This paper introduces an integrated Neogene microfossil biostratigraphic chart developed within post-merger BP for the Gulf of Mexico Basin and is the first published industrial framework “fully-tuned” to orbital periodicities. Astronomical-tuning was accomplished through a 15-year research program on the Ocean Drilling Program’s (ODP) Leg 154 sediments (offshore NE Brazil) with sampling resolution for calcareous nannofossils and planktonic foraminifera ∼20 k.y. and 40 k.y. (thousand year), respectively. This framework extends from the Late Oligocene (25.05 Ma) to Recent at an average Chart Horizon resolution for the Neogene of 144 k.y., approximately double that of published Gulf of Mexico biostratigraphic charts and a fivefold increase over the highest resolution global calcareous microfossil biozonation. Such resolution approximates that of fourth to fifth order parasequences and is a critical component in the verification of seismic correlations between mini-basins in the deep-water Gulf of Mexico. Its utility in global time-scale construction and correlation has been proven, in part, by application of the scheme in full to internal research for the Oligocene–Miocene boundary interval on the global boundary stratotype section and point (GSSP) in northern Italy and offshore wells in the eastern Mediterranean Sea. This step change in Neogene resolution, now at the level of cyclostratigraphy (the orbital periodicity of eccentricity) and the magnetostratigraphic chron, demonstrates the potential for calcareous microfossil biostratigraphy to more consistently reinforce correlations of these time scale parameters. The integration of microfossil disciplines, consistent taxonomies, and rigorous analytical methodologies are all critical to obtaining and reproducing this new level of biostratigraphic resolution.

It had long been realized that there were only a limited number of published geologic ages available for bioevents in GoM industrial schemes. Coupled with the late twentieth century “cyclostratigraphic revolution” and the availability of such reference sections, an internal research program was initiated in 2002 with the goal to derive astronomically-calibrated geologic ages for the entire BP Neogene biostratigraphic framework. Efforts culminated in the first “BP Gulf of Mexico Neogene Astronomically-tuned Time Scale” (BP GNATTS) in early 2007. Subsequent research through 2012 extended calibration into the lower Oligocene through sampling and study of the base Neogene GSSP in northern Italy and ODP Leg 154 cores, offshore NE Brazil ( Fig. 1 ). In 2016, efforts were refocused on publication of BP GoM taxonomy and biostratigraphy, including new research on ODP Leg 154 materials to more thoroughly document biostratigraphic events for publication.

Advances in timescale construction, combined with the upswing in oil prices around the beginning of the twenty-first century, provided new opportunities for industrial biostratigraphy in the GoM. BP America staff from the three heritage companies (BP, Amoco, and Arco Vastar) were charged with producing a single Neogene GoM chart from three independently-derived GoM biostratigraphic frameworks. Staff exchanged taxonomic concepts and methodologies, which accounted for some differences between company schemes; remaining discrepancies were solved through in-house analyses of well samples. The resulting improved and unified internal framework, completed in 2007, aided exploration and development efforts within the stratigraphically and structurally complex GoM deep-water (DW).

Cenozoic stratigraphic research was in the midst of a revolution during the 1990’s. Research on, and formal ratification of, reference outcrop sections known as global boundary stratotype section and point (GSSP) resulted in precise global definitions of stratotype boundaries (see stratigraphy.org ). The scaling of geologic time shifted from geomagnetic polarity time scales (GPTS) with the first applications of astronomical “tuning” ( Lourens et al.,1996 ; Laskar et al., 2004 ) of sedimentary cycles in the construction of higher temporal resolution Late Neogene timescales (see Hilgen et al., 1997 ). Today, Neogene calcareous microfossil biozonations (e.g., Backman et al., 2012 ) are founded almost entirely by astronomical ages. Although the accuracy and precision of ages for Neogene planktonic microfossil events also improved by an order of magnitude with these techniques, the resolution of global biozonations (∼750–1200 k.y.) are still coarser than the lower limits of astronomical tuning achieved by the dominant 405 k.y. eccentricity cycle (see Hinnov, 2013 ).

