Global Eustatic Events Research Articles

Coombs Hill: A Late Devonian fossil locality in the Witpoort Formation (Witteberg Group, South Africa)

Coombs Hill, a new fossil locality in the Witpoort Formation (Witteberg Group) of South Africa, preserves a record of Famennian (Late Devonian) life in Gondwana. Fossil plants collected at Coombs Hill are preliminarily assigned to several classes. Shelly invertebrates include a variety of bivalve mollusc forms, some of which appear to be preserved in life position. Biodiversity at Coombs Hill is comparable to that of the well-known Waterloo Farm lagerstätte in ordinal diversity, but exhibits differences in species composition. Ongoing taxonomic analysis will provide a rare window into the ecology of high-latitude environments during this pivotal stage of Earth history, which immediately preceded the end-Devonian extinction. Sandstone dominated sedimentary facies at Coombs Hill suggest a high-energy coastal marine setting, with brackish back-barrier estuarine/lagoonally derived fossiliferous mudstones. Exact stratigraphic placement within the Witpoort Formation is hampered by structural deformation, and precise age comparisons with Waterloo Farm are currently tenuous.Significance:
 
 A new fossil locality at Coombs Hill comprises the second known site with a suite of well preserved continental and marginal marine fossils from the Witpoort Formation, providing an exceptionally rare example of high-latitude life during the critical latest Devonian Famennian age.
 Several new plant taxa will be diagnosed from this locality, which also gives important insights into the morphology of Archaeopteris notosaria, South Africa’s earliest known tree.
 Discovery of a second palaeontologically significant site in the Witpoort Formation provides impetus for further structural and sedimentary facies analyses to align the unit with datable global eustatic events, and to clarify its internal chronology.

Eustatic and local tectonic impact on the Late Ordovician – early Silurian facies evolution on the SW margin of peri-Baltica (the southern Holy Cross Mountains, Poland)

AbstractThis paper provides insight into the Late Ordovician to earliest Silurian evolution of sedimentary environments in the southern Holy Cross Mountains (SE Poland), which at that time were a part of the SW periphery of Baltica. The facies layout in this area was influenced by the basement block faulting, which differentiated the basin bathymetry into submarine horst and grabens, controlling facies distribution. However, the local tectonism was insufficient to fully mask the global eustatic events. Therefore, it is possible to correlate some facies changes in the Upper Ordovician and lower Llandovery sedimentary record of the southern Holy Cross Mountains with eustatic and palaeoceanographic changes reported worldwide. The most noticeable influence of eustasy on the sedimentary record in the studied area occurs at the Ordovician/Silurian boundary. It is manifested by Hirnantian regressive coarse-grained clastic sediments overlain by a post-glacial anoxic/dysoxic interval represented by the Rhuddanian transgressive black cherts and shales. It is noteworthy that the pre- and post-Hirnantian sedimentary environments in the southern Holy Cross Mountains were affected by upwelling induced by the SE trade winds.

Отражение глобальных событий франского века в разрезе западного склона Полярного Урала

For the first time we carried out a detailed comprehensive study of Frasnian stage in the lower сurrent of the Malaya Usa River located within the Eletskaya structural-formational zone of the Polar Urals. The Middle-Late Frasnian age of these rocks was determined by conodonts in the sequence from hassi-jamieae to Late rhenana of standard conodont zones. We analyzed the distribution of conodont biofacies in the section. Polygnathus-Ancyrodella biofacies is most developed in the interval of the hassi-jamieae zones. We found increasing biodiversity of conodonts and prevalence of deep-water Palmatolepis biofacies in the interval of the Early rhenana-Late rhenana subzones. We studied the material composition and secondary changes in the carbonate rocks of the section, which allowed reconstructing primary sedimentary structures and sedimentation environments. In general, the deposits were formed within a narrow intra-shelf depression, to which intra-basin clastic material actively entered. At some intervals of time, more deep-water clay-carbonate sedimentation conditions, which were associated with the global eustatic events at the jamieae-Early rhenana (Semichatovae transgression) and Early rhenana-Late rhenana (Lower Kellwasser transgression) are noted. The analysis of isotope-geochemical data presented that the early stage of the Semichatovae transgression corresponded to a slight shift in the isotope ratio of carbon from 1.5 to 1.8‰, after which the isotope composition lowered to 0.4‰. The subsequent variation changes from 0.8 to 1.7‰ gradually decreased, reaching the value of 0.9-1.0‰. At the level of the Lower Kellwasser transgression, a slight positive shift in the isotope ratio of carbon from 0.1 to 0.7 with subsequent variations in the range of 0.5-0.7‰ is noted. Similar changes in the isotope composition of carbon at the event levels of Semichatovae and Lower Kellwasser are observed world-wide and mostly developed in the sections of Central and Southern Europe, North America and North Africa.

