Timothy J. Bralower

Calcareous nannofossil zonation of the Jurassic-Cretaceous boundary interval and correlation with the geomagnetic polarity timescale

Abstract Calcareous nannofossil stratigraphy has been investigated in six European land sections and at two Deep Sea Drilling Project Sites (391C and 534A) ranging from the Upper Kimmeridgian to the Lower Valanginian. Most of the sections contain a continuous record of the Jurassic-Cretaceous boundary interval, a time of rapid increase in nannofossil abundance and diversity and allow development of a revised nannofossil zonation scheme for this period. Numerous nannofossil lineages have been studied in detail and the results help to increase potential stratigraphic resolution in this interval. The proposed scheme consists of six zones which are units recognizable in all sections regardless of nannofossil preservation and eleven subzones determinable in all but the most poorly preserved material. Thirty-eight additional biohorizons are proposed based on the stratigraphy of the two DSDP sites. These can be accurately established only in moderately well-preserved material but spotty occurrences of these taxa in other sections may be stratigraphically useful. Most of the sections studied have been investigated magnetostratigraphically and the results of this study allow correlation between nannofossil zonation and the geomagnetic polarity time scale. Magnetostratigraphy indicates that many nannofossil events can be quite precisely determined and are not significantly time-transgressive. Finally, the magnetostratigraphic definitions of two stage boundaries are refined. The Kimmeridgian-Tithonian and Berriasian-Valanginian boundaries are placed within Chrons CM22n and CM15n respectively. Two new genera, Faviconus and Umbria , and seven new species are described: Umbria granulosa, Rhagodiscus nebulosus, Cretarhabdus octofenestratus, Faviconus multicolumnatus, Nannoconus infans, Nannoconus compressus and Nannoconus wintereri . Eight subspecies and three taxonomic emendations are also described.

M-sequence reversals recorded in DSDP sediment cores from the western Mid-Pacific Mountains and Magellan Rise

New paleomagnetic and paleontologic data from Pacific DSDP Sites 463 and 167 define the magnetic reversals that predate the Cretaceous Normal Polarity Superchron (K-N). Data from Mid-Pacific Mountain Site 463 provide the first definition of polarity chron M0 in the Pacific deep-sea sedimentary record. Foraminiferal biostratigraphy suggests that polarity chron M0 is contained entirely within the lower Aptian Hedbergella similis Zone, in agreement with foraminiferal data from the Italian Southern Alps and Atlantic Ocean. Nannofossil assemblages also suggest an early Aptian age for polarity chron M0, contrary to results from the Italian Umbrian Apennines and Southern Alps, which place polarity chron M0 on the Barremian-Aptian boundary. Biostratigraphic dating discrepancies caused by the time-transgressive, preservational, or provincial nature of paleontological species might be reconciled by the use of magnetostratigraphy, specifically polarity chron M0 which lies close to the Barremian-Aptian boundary. At Magellan Rise Site 167, five reversed polarity zones are recorded in Hauterivian to Aptian sediments. Correlation with M-anomalies is complicated by synsedimentary and postsedimentary sliding about 25 m.y. after basement formation, producing gaps in, and duplications of, the stratigraphic sequence. The magnitude and timing of such sliding must be addressed when evaluating the stratigraphy of these oceanic-rise environments.

Significance of Upper Triassic nannofossils from the Southern Hemisphere (ODP Leg 122, Wombat Plateau, N.W. Australia)