The first industrial applications of microfossil biostratigraphy along the U.S. Gulf of Mexico Coast began with benthic foraminifera nearly a century ago ( Loutit et al., 1988 ) and progressed from onshore to deep-water in response to exploration focus ( Martin, 2013 ). Today, planktonic foraminifera and nannofossils are the primary groups used for time correlation in deep water wells and the construction of global Cenozoic timescales. Integrated Gulf of Mexico (GoM) industrial biostratigraphies and published global biozonations utilizing these two planktonic groups date back half a century, near the inception of a research coring program in the world’s deep-sea basins (Deep Sea Drilling Project). During this time, many industrial staffs dedicated to the GoM developed their own internal Cenozoic biostratigraphic frameworks. The Deep Sea Drilling Project (DSDP) was later rebranded the Ocean Drilling Program (ODP) in 1983 and the Integrated Ocean Drilling Program (IODP) from 2003 to 2013.

Microfossils are an important, arguably integral tool in subsurface petroleum exploration. Conventional exploration has reached into new regions and basins, while further development takes place in stratigraphically- and structurally-complicated fields with increased requirements for finer correlation in reservoir intervals (i.e., the need for increased resolution in the expanded sections that are typically targeted in offshore exploration). Application of published Cenozoic global biozonations ( Blow, 1969 ; Martini, 1971 ; Okada and Bukry, 1980 ) in deep-water exploration was not ideal, especially with the combined effects produced by sediment dilution on microfossil recovery, different taxonomic concepts, and varied methodologies. This necessitated improvement beyond published global biozonations and stimulated petroleum companies to support research that improved their biostratigraphic databases and frameworks.

The BP GNATTS framework has been extended to the Mediterranean for the Oligocene–Miocene boundary interval. The first Mediterranean locality is the base Neogene GSSP of Lemme-Carrosio in northern Italy ( Bergen et al., 2009 ; de Kaenel and Villa, 2010 ). Thirty-nine samples were collected from this section in Fall 2008 with aid from the University of Parma. Biostratigraphic research on the GSSP was completed in 2009 and the nannofossils reexamined in late 2017 as part of this study. The second location in the eastern Mediterranean Sea are three exploration wells analyzed by the first two authors.

There are several thousand wells in the BP GoM biostratigraphic database. The vast majority are located onshore and on the continental shelf; several hundred wells are located in deep water. Deep-water exploratory drilling began in the 1970s and encountered progressively older section through time. By the 1990s, several major DW Miocene reservoirs had been discovered in the GoM.

Biostratigraphic events derived from sample analyses are often evaluated relative to their resolution, reliability, and synchronicity. Resolution is defined herein by the number of events per unit of geologic time. Sample resolution is the number of samples per unit of geologic time. For ODP Leg 154, where samples are related to geologic time through cyclostratigraphic methods, sample resolution equates to the precision of derived bioevents and estimates of error in their geologic age. Reliability refers to bioevents either within a biostratigraphic scheme or an individual well or section. For BP GNATTS, the reliability of biostratigraphic markers is evaluated relative to the number of times a bioevent has been reproduced in sequence between wells. For events used by all three heritage companies in the GoM, marker events in these schemes have been tested in hundreds to possibly thousands of wells. For post-merger BP, largely in the DW GoM, marker events in BP GNATTS have been tested in tens to hundreds of wells. The reliability of a microfossil top (highest occurrence) or base (lowest occurrence) in an individual section (well, core, or outcrop) is also related to its persistence in occurrence. Taxa that are very rare and sporadic in occurrence at the ends of their stratigraphic ranges are considered less reliable. Such taxa are certainly less desirable as marker events in biostratigraphic schemes, especially if they are sporadic in occurrence in fossil-rich sections (e.g., ODP Leg 154 cores) not affected by sediment dilution. Bioevents defined by significant abundance changes in taxa are determined more quickly in sample analyses, but must be evaluated relative to facies changes. This is certainly true in a large sedimentary basin such as the GoM, where lateral changes in microfossil abundances can occur between an original wellbore and its bypass or sidetrack hole. The synchronicity of microfossil events used in BP GNATTS has been tested in a number of ways. First, is their reproducibility in sequence within an enumerated Neogene framework having an average resolution of 144 k.y. Second, is their application within much higher resolution biostratigraphic frameworks at field scale or individual well bores, where correlations are further constrained by geologic log correlations. Third, is that the large majority of events in BP GNATTS (∼86%) have been reproduced in sequence from research on cores in the western tropical Atlantic Ocean (ODP Leg 154) and western Mediterranean Sea (ODP Leg 161).