Palynostratigraphy of the Devonian–Carboniferous transition in the Tulong section in South Tibet: A Hangenberg Event sequence analogue in the Himalaya-Tethys zone

Palynological assemblages from the Tulong succession in South Tibet enabled the recognition of three miospore biozones within the Zhangdong and ‘Yali’ formations. In ascending order, they are the Retispora lepidophyta–Verrucosisporites nitidus (LN) Biozone, the Vallatisporites vallatus–Foveosporites pellucidus (VP) Biozone and the Rugospora polyptycha–Tricidarisporites arcuatus (PA) Biozone. The LN Biozone in the Tulong section can be correlated to the LN Biozone of Western Europe, based on the first and last appearances of many index miospore species. However, Tournaisian palynological assemblages of Western Europe and South Tibet show compositional differences. The VP and PA biozones of South Tibet are correlated tentatively to the VI–HD and BP–TC biozones of Western Europe based on the occurrences of rare miospore index species in both regions. The Zhangdong and ‘Yali’ formations are now dated as Famennian to possibly earliest Visean. This age assessment is supported by rare conodonts and ammonoids recovered from intercalated limestones in the basal part of ‘Yali’ Formation. In the Everest region, the Devonian–Carboniferous boundary is now placed c. 10 m above the lower boundary of the ‘Yali’ Formation, which was originally suggested by Lin and Qiu (1985) to be position of the D–C boundary. Unlike western Gondwana, i.e. Bolivia and Brazil, the Himalaya-Tethys zone at the northeastern margin of Gondwana does not show obvious lithological evidence for glaciations. The new palynostratigraphic data, including information on sea-level changes reflected by variations in palynological composition and lithology around the Devonian–Carboniferous boundary allow, an interregional, approximate correlation with the classical Hangenberg Event sequence in the Rhenish Massif. The analogous Hangenberg Event sequence in South Tibet further supports the hypothesis that the rapid Hangenberg Event regression/transgression was a global eustatic event of significant magnitude. The co-occurrence of probably reworked Frasnian Geminospora lemurata and Archaeoperisaccus spores in the Himalaya-Tethys zone indicates the palaeogeographical proximity of the northern margin of Gondwana to the South China Block in the palaeoequatorial belt.

Stratigraphical and sedimentary characters of Late Cretaceous formations outcropping in central and southern Tunisia, Tethyan southern margin

The main goals of our approach are to identify some local to global events in relation with tectonic instabilities and/or sea-level changes, occurring during the deposition of Cenomanian-Coniacian carbonate series in Tunisia. Several sections surveyed in Central-Southern Tunisia, along a North-South transect extending from Sidi Bouzid to Gafsa area, show that the Cenomanian-Coniacian series include rudist-rich facies associated to other shallow marine to deeper deposits.Detailed sedimentological studies supported by new biostratigraphical data (provided by H. Bismuth, oral comm.), have allowed to add more precisions on the lithostratigraphical stacking and thus on the Central Tunisia Stratigraphic Chart. Some carbonate members such as the Middle Turonian Bireno and the Late Turonian-Coniacian Douleb have been identified in certain localities for the first time. Indeed, these members were never described before at Jebel el Kébar and Jebel Meloussi.In the Sidi Bouzid area, especially at Jebel el Kébar, the Cenomanian-Coniacian carbonate members are characterized by frequent and rapid changes, related to the existence of highs (horsts, probably) and depressed depositional domains (grabens, probably), which formed during the deposition of the two lower Units of the Middle Turonian Bireno Member. Above, the Late Turonian to Coniacian deposits, have tended to seal the irregular paleotopography affected, at least locally, by Middle Turonian extensional tectonic movements. They could be related, in contrast, to a drowning linked to a sea level rise.Similar events were described abroad during Late Turonian times; a partial drowning of carbonate platforms was already identified in other localities of the African Tethyan margin.However, the global drowning corresponding to the C/T event was not identified in the present study, although previous works have described this event North of the studied sector. As demonstrated in other localities, a global eustatic event could be locally interrupted by tectonic events, which could mask the eustatic message and leave their record in the deposit cycle.

