New data from a continuously cored succession, the Mossy Grove core, near Jackson, central Mississippi, recovered
The Eocene–Oligocene transition (EOT:
Dinocysts have been widely used in biostratigraphic and palaeoenvironmental studies (e.g. Bujak and Williams, 1985; Duxbury and Vieira, 2018; Kothe, 1990; Powell, 1992; Pross et al., 2010; Stover et al., 1996; Vieira et al., 2018), including several detailed studies of the EOT (e.g. Brinkhuis and Biffi, 1993; Brinkhuis, 1994; Jaramillo and Oboh-Ikuenobe, 1999; Van Mourik et al., 2001; Oboh-Ikuenobe and Jaramillo, 2003; Houben et al., 2012, 2013, 2018). A major compilation of Late Cretaceous–Neogene calibrated dinocyst bioevents, from both Northern Hemisphere and Southern Hemisphere, demonstrates a strong climatic control on the timing of bioevents, which are commonly diachronous (Williams et al., 2004) . Dinocyst biostratigraphic studies thus need to consider comparable realms in similar climatic zones when selecting appropriate dinocyst bioevents (Williams and Bujak, 1985). There are existing dinocyst biostratigraphic studies of the Eocene–Oligocene from both the Northern Hemisphere (Brinkhuis and Biffi, 1993; Bujak and Mudge, 1994; Egger et al., 2016; Mudge and Bujak, 1994; Śliwińska et al., 2012; Thomsen et al., 2012; Wilpshaar et al., 1996) and the Southern Hemisphere (Bijl et al., 2018; Wilson, 1988). At the Massignano Eocene–Oligocene Global Stratotype Section and Point (GSSP), there are two successive influxes of cool-water high-latitude organic-walled dinoflagellate cyst (dinocyst) species (Brinkhuis and Biffi, 1993), the first of which correlates directly with the EOB and the second with the onset of a more severe cold episode and inferred sea level lowstand. In parallel, quantitative analysis of dinocyst distribution patterns from the “Massicore”, central Italy (Van Mourik and Brinkhuis, 2005), revealed biotic turnover potentially associated with latitudinal and climatic zone changes through the Eocene–Oligocene transition. In the Massicore, dinocyst assemblages are interpreted to represent substantial cooling on the first step, with little subsequent change on the second step (Houben et al., 2012). Records from the Antarctic Margin show decreasing dinocyst species diversity through the Eocene and at the EOB (Mohr, 1990), interpreted as the progressive development of cold surface waters. More recent dinocyst studies (Houben et al., 2013) document a sudden regime shift in zooplankton-phytoplankton interactions in the Southern Ocean associated with the earliest Oligocene glaciation of Antarctica, likely triggered by cooling, ice-sheet expansion, and sea-ice formation.
Eocene to Oligocene strata are well represented in outcrop and the shallow
subsurface across the Gulf Coastal Plain of Mississippi and Alabama, as
well as in the offshore systems of the northern Gulf of Mexico
(Hosman, 1996). In these areas, dinocyst assemblages have an
instrumental importance for site-to-site stratigraphic correlation and
biochronology (Jaramillo and
Oboh-Ikuenobe, 1999). Existing palynological work from five sites across
southern Mississippi and Alabama allowed the correlation of stratigraphic
sequences through the upper Eocene and the lower Oligocene
(Jaramillo and Oboh-Ikuenobe, 1999).
They observed a maximum flooding surface, estimated to be
While considerable attention has focused on EOT sections in southwestern Alabama (Jaramillo and Oboh-Ikuenobe, 1999; Katz et al., 2008; Mancini, 1979; Miller et al., 1993, 2008; Quaijtaal and Brinkhuis, 2012; Tew and Mancini, 1995; Wade et al., 2012) and southeastern Mississippi (Jaramillo and Oboh-Ikuenobe, 1999; Quaijtaal and Brinkhuis, 2012; Tew and Mancini, 1995), where the Yazoo Formation is lithologically heterogenous and divided into multiple constituent members, in central and western Mississippi the Yazoo Formation is a thick and relatively homogeneous succession of calcareous clays. For micro-palaeontological and palaeoenvironmental studies, these successions have the potential to yield some of the best marine records of the late Eocene in all of North America, if not globally. The relative lack of study of these successions appears to stem from early problems in determining a robust chronostratigraphic framework and the failure to constrain the EOB within the upper Yazoo Formation (Obradovich et al., 1993). The purpose of this study is to provide a detailed dinocyst biostratigraphic framework through an expanded EOT succession from the US Gulf Coast, as the basis for future studies of community change, extinction, and palaeoenvironments based on these assemblages. Such studies of the response of organic-walled cyst-producing dinoflagellate communities across this interval are important for an understanding of the nature of the reorganization of planktonic ecosystems through this major transition.