The final critical factor to achieving our results was the setting. These innovations were possible because the biostratigraphy team worked together daily in an integrated environment with geologists and geophysicists. These interactions gave biostratigraphers time to share concepts, identify business needs, fully integrate biostratigraphy with the description of the subsurface (geology), and provide essential one-on-one training for the next generation.

Abundance estimates (total assemblage and individual taxa) are routine for well and research analyses. For nannofossils, the cascading count system of Styzen (1997) is GoM industry standard (specimens/100 fields of view estimates) outside of BP. Within BP, a 0–100 based counting system has been employed for decades for the sole practical purpose in having to work with hardcopy histogram data (estimates based on specimens per field-of-view at 1000×). For foraminifera, relative abundance categories (10 categories between Present to Flood, with quantitative values estimated within categories) per BP standard were used in well analyses and the original research. The foraminifera research data was converted to presence/absence for this publication because it was not possible to normalize all count data from different researchers over a fifteen-year time span.

The primary ODP Leg 154 research was done by Eric de Kaenel and Jim Bergen for nannofossils (1202 samples), Sheila Barnette and Steve Truax for foraminifera (618 samples). The remaining authors re-examined selected research samples in the months prior to completion of an internal 2007 GoM Neogene chart and again for this publication. For nannofossils, 45–60 minutes were typically needed to document rare “marker” taxa, corresponding to ∼1000 fields-of-view at 1000× magnification. For planktonic foraminifera, similar analysis times are recommended. Barnette examined 519 samples from the upper Oligocene (24.303 Ma) to lower Pliocene (4.128 Ma). BP staff examined 253 samples for the presence/absence of marker species, while also extending stratigraphic coverage into the upper Oligocene (24.900 Ma) and lower Pleistocene (1.720 Ma).

Time is the most determinative factor in sample analyses because fossils are very rare at the ends of their stratigraphic ranges. In clastic settings such as the GoM, analysis times are skewed toward samples within abundance peaks, which maximizes results and makes it possible for an experienced analyst to average up to two samples per hour. For pelagic and hemipelagic settings, collecting accurate abundance estimates and range data for rare taxa from fossil-rich assemblages is more time consuming.

The preparation and examination of samples are two critical factors to results. Consistency is of primary importance in sample preparation because microfossil abundance estimates are fundamental. ALS Ellington and Associates, Inc. (Houston, Texas, USA) prepared all well samples and foraminifera ODP core samples, whereas ODP core samples taken for nannofossils were prepared by the research scientists. Historically, BP utilized microfossil abundance peaks in well correlation and the identification of event horizons within these peaks for use in basin-wide and field scale schemes. This methodology is outlined in Armentrout (1996) .

In 2009, the International Union of Geological Sciences (IUGS) ratified the Quaternary System (Period) as a formal stratigraphic unit ( Gibbard and Head, 2010 ), truncating the top of the Neogene System and abandoning the informal term Tertiary. We have chosen a twofold division of the Cenozoic, referring the Miocene through Holocene Series (Epochs) to the Neogene, following decades of conventional use in marine micropaleontology and that advocated by Hilgen et al. (2012) for the Neogene Period. To us, the terms Quaternary and Tertiary should be paired and are an alternate way to subdivide the Cenozoic. For further clarification, we also follow Hilgen et al. (2012) by placing the Pliocene–Pleistocene boundary at the top, as opposed to the base, of the Gelasian Stage. We have maintained an age of 23.03 Ma for the Oligocene–Miocene boundary, following both Hilgen et al. (2012) and Ogg et al. (2016) . Stadial terminology is not typically used in the GoM Basin, where Series/Epochs (e.g., Pliocene) are the basic stratigraphic units.