Late Quaternary sea-level changes of the Persian Gulf

Late Quaternary reflooding of the Persian Gulf climaxed with the mid-Holocene highstand previously variously dated between 6 and 3.4ka. Examination of the stratigraphic and paleoenvironmental context of a mid-Holocene whale beaching allows us to accurately constrain the timing of the transgressive, highstand and regressive phases of the mid- to late Holocene sea-level highstand in the Persian Gulf. Mid-Holocene transgression of the Gulf surpassed today's sea level by 7100–6890calyr BP, attaining a highstand of >1m above current sea level shortly after 5290–4570calyr BP before falling back to current levels by 1440–1170calyr BP. The cetacean beached into an intertidal hardground pond during the transgressive phase (5300–4960calyr BP) with continued transgression interring the skeleton in shallow-subtidal sediments. Subsequent relative sea-level fall produced a forced regression with consequent progradation of the coastal system. These new ages refine previously reported timings for the mid- to late Holocene sea-level highstand published for other regions. By so doing, they allow us to constrain the timing of this correlatable global eustatic event more accurately.

Small-scale cyclic deposition in the Frasnian (Upper Devonian) of the Holy Cross Mountains, Poland

Abstract In sections exposing Frasnian limestones at five outcrops in the Holy Cross Mountains, five lithofacies (L1 to L5) that represent upper slope to basinal environments are identified. These lithofacies are characterised by dark-coloured micritic limestones-marly shale couplets with many light-coloured intercalations of fine- to coarse-grained limestones (= event beds). This lithofacies pattern characterises mostly low-energy domains punctuated by storm episodes. In addition, these upper-slope to basinal lithofacies are arranged into small-scale, coarsening-upward beds and cycles. The cycles are locally composed of fining/thinning-upward beds. The small-scale cycles have a calculated duration of 19 to 42 kyr. The differential thickness of beds and cycles within and between sections was probably caused by differential subsidence and local tectonics. Possible evidence of tectonic activity is also related to a difference in number of cycles recorded in the time-equivalent sections. The recognised cyclicity shows sea-level fluctuations and a few deepening episodes. Some of them are correlated with the Timan global eustatic events. However, local tectonics and episodic subsidence may have played a significant role in recording brief deepening pulses. Thus, low-amplitude sea-level changes were major factors in platform generation and evolution in the Frasnian of the Holy Cross Mountains modified by local, block-related subsidence.

Incised valley fill interpretation for Mississippian Black Hand Sandstone, Appalachian Basin, USA: Implications for glacial eustasy at Kinderhookian–Osagean (Tn2–Tn3) boundary

Lower Mississippian strata of east-central Ohio are predominantly fine-grained marine deposits of the Cuyahoga and Logan formations. Within these sediments is the Black Hand Sandstone of the Cuyahoga Formation. The Black Hand Sandstone is a multistory, crossbedded, coarse-grained conglomeratic sandstone. The contact between the Black Hand Sandstone and the subjacent Cuyahoga Formation is sharp and scoured, with intraclasts of the Cuyahoga Formation incorporated into the basal Black Hand Sandstone. The Black Hand Sandstone was previously thought to represent a distributary channel deposit; however, the combination of lithofacies and architectural elements indicates deposition in a braided stream setting. The Cuyahoga Formation was deposited in a shallow marine setting. The erosional basal contact of the Black Hand Sandstone and the juxtaposition of fluvial and marine sediments suggests a sequence boundary. The geographic distribution of the Black Hand Sandstone combined with the evidence for a sequence boundary suggests deposition in an incised valley. The age of the Black Hand Sandstone is key to inferring the causes of valley incision. The Black Hand Sandstone is nearly devoid of body fossils, necessitating a biostratigraphic analysis of the surrounding Cuyahoga and Logan formations. Analysis indicates the Logan Formation is early Osagean age. Data from the Cuyahoga Formation suggest a Kinderhookian age with a possible transition to the Osagean in the uppermost Cuyahoga Formation. This constrains the age of the Black Hand Sandstone to the transition at the Kinderhookian–Osagean boundary. Recent reports indicate late Kinderhookian (Tournaisian, Tn2) Gondwanan glaciation based upon tillites and sharp excursions in stable-isotope curves. A glacio-eustatic fall in sea level is inferred to have caused incision of the Cuyahoga Formation, followed by deposition of the Black Hand Sandstone and Logan Formation during the subsequent sea level rise. The associated unconformity correlates to the sequence boundary at the Kinderhookian–Osagean boundary in the stratotype area of North America, and the correlative Tn2–Tn3 boundary worldwide, supporting the hypothesis of a global eustatic event at this time.