A taxonomic and biostratigraphic investigation has been carried out on Upper Triassic (Carnian-Rhaetian) nannofosils from Sites 759, 760, 761 and 764 drilled on the Wombat Plateau during ODP Leg 122. The recovery of continuous sequences containing well preserved nannofossils has enabled us to refine the previous taxonomy and biostratigraphy of this interval. Fossil assemblages are of two major types: (1) previously described calcareous taxa were recovered at Sites 761 and 764; and (2) sideritic forms, which may represent diagentic replacement of calcareous nannofossils, were observed in material from Sites 759 and 760. The sideritic forms proved difficult to study taxonomically due to inadequate optical properties. Calcareous nannofossil assemblages in the Upper Triassic are dominated by Prinsiosphaera trisassica. We show that the multitude of identities of this species in the light microscope are the result of selective etching on a layered structure. We propose an evolutionary lineage for the earliest known coccoliths, with Crucirhabdus primulus as the ancestor. This species gave rise to C. minutus and Archaeozygodiscus koessenensis. The Upper Triassic can be subdivided based on the sequential first occurrences of C. primulus and Eoconusphaera zlambachensis in the upper Norian. The late Norian and Rhaetian were times of slow evolution of calcareous nannofossils. However, we noted three morphmetric changes in this time-interval which possess biostratigraphic utility: (1) P. triassica increases in diameter from an average of 6-mu-m to over 9-mu-m; (2) E. zlambachensis evolves from a stubby to an elongated shape; and (3) C. primulus increases in size. Upper Triassic assemblages from the Wombat Plateau are similar in composition and diversity to those which have been described in detail from the Alps. In both areas, nannofossiliferous sediments interfinger with massive limestones deposited in reef and peri-platform environments. Stable isotopic analyses of Wombat Plateau nannofossil assemblages indicate that they thrived in open ocean conditions. Biostratigraphy allows sequence chronostratigraphic interpretation of ODP Site 761 and supports the chronostratigraphic cycle charts of Haq et al. (1987).

Mid-Tertiary Braarudosphaera-Rich Sediments on the Exmouth Plateau

Nannofossil assemblages enriched in Braarudosphaera occur in lower Oligocene to lower Miocene sediments at Ocean Drilling Program Sites 762 and 763 on the central Exmouth Plateau. Braarudosphaerids appear here rather abruptly in the lower Oligocene (in Zone NP21). They reach their greatest numbers in the lower Oligocene (in Zones NP22 and NP23), where they comprise up to 10% of some samples. Braarudosphaera bigelowii is the overwhelmingly dominant species, occurring together with rare specimens of B. discula and Micrantholithus pinguis. The holococcoliths Peritrachelina joidesa and Lantemithus minutus are also associated with the Braarudosphaera enrichment. There are two populations of B. bigelowii: one of normal size (10-14 μm) and one of large specimens (20-22 μm). The larger braarudosphaerids are more common than the smaller forms. Braarudosphaera-nch sediments are absent at Wombat Plateau sites during the same time interval. We attribute this to latitudinal control, because the Wombat sites are about 4° north of the central Exmouth Plateau sites. We believe that the occurrence of braarudosphaerids is related to an Oligocene to early Miocene oceanographic event on the Exmouth Plateau. We suspect that mid-ocean up welling of cool, low-salinity, nutrient-rich water along a divergent zone created the Braarudosphaera-nch sediments in the South Atlantic and Indian oceans.

Lower cretaceous calcareous nannofossil stratigraphy of the Great Valley Sequence, Sacramento Valley, California

The calcareous nannofossil stratigraphy of four sections in the Great Valley Sequence of the Sacramento Valley, California, has been investigated in detail. The ages of these sections, which have independently been studied by ammonite, pelecypod and radiolarian biostratigraphers, range from Berriasian to Albian. Nannofossil stratigraphy is more directly comparable to that of stratotype sequences in Europe than are the other fossil groups, but the ages given are generally similar. Two major discrepancies do exist, however. The results of this study indicate that the pelecypod, Buchia aff. B. okensis Zone is Berriasian in age, and not Tithonian as previously believed and that the B. pacifica Zone is partly Berriasian and not entirely Valanginian. Nannofossil data provide useful age calibrations for ammonite stratigraphy. The study illustrates that nannofossil biostratigraphy can be effectively used to date sediments deposited in convergent margin settings. One new species, Nannoconus paskentaensis , is described.

Relationship between sequence boundaries and the evolutionary history of planktonic foraminifera, calcareous nannofossils, and reef communities in the mid-Cretaceous (Barremian-Cenomanian)