Abundance categories for calcareous nannofossils ( Fig. 3 ) are based on estimates of the number of specimens per fields-of-view (FOV) at 1000× magnification, where from lowest to highest are: P (present) being 1 specimen in >100 FOV; R (rare) then being 1 specimen in ≤100 FOV; F (few) then being 1 specimen in ≤10 FOV; C (common) then being ≥1 specimen per FOV; and A (abundant) then being ≥10 specimens per FOV.

The following standard abbreviations ( Figs. 3 and 4 ) are used for bioevent terminology: LO (lowest occurrence) and HO (highest occurrence); abundance modifiers are: R (regular or persistent) for LRO and HRO, I (increase) for LIO and HIO, F (few) for LFO and HFO, C (common) for LCO and HCO, A (abundant or acme) for HAO and LAO, and Ab (absence) for LOAb and HOAb. Other paired terms utilized herein include EXIT and RE (re-entry), disappearance (DA) and reappearance (RA), and increase (INC) and decrease (DEC). For planktonic foraminifera, coiling directions are abbreviated as S (sinistral) or D (dextral). Event terminology is in a “top-down” or “down-hole” sense because of its use in drilling ( Figs. 4 and 5 ). These terms should not be confused with those used in a “bottoms-up” or depositional sense for deep-sea research cores (DSDP, ODP, IODP) and outcrops (see Appendix).

The documentation of fossil appearances (first occurrences) and extinctions (last occurrences) in a stratigraphic section is of primary importance, where they are expressed as lowest occurrences (bases) or highest occurrences (tops), respectively. Two-thirds of the microfossil events used in BP GNATTS are lowest or highest occurrences, with the latter being far more predominant. Ninety percent of the bioevents utilized among the six Neogene calcareous microfossil biozonations discussed herein (see “Calibration, Global Biozonations” section) are based on microfossil appearance or extinction events, including all zonal bioevents for planktonic foraminifera. For nannofossils, age estimates in the Backman et al. (2012) biozonation were chiefly derived from astronomically-tuned cyclostratigraphies using the semiquantitative methods of Backman and Shackleton (1983) . These counting methods provide precise and reliable bioevents that are easily determined; however, they emphasize speed, which is counter-intuitive to documenting the rare and sporadic occurrences that typify the stratigraphic extremities of individual species. We believe these semiquantitative methods actually have produced both initial and final abundance changes as proxies for nearly all of the Neogene nannofossil appearance and extinction events in Backman et al. (2012) . Thus, we introduce two biostratigraphic events for these abundance changes: (1) the lowest increase occurrence for the initial abundance increase; and (2) the highest increase occurrence for the final abundance decrease. This fundamental difference of opinion about the stratigraphic expression of nannofossil appearances and extinctions explains why many of our geologic ages are outside those ages presented in Backman et al. (2012) , many of which were derived from the same reference cores (ODP Leg 154).

Historically, GoM microfossil events have been limited to fossil tops (highest occurrences) and abundance increases (in a down-hole or drilling sense) because ditch-cuttings routinely “cave” down the well bore. Down-hole caving is less problematic in modern DW GoM wells relative to those drilled before the latter part of the twentieth century. We speculate that this could be due to the use of synthetic muds that do not break down the ditch-cutting samples and by improved drilling parameters and pressure predictions. Remobilization and redeposition of sediments, the latter referred to as reworking by microfossil specialists, are more problematic in a terrigenoclastic basin with salt tectonism such as the GoM. In the GoM, the reworking of microfossils is most often observed as sporadic occurrences of rare specimens involving only a few species. Such processes are easier to recognize in wells when applying a much higher resolution biostratigraphic scheme such as BP GNATTS, which then provides a foundation for still higher resolution biostratigraphic schemes developed for expanded reservoir sections in the GoM. When all this is considered along with the consistency provided in sample preparations, it is now possible to more fully utilize the entire abundance profile of individual species ( Fig. 3 ) in correlation. Biostratigraphic resolution can be further improved by utilizing non-standard marker taxa and describing new species.