The Gulf Coastal Plain (Fig. 1) is one of the largest physiographic provinces in North America. In the subsurface the Mississippi embayment consists of a structurally complex basin with thick Jurassic-to-Holocene deposits (Cushing et al., 1964; Hosman, 1996). Within these sequences, the most continuous Paleogene successions are found between central and southern Mississippi and Alabama (Cushing et al., 1964; Hosman, 1996).
Palaeogeographic reconstruction showing location of the Mossy Grove, #1 Wayne and #1 Young cores, as well as St. Stephens Quarry outcrop. Main North American palaeogeography at
The upper Eocene to lower Oligocene stratigraphy of the US Gulf Coast is
split into two groups (Fig. 2) – the Jackson
(
Simplified stratigraphic correlation chart of upper Eocene to lower Oligocene sedimentary rocks in the Mississippi–Alabama area (modified from Pasley and Hazel, 1995; Tew and Mancini, 1995; Hosman, 1996).
The upper Eocene to lower Oligocene lithostratigraphy outlined above has
been interpreted within two distinct sequence stratigraphic models. The
first argues that the Yazoo–Bumpnose contact coincides with a maximum
flooding surface throughout the region
(Baum and Vail,
1988; Echols et al., 2003; Jaramillo and Oboh-Ikuenobe, 1999; Loutit et al.,
1988; Mancini and Tew, 1991; Tew, 1992). The second, and more recent
(Miller et al.,
2008), supports the association of the Yazoo–Bumpnose contact
(Dockery, 1982) with a low stand sequence boundary, which is
linked to the increasing
Stratigraphic correlation between the Mossy Grove core and the other three sections from Mississippi and Alabama (Jaramillo and Oboh-Ikuenobe, 1999). Lithology at the Mossy Grove follows Dockery III et al. (1991) and the timescale for all sections follows Westerhold et al. (2014). See discussion about dinocyst bioevents in Sect. 3.2.
Even at the St. Stephens Quarry section, southwestern Alabama, which is a
widely studied reference for the EOT
(Katz
et al., 2008; Miller et al., 2008; Wade et al., 2012), establishing a robust
biostratigraphy and sequence stratigraphy has been difficult
(Miller et al.,
2008). The low abundance and generally poor preservation of hantkeninids in
the shallow-water Gulf Coast successions make the accurate placement of
their last occurrence (LO) problematic
(Miller et al.,
2008). This difficulty is further aggravated by the reworking of upper
Eocene calcareous microfossils up into lowest Oligocene sediments as a
result of the major regression across the EOT (Bybell and Poore,
1983). Nevertheless, a middle-to-late Priabonian age was assigned to the
Pachuta Member at SSQ, based on the LO of
The Mossy Grove borehole was drilled in September 1991 at Mossy Grove, Hinds
County, Mississippi. It consists of a 161.6 m (530 ft) continuously cored
succession, with near full recovery (Dockery
III et al., 1991). At its base it recovered
Establishing a robust age model for the Mossy Grove core has required
significant effort and the integration of dinocyst, calcareous nannofossil,
and radiometric dating techniques. Initial age models were based on
planktonic foraminifera assemblage data
(Fluegeman, 1996; Fluegeman et al.,
2009), but due to low planktonic foraminifera abundances, especially in the
upper part of the core (< 91.4 m; 300 ft), this age model relied
upon poorly calibrated secondary markers. For instance, the direct placement
of the EOB, based on the last occurrence of the Hantkeninidae, is
problematic as the record of
Radiometric
Altogether, 112 sediment samples were collected from the Mossy Grove core at
With a Zeiss transmitted-light microscope, at least 200 dinocyst specimens were counted in each sample, along with the respective number of spores, pollen, algae (Prasinophyceae and Chlorophyceae), zoomorphs/zooclasts, phytoclasts, and amorphous organic matter. Once 200 specimens were counted, the remaining fields of the coverslip were scanned for very rare dinocysts. Only palynomorphs that were more than 50 % complete and not obscured either by air bubbles or organic debris were considered. We used the following abundance scheme: 0 % – barren (B); 0 %–1 % – trace (T); 1 %–10 % – few (F); 10 %–25 % – common (C); > 25 % – abundant (A). Preservation was qualitatively categorized as good (G), moderate (M) or poor (P).