Corrélation des successions de l'Albien par les ammonites entre la Province Nord-Pacifique et les Provinces européenne et arctique du Domaine boréal : zonation, eustatisme et paléobiogéographie

The vertical distribution of Albian ammonites of the North Pacific Province is described on the basis of several sections located in Northern California. Nine Albian ammonite assemblages are identified, compared to the 4 previously described by Murphy in 1956. Owing to several gaps of observation possibly more assemblages could exist. In the North California Albian history, the first results show a succession of confinement periods – with numerous endemic faunas – followed by periods of open communications with other faunal provinces. As an example, the base of the Middle Albian is marked by exotic ammonites coming from the Tethyan realm ( Oxytropidoceras, Lyelliceras) and from the Arctic Province of the Boreal realm ( Gastroplites, Pseudopulchellia). These ammonites – except Oxytropidoceras – were not known in that area and are described relatively to their palaeobiogeographical interest. Although rare these migrant faunas give valuable elements to correlate these various provinces. Planktonic foraminifera confirm the existence of open sea communications during the uppermost Albian. With such faunal links, a comparison is proposed between the Albian ammonite zonation of Northern California and the standard and phyletic ones of Europe. This attempt suggests that the major connections between the three faunal provinces are established at second order peak transgressions in the Early and Middle Albian, and during the sea-level high of the Late Albian transgressive period. So, it seems that the vertical distribution of Albian non-endemic ammonites of North California and Europe is largely controlled by global eustatic events. Two palaeobiogeographic maps for Early and Middle Albian and one map for the late Late Albian show the migration outer of these exotic ammonites.

Evolution of the early Paleozoic Cordilleran margin of Laurentia: tectonic and eustatic events interpreted from sequence stratigraphy and conodont community patterns

Several field seasons in the Canadian Cordillera have allowed the measurement, description and sampling of over 20 000 m of lower Paleozoic strata from 26 stratigraphic sections across four platform-to-basin transects, with the recovery of over 100 000 conodonts from more than 1200 4–5 kg samples. This work was part of the Lithoprobe Slave – Northern Cordillera Lithospheric Evolution (SNORCLE) project but is also being extended through a Pan-Lithoprobe project to understand the tectonic and eustatic response of much of the Laurentian plate through the early Paleozoic. Based on the abundant field data, the complex stratigraphic framework is interpreted in terms of sequence stratigraphy and a derived relative sea-level curve. Using detailed conodont taxonomic and biostratigraphic results, cluster analysis of conodont distributional data identified an evolving series of conodont communities through space and time. These communities were partitioned across the platform-to-basin gradient and provide an additional sensitive indicator of relative sea-level change. These two independent approaches generated comparable eustatic curves for this Cordilleran Laurentian margin during much of the early Paleozoic and identified some global eustatic events noted by earlier workers. This part of Laurentia was not a simple passive margin during the early Paleozoic, but rather was affected by four main tectonic events complicated by six principal eustatic changes. Some success was achieved in filtering the global and regional tectonic–eustatic effects and in proposing causes for some of the events.

The Cenomanian-Turonian transition on the Peruvian margin

In Peru, various deposittonal sequences can be recognized during the late middle Cenomanian to early Coniacian timespan, most of which seem to have an eustatic origin. For instance, the early late Cenomanian, late early Turonian and early Coniacian maximum floodings identified in Peru coincide with well-known global eustatic events. The Mammites nodosoides-bearing marls probably correspond to the major Cretaceous marine transgression on the Peruvian margin. However, tectonic movements may have occurred during late Cenomanian times. The widespread hiatus of lalest Cenomanian to earliest Turonian age, probably owing to a significant lowering of sea level, may have been amplified by tectonically-induced erosion. Restricted to disaerobic conditions reach a maximum close to the Cenomanian-Turonian boundary and coincide with the well-known oceanic anoxic event. However, anoxia seems to have been less important than elsewhere, probably because of better oceanic circulation. Large benthonic foraminifera are represented by Perouvianella peruviana, Archaecyclus sp. and species of Sellialveolina. The remarkable persistence of P. peruviana through the Cenomanian-Turonian boundary crisis could be related to the less drastic anoxic conditions, responsible for continuous carbonate shelf sedimentation.