The terminology of BP GoM Chart Horizons has a long history prior to the BP mergers and has evolved somewhat haphazardly. Letter designations refer to the Pleistocene (PS), Pliocene (P), Miocene (M), and Oligocene (O) Epochs. Horizons within each epoch are then numbered bottoms-up, akin to global biozonations. Epoch enumerations in the original BP framework ranged from 1 to 100, but this is no longer true because epoch boundary criteria have changed over the decades. Some Chart Horizons are subdivided into upper (U) and lower (L) and/or subdivided by lettering (from A to D), both in a top-down sense. Horizons not yet proven across the entire basin are referred to as “Locals.” For the Neogene, there are 160 Chart Horizons and 14 Locals. Eight Oligocene Chart Horizons (O85-O75) are also included on BP GNATTS.

BP biostratigraphic Chart Horizons are both chronostratigraphic terms and mappable surfaces in the GoM. In application, a Chart Horizon (Fig. 2) is a surface that includes section down to the top of the next Chart Horizon (a top-down or drilling sense). A biozone is bounded by two surfaces and its application functions in a similar manner. A BP biostratigraphic Chart Horizon is set apart from a biozone in that it is multidisciplinary (i.e., both nannofossil and foraminifera events) and often utilizes multiple event criteria and intra-horizon (or intra-zonal) events.

Industrial historic placement of Neogene Epoch boundaries in the GoM Basin are lithologic, corresponding to major mappable seismic surfaces. Placement of microfossil biostratigraphy relative to these surfaces has varied among companies, partially caused by different global usages of boundary microfossil criteria. This has sometimes caused confusion in communication between companies and partners; for example, when stating “a well has penetrated the Oligocene.” Various GoM microfossil events that have been used to pick the top Oligocene vary over 3.55 m.y. ( Bergen et al., 2009 ), representing the interval spanned by BP Horizons M4 to O79 (Fig. 2). Within BP, this boundary has moved from a purely lithologic definition in the twentieth century (M4/old O90), to a “global” definition in 2001 (O80), to where it is now and calibrated internally to both the base Neogene GSSP in northern Italy and the ODP Leg 154 chronometer between Horizons LM3C and O85 (old M2).

We have used the ages for stage boundaries in accordance with the most recent geologic time scale of Ogg et al. (2016) , who followed the IUGS and positioned the base of the Pleistocene Series—and Quaternary System—at the base of the Gelasian Stage. However, we have chosen to maintain a 3-fold subdivision of the Pliocene and place the Pliocene-Pleistocene boundary at the top of the Gelasian. This conforms to placement within previous GoM charts (Shell and Texaco) and global planktonic foraminifera and nannofossil biozonations.

There are 468 Neogene events on BP GNATTS, which yielded an effective resolution of 101 k.y. for nannofossil events, 397 k.y. for planktonic foraminifera events, and 344 k.y. for benthic foraminifera. About 60% of these events are fossil tops. Chart Horizon resolution is highest in the Pleistocene (67 k.y.) and lowest in the Early Miocene (261 k.y.)—also typical of global calcareous microfossil biozonations and other GoM frameworks.

Most of the 174 Chart Horizons (including Locals) are defined by multiple events and various combinations of events from each of the three microfossil groups. The age for a Chart Horizon is the youngest geologic age determined for either a planktonic foraminiferal or calcareous nannofossil event associated with that Chart Horizon (Fig. 2; Table S1). Direct age calibrations to ODP Leg 154 have been made for 86% of the Chart Horizons. An extreme example of an indirect calibration would be Horizon M82, where the HO of the nannofossil Discoaster bellus in the GoM has been tied to its HCO in the ODP Leg 154 research.