Samples for the analysis of calcareous nannofossils were taken at
Nearly all the core samples contain abundant and very well-preserved
dinocysts, with only a few samples being scarce or rare. A
semiquantitative distribution of all dinocysts recognized in the
investigated section is shown in Fig. 4.
Semiquantitative range chart of dinocyst taxa encountered at the Mossy Grove core, central Mississippi, US Gulf Coastal Plain, including calcareous nannoplankton zonation.
For this study we present the first calcareous nannofossil biostratigraphy
of the Mossy Grove core, with biohorizons calibrated to the global zonation
scheme of Agnini et al. (2014). We updated these absolute ages
(Agnini et al., 2014) to the
timescale of Westerhold et al. (2014). Three of these bioevents constrain the base and top of the core: the
absence of
List of bioevents, biohorizons, and calibrated ages, updated to the timescale of Westerhold et al. (2014). Legend: CN – calcareous nannofossils (this study); PF – planktonic foraminifera (Fluegeman et al., 2009); RD – radiometric dating (Obradovich et al., 1993; Obradovich and Dockery III, 1996).
Dinocysts dominate the palynological assemblages in the studied material. In total, we identified 52 genera and 70 species, with nomenclature following Williams et al. (2017) and references therein. A full species list is provided in Appendix 1. The rich and well-preserved dinocyst assemblages provide the basis for a detailed assessment of upper Eocene dinoflagellate biostratigraphy in this succession, which is developed with reference to dinocyst biostratigraphic data from offshore eastern Canada (Egger et al., 2016; Williams, 1975, 1977), the Gulf Coast Plain (Houben et al., 2018; Jaramillo and Oboh-Ikuenobe, 1999), offshore Florida (VanMourik et al., 2001), the Norwegian and Greenland seas (Eldrett et al., 2004; Manum, 1976; Manum et al., 1989; Williams and Manum, 1999), the North Sea (Bujak and Mudge, 1994; Gradstein et al., 1992; Hansen, 1977; Heilmann-Clausen and Van Simaeys, 2005; Mudge and Bujak, 1996; Śliwińska et al., 2012), the Hampshire Basin (Costa et al., 1976), the central Mediterranean region (Brinkhuis, 1994; Brinkhuis and Biffi, 1993; Van Mourik and Brinkhuis, 2005; Wilpshaar et al., 1996), northwestern Europe (England, Belgium, and Germany; Costa and Downie, 1979; Kothe, 1990), the Tasmanian Gateway (Stickley et al., 2004), and the wider Northern Hemisphere (Williams, 1993). Although there have been studies of dinocyst biohorizons across the EOB interval in Gulf Coast sections from Alabama (e.g. Jaramillo and Oboh-Ikuenobe, 1999; Houben et al., 2018), these do not have the stratigraphic coverage of the upper Eocene or the quality of dinocyst preservation provided by the Mossy Grove core material. Age assignments are based on the global compilation of age-calibrated bioevents (Williams et al., 2004), with a total of 23 potentially useful dinocyst biohorizons recognized in the Mossy Grove succession (Table 2), including 7 first occurrences (FO), 1 first common occurrence (FCO), 14 last occurrences (LO), and 1 last common occurrence (LCO) event.
List of dinocyst bioevents recorded in the section. Reliability rating and absolute ages were assigned for selected events. Timescale follows Westerhold et al. (2014). The last column presents ages calculated from the age–depth model provided by this study (see Sect. 3.4). Legend: FO represents first occurrence; LO represents last occurrence; FCO represents first common occurrence; LCO represents last common occurrence; A represents acme;
References for chosen ages:
At least 15 of the 23 biohorizons within the Mossy Grove succession could be correlated to other mid-latitude areas in the Northern Hemisphere, and we discuss each of these – from the base of the succession upwards – in the text below. Our estimation of the reliability of each bioevent as highly, moderately, or lowly reliable; it is based on a combination of the clarity of taxonomic definition of a species or genus, as well as the number of reports of the associated biohorizon at other localities, and the consistency of stratigraphic position between them. The estimated age of each bioevent is also given, along with a rationale for the chosen calibration. Ages are first provided according to the original assignment and timescale but then, for consistency, are all updated to the Westerhold et al. (2014) timescale, which is given in brackets. Where there is evidence for latitudinal diachroneity of an event, we preferentially choose age calibrations from locations closest to the study site. Standard nannoplankton (Agnini et al., 2014; Martini, 1971) and foraminiferal (Berggren et al., 1995; Berggren and Pearson, 2005; Wade et al., 2011) biozonation schemes were used when discussing selected dinocyst bioevents.
