3 = mid N8 to upper N9, and BBS-1 to BBS-5 = N12 through N14. The durations of hiatuses between successive third-order sections are approximately 1.0 to 1.5 Ma, whereas those between fourth-order sequences cannot be resolved biostratigraphically.