The geologic ages for BP GNATTS were derived through orbital scaling of ODP Leg 161 (Horizons PS107-LPS60) and ODP Leg 154 (Horizons PS50-O75) based on our internal research on these cores. The base Holocene age is assigned to PS108. Direct ages are those based on the same type of event in both the research and the GoM. For example, the highest occurrence (HO) of the nannofossil Discoaster brouweri is used in both BP GNATTS and the research on the ODP Leg 154 cores. Ages are considered indirect when a GoM event (e.g., HO or HCO) is sequenced to different event type (e.g., HRO or HIO, respectively) in the research. Associated ages are those assigned ages only through correlations established in the GoM framework; all benthic foraminifera events fall into this category. Bioevent ages derived from sampling of ODP Leg 154 and Leg 161 are maintained at three decimal precision, when expressed in mega-annums (Ma). Errors for these ages are the differences in assigned ages for the next sample analyzed upwards or downwards in the composite section (Supplementary Materials; Tables S5–S16, S18–S23).

Three columns outside biostratigraphy were included for reference and calibration. The Earth’s major orbital eccentricity periodicities (∼100 k.y. and 405 k.y.) derived from the Laskar et al. (2004) solution are shown near the left side of BP GNATTS. Geomagnetic polarity appears to the left of the orbital scale (from Ogg et al., 2016 ; Hilgen et al., 2012 ). The eustatic sea level sequences and transgressive-regressive cycles at the right side of the chart are calibrated to BP GNATTS through TimeScale Creator GTS 2016 by Purdue University, West Lafayette, Indiana, USA, ( engineering.purdue.edu/stratigraphy/tscreator ) founded on the SEPM Chart #2 by Hardenbol et al. (1998) .

BP GNATTS has utility beyond a biostratigraphic framework for the GoM Basin, where it has impacted the entire value chain in the GoM from exploration to production and is the foundation for yet higher resolution, reservoir biostratigraphic frameworks. BP GNATTS has been tested in selected portions of the geologic column outside the GoM Basin, in both research and industrial settings. Extending this improved resolution geographically has new found application for the interpretation and correlation of cyclostratigraphic, magnetostratigraphic, and eustatic records.

BP GNATTS is a step-change in stratigraphic resolution (144 k.y.) relative to published Neogene calcareous microfossil schemes. It doubles stratigraphic resolution relative to published industrial GoM Neogene biostratigraphic charts ( Styzen, 1996 ; Lawless et al., 1997 ) and is distinguished from all other industrial schemes by being “fully-tuned” to orbital periodicities. Neogene resolution for nannofossil events marking BP GNATTS Chart Horizons is 142 k.y., whereas the effective resolution for all nannofossil events on BP GNATTS is 101 k.y. These are five- and four-fold increases in resolution relative to the zonal and event level resolution of the Backman et al. (2012) global scheme. For planktonic foraminifera, Neogene resolution on BP GNATTS Chart Horizons is 435 k.y., doubling that of the highest resolution global biozonation of Berggren et al. (1995a , 1995b) .

Per BP methodology, microfossil abundance peaks are the fundamental unit used in correlations. In theory, biostratigraphic resolution is then only limited by the number of abundance cycles that can be uniquely defined by their microfossil content and stratigraphic position. The influence of facies on microfossil recovery and preservation is usually a primary practical limitation on biostratigraphic resolution, affecting the abundances of species to entire microfossil groups, as well as the number of taxa that may be utilized in correlations. This is less problematic in DW GoM Neogene, although significant changes in facies affecting microfossil recovery and diversity can exist between an original hole and its sidetrack or bypass holes. One of the most important influences on stratigraphy in DW GoM is salt tectonism. This structural component affects the placement, orientation, and continuity of stratigraphic section. The effects of salt can be dramatic, involving thousands of feet of repeated, folded, or inverted section. The mixing of fossil assemblages and remobilization of sediments through salt movement must always be considered in biostratigraphic analyses and interpretations in the GoM.

Salt tectonism and redeposition can cause discontinuities in the sequence of biostratigraphic events. The effects of such geologic processes are better detected by higher resolution schemes utilized in well-to-well correlation (see Denne, 2009). Detection of “out-of-sequence” events such as reworking, down-hole caving of drill cuttings, faulting, and repeated stratigraphic section is much more likely with improved resolution. Higher resolution biostratigraphy increases the number of correlations between sections, which in turn, improves the precision and accuracy of these correlations. Errors in correlation are not always obvious when lower resolution biostratigraphic schemes are applied to expanded GoM sections. In such settings, biostratigraphic events can appear to be in sequence, but in reality, there may be errors of hundreds to more than a thousand feet because of a combination of high sedimentation rates, reduced microfossil recoveries, and the low number of events.