s and Program RElATIONSHIP BETWEEN SEQUENCE BOUNDARIES AND THE EVOLUTIONARY HISTORY OF PLANKTONIC FORAMINIFERA, CALCAREOUS NANNOFOSSILS, AND REEF COMMUNITIES IN THE MID-CRETACEOUS (BARREMIAN-CENOMANIAN) LECKIE·, R. Mark, Dept. of Geol/Geog, Univ. of Mass., Amherst, MA; SCOIT, Robert W., Amoco Prod. Co., Tulsa, OK; BRALOWER, Timothy J., Dept. of Geol., Univ. of N. Carolina, Chapel Hill, NC; SLITER, William V., U.S. Geol. Surv., Menlo Park, CA 179 Planktonic foraminifera first evolved in the middle Jurassic but did not experience a major radiation until the mid-Cretaceous. The mid-Barremian to late Aptian was characterized by a steady increase in species richness and by the appearance of new morphological forms including planispiral coiling, clavate and radially elongate chambers, and culminating in the first appearance of taxa with complex apertural structures and the keeled morphotype in late Aptian time. This broad interval of radiation was abruptly ended by evolutionary turnover and low diversification rates in the latest Aptian and early Albian prior to a second explosive episode of radiation in the middle and late Albian. The evolutionary history of mid-Cretaceous calcareous nannofossils generally parallels the trends observed in planktonic foraminifera, although the latest Aptian-early Albian turnover event is not as pronounced. Reef communities in the Caribbean/Gulf of Mexico and Mediterranean provinces show a change in dominance from coral-algal-rudist reefs in the Barremian-early Albian to rudist domiance by the late Albian time. These changes in calcareous plankton and reef communities are related to complex oceanographic changes of the mid-Cretaceous including structure of the upper water column, productivity, sea level, atmospheric and oceanographic circulation, and changes in the chemistry of the ocean. Changes in eustatic sea level influenced many of these factors including nutrient delivery to the oceans, climate, sites and rates of deep water formation, and ocean chemistry. What is the relationship between changes in sea level, as expressed by major seismic sequence boundaries, and the changes observed in marine biota? We have compared major changes of eustatic sea level within this interval of ge~erally rising global sea level (Scott et al., 1988), with equivalent sequence boundaries (Haq et al., 1988) and the records of calcareous plankton (Roth, 1987; Leckie, 1989) and reef communities (Scott, 1988). What is most striking about these relationships is the apparent lack of direct correlation between sequence boundaries and turnover events in the marine biota. The calcareous plankton alternate in phase between relatively high rates of diversification and low rates of diversification, with the major sequence boundaries falling within intervals of change rather than at intervals of change. However, we acknowledge the potential of missing or condensed intervals in deep sea settings which may influence the record of evolutionary rates (e.g., Loutit, et al., 1988). Only the basal Albian sequence boundary appears to correlate with a major turnover event in the planktonic foraminifera, and the rapid change in Gulf Coast reef communites between the middle and upper Albian may correlate with a eustatic sea level change and a major sequence boundary. Based on high-resolution calcareous nannofossil, planktonic foraminiferal, sedimentologic, and geochemical data of Bralower et al. (submitted), the lower Aptian, basal Albian, and lower upper Albian sequence boundaries appear to correlate more closely with widespread oceanic dysoxic/anoxic events OAEla, OAE1b, and OAElc, respectively. The correlations between evolutionary events, anoxic events, and sequence boundaries must be considered tentative at this time because major disparities exist between the correlation of calcareous plankton zones and mid-Cretaceous chronostratigraphic units used by Haq et al. (1988) and Bralower et al. (submitted).

Mesozoic sea level fluctuations documented on Exmouth Plateau off northwestern Australia

The Exmouth Plateau is uniquely suited to the study of sea level changes because of the existence of an extensive seismic grid and industry well sites, an extended Mesozoic stratigraphic record punctuated with several major unconformities, and the relatively protected position of this high plateau. Thus, documenting sea level fluctuations was one of the major objectives of drilling on the Exmouth Plateau. This documentation depends on their ability to (1) isolate the tectonic overprint from the eustatic signal by retracing the subsidence histories of the drill sites and (2) accurately date the unconformities. Two transects of sites were drilled, one with four sites on the Wombat Plateau and the other with two sites on the central Exmouth Plateau, with one site located relatively proximally and another distally to the source of sediment supply. Preliminary shipboard work indicates that the age of Mesozoic unconformities can be accurately constrained and the subsidence-related tectonic events can be effectively isolated from sea level fluctuations. Sequence stratigraphic analysis of seismic, lithofacies, biofacies, and well-log data document important Upper Triassic sequence boundaries on the Wombat Plateau between the middle and upper Carnian (Norian-Rhaetian boundary) and in the upper most Rhaetian, whose timing and relative magnitude conformmorexa0» well with the eustatic cycle chart. The sequence boundary and systems tracts recognized in the central Exmouth Plateau Barrow Group equivalent strata (Berriasian-Valanginian) also correspond well with the global cycle chart. These preliminary results are of considerable importance in providing a test of the validity of the eustatic model.«xa0less