The palynological assemblages of the Mossy Grove core show strong evidence
for sea level fall associated with the EOT interval manifest in the form of
increased flux of terrestrial plant material but also in the influx of
clearly weathered and reworked acritarchs and continental palynomorphs
towards the top of the succession. This includes reworked Late Cretaceous
taxa such as
On a regional scale, some distinctive events found at Mossy Grove are very
similar to those detected in five sections from southeastern Mississippi and
southwestern Alabama (Jaramillo and
Oboh-Ikuenobe, 1999). Graphic correlation across five sections (St. Stephens
Quarry, R2089, #1 Young, #1 Wayne, and #1 Ketler) allowed age
determination of the main dinocyst events, including the FO of
It is notable that well-preserved specimens of
Using these ages, we present a significantly refined age model for the Mossy Grove core (Fig. 5) based on dinocyst bioevents as discussed in the previous section (Table 2), integrated with new age constraints from calcareous nannofossil assemblages as well as existing radiometric dates. We show previously published foraminiferal-based age control for reference (Table 1).
Refined age–depth model of the Mossy Grove core. Lithology follows Dockery III et al. (1991) and the timescale follows Westerhold et al. (2014). Best fit lines for the models of Fluegeman et al. (2009) and this work are represented as green and red lines, respectively. Sedimentation rates are also indicated. The shaded area in light red reproduces an error of
This new age model can be interpreted to represent sedimentation within two
intervals with distinct rates: between the bottom of the analysed section,
i.e.
Another direct consequence of this model is the reassessment of the age of
some dinocyst bioevents. As already mentioned, the extinction of
Comparisons with nearby sites in Mississippi and Alabama
(Houben et al., 2018;
Jaramillo and Oboh-Ikuenobe, 1999) (Fig. 3) show a
similar sequence of bioevents as the Mossy Grove section. The following key
events within the Gulf Coast Plain show a high degree of consistency (from
the older to the younger): FO
The Mossy Grove core recorded a rich upper Eocene to lower Oligocene section
with excellently preserved palynomorph content, including 52 genera
and 70 species of dinocysts identified. Furthermore, rich diversity in
other palynological groups was observed, revealing the enormous potential of
the study site for the elaboration of future palynofacies models, useful in
palaeobathymetric studies. High-resolution analysis (
All semi-quantitative data are provided within the body of the article.
“
Illustration of taxa and sample or slide number. (1)
Illustration of taxa and sample or slide number. (1)
Illustration of taxa and sample or slide number. (1–4)
Illustration of taxa and sample or slide number. (1, 2)
Illustration of taxa and sample or slide number. (1)
Illustration of taxa and sample or slide number. (1, 2)
Illustration of reworked taxa and sample or slide number. (1)
MADLM conceived the study, processed and analysed samples, interpreted results, and wrote and edited the article; GH analysed the samples and participated in interpretation of the article; TDJ collected samples and participated in interpretation, writing, and editing of the article.
The authors declare that they have no conflict of interest.
We thank Carlos d'Apolito Júnior (Federal University of Mato Grosso, Brazil) and Manuel Vieira (Shell, UK), as well as James A. P. Bendle and Kirsty M. Edgar (University of Birmingham, UK) for their numerous and helpful suggestions, which have greatly improved this work; we thank Roger Burgess (University of Aberdeen, UK) for the initial pilot study, revealing the excellent level of preservation of the samples. We also thank PetroStrat Ltd, particularly Gary Smith, Marcel Polling, Neil Campion, and Paul Cornick, for allowing us access to their facilities during the slide preparation step.
This research has been supported by the National Council for Scientific and Technological Development (CNPq, Brazil) (grant no. 206218/2014-1) and the Natural Environment Research Council (NERC, UK) (standard grant NE/P013112/1).
This paper was edited by Francesca Sangiorgi and reviewed by Henk Brinkhuis, Kasia K. Sliwinska, and two anonymous referees.