The BP GNATTS chart has been built upon a succession of BP heritage GoM Neogene biostratigraphic charts. These heritage charts have never been static frameworks. Efforts in twenty-first century post-merger BP focused on improving both the reliability and resolution of GoM Chart Horizons, which are integrated into subsurface mapping and wells. BP GNATTS is strengthened by the next level of biostratigraphic data, whose entire detail is beyond the scope of this publication. However, three examples are presented below.

Both the resolution and reliability of Chart Horizons are enhanced by the addition of biostratigraphic events through well analyses. The evolution in the definition and recognition of two upper Miocene Chart Horizons (M70 and M68) is presented in Table 3. In pre-merger BP, three events marked the M70 Horizon and a single event marked the M68 Horizon; none were directly calibrated to geologic time. One of these four events could not be proven in post-merger BP through analyses of DW GoM wells, nor by research on ODP Leg 154 cores (the HO of the nannofossil Helicosphaera walbersdorfensis is one of only two Neogene nannofossil events in heritage company schemes that could not be confirmed post-merger). A new microfossil abundance cycle (i.e., Local), observed in several wells across DW GoM, has been included on BP GNATTS. Six calcareous microfossil events, all calibrated to geologic age through ODP Leg 154 research, now marked two Chart Horizons and one Local on BP GNATTS. Four additional microfossil events for Chart Horizon M70, three of which have been tied to geologic age in our research, have been observed in a limited number of DW GoM wells and were not included on BP GNATTS (Table 3).

The use of multiple types of biostratigraphic events, as in Chart Horizon M70, provides additional benefits. Using multiple microfossil groups for confirmation is standard practice in the GoM. Abundance changes of individual taxa or taxa groups have practical utility in their speed of recognition, but must be evaluated relative to lateral changes in facies in the GoM that can affect microfossil abundances. The sequencing of fossil bases (lowest occurrences) into the biostratigraphic framework has proven critical to interpretations of redeposition in DW GoM wells.

A second way to illustrate the use of different types of events and biostratigraphic resolution beyond BP GNATTS is through the lens of an individual taxon (Table 4). Standard industrial biostratigraphic schemes are founded on microfossil tops (HOs), supplemented by down-hole abundance increases in taxa, due to concerns about the caving of materials in well bores. We have observed that down-hole caving is not generally problematic in DW GoM wells drilled since the turn of the century. At first pass, this brings into play the use of fossil bases (LOs) and down-hole abundance deceases (DECs); this effectively doubles biostratigraphic resolution and makes full calibration to published academic biozonations possible. The use of multiple events for a single taxon has been taken to the extreme for the nannofossil Reticulofenestra pseudoumbilicus (Table 4). Here, twenty events recognized among DW GoM wells have been calibrated either directly through the ODP Leg 154 research or by “association” with marker events on BP GNATTS through well analyses. Ten of the more proven events mark Chart Horizons on BP GNATTS. Four events are related to the exit and re-entry (RE) of R. pseudoumbilicus from the GoM Basin during the early Middle Miocene and early Late Miocene. The timing for three of these four events involving the two disappearances of R. pseudoumbilicus is different between the GoM and the ODP Leg 154 sites near the equator (see Table 4). The disappearance of R. pseudoumbilicus in the early Late Miocene has long been studied and considered by some to represent a significant reorganization in the Neogene carbonate producing community (Rio et al., 1990b; Young, 1990; Takayama, 1993; Raffi and Flores, 1995; Backman and Raffi, 1997; Kameo and Bralower, 2000; Krammer et al., 2006). For the planktonic foraminifera Catapsydrax dissimilis, two of the five events recognized in GoM wells have been incorporated into BP GNATTS (Table 4).