Sea-level changes in a back-arc-foreland transition: the late Carboniferous-Permian Karoo Basin of South Africa

The late Carboniferous to early Permian succession in the Karoo Basin consists in ascending stratigraphic order of diamictites, mudrocks, greywackes and immature sandstones. The diamictites are interpreted as glacigene deposits laid down by a marine ice sheet on a shelf. Post-glacial suspension-settling of predominantly mud occurred on the basin floor during open marine and later brackish conditions. Basin floor fan deposits accumulated in the foredeep, whereas deltaic mud, silt and sand finally infilled the basin. Glacigene sedimentation occurred in a back-arc basin during the late Carboniferous and early Permian. The post-glacial succession accumulated in a foreland basin in which the rising orogenic belt was the predominant sediment source. Relative sea-levels, following the major Namurian regression, apparently remained low during the extensive Gondwana glaciation. Marine ice sheets are highly sensitive to sea-level fluctuations: and minor sea-level rises, probably climatically controlled, were recorded as interglacials when parts of the marine ice sheet collapsed. Final collapse of the marine ice sheet was associated with a major transgression. Post-glacial mudrock deposition was characterised by relatively high sea-levels, except for the period of basin floor fan deposition when there was a sea-level lowstand. Final infilling of the basin by prograding deltas was accompanied by a major regression. The late Carboniferous to early Permian sea-level fluctuations in the Karoo Basin show correspondence with global eustatic cycles reflecting low tectonic interference with sea-level patterns. However, late Permian sea-levels were predominantly tectonically controlled as a result of foreland basin evolution. Possible global eustatic events complemented apparent sea-level changes within the basin during this period.

Paleocene reservoirs of the Everest trend

The Everest trend of stratigraphically-trapped Paleocene gas accumulations was first recognized by the Amoco Group in the late 1970s and tested by the 22/10a-2 well in 1981. The well discovered gas–condensate pay in submarine fan sandstones of the Maureen and Andrew formations of the Montrose Group. These sandstones are absent in the 22/10-1 well, some 8 km to the east, where the entire Paleocene section above the Ekofisk Formation consists of shales of the Rogaland Group. A large stratigraphic trap was mapped, relying on up-dip shale-out to the east, and dip drape over a structural nose in the underlying Jaeren High basement block. No sandstones were found in the age-equivalent section of the Forties Formation in the 22/10a-2 well and so a further, down-dip, stratigraphic accumulation was predicted and subsequently successfully tested by the 22/10a-4 well in 1984. These discoveries are now named the Everest Complex. Simultaneously, the play concept was proven to the north by the Sleipner Øst discovery in 1981, where shale-out of the sandstone occurs over the Ling High. A third, similar, accumulation between these two fields was discovered in 1982 by Well 22/5b-2, which forms part of the Fleming accumulation. Reservoirs, which consist of poor to moderately sorted quartz-rich arenites, exhibit a wide variety of sedimentary facies produced by turbiditic and gravity flow processes. The distribution and form of the sandbodies is controlled by a combination of underlying palaeotopography, regional tectonism and global eustatic events. Correlation of reservoir units is accomplished by integration of wireline logs, detailed biostratigraphy and seismic facies analysis The commercial importance of the trend is reflected in the reserves proven to date, which are in excess of 4 trillion cubic feet (TCF) of wet gas.

Sequence stratigraphy and eustatic sea-level change: the role of micropalaeontology

Abstract. Following the May 1992 meeting in Dijon, which initiated an international project on the “Sequence Stratigraphy of European Basins”, it seems an appropriate time to consider the contribution micropalaeontology can make to the science of sequence stratigraphy. In this short note, we assume that readers are familiar with sequence stratigraphic terminology; if not, see Van Wagoner et al. (1988).WHAT ARE THE CHALLENGES FACING SEQUENCE STRATIGRAPHY?Demonstrating global eustatic sea-level change. We accept that the basic sequence stratigraphy model put forward by Peter Vail and his colleagues (see Van Wagoner et al., 1988 for a summary) is a powerful tool for describing many sedimentary successions, and that the associated eustatic sea-level curve (Haq et al., 1987) has some validity. Our own observations on numerous sedimentary sequences around the world suggest that local and global eustatic events exist, and that relative sea-level curves can be constructed, but it should be remembered that the timing and magnitude of many global eustatic events are still to be established. As most workers in the field will be aware, much of the evidence to support the Haq et al. curve has not been published. The Sequence Stratigraphy of European Basins Project will go some way to rectify this, but it should be borne in mind that there can be an unfortunate tendency to use the Haq et al. curve for dating in its own right - i.e. fitting relative sea-level changes seen in a succession to the curve. If this is done, then the global . . .

The late Devonian trilobite crises

Mid‐Devonian to end‐Late Devonian trilobites of different taxonomic categories are updated as to their actual stratigraphical range with respect to the internationally defined stage boundaries. The main palaeogeographical and ecological occurrences are summarized. Numerical analyses emphasize the clear relationship between fluctuations in diversity and global eustatic events. Already declining in diversity from the early mid‐Devonian, shallow‐water communities became most restricted during the mid‐Givetian Taghanic transgression. After a phase of adaptive radiation, off‐shore trilobite communities were severely affected during the mid‐ and end‐Late Devonian crises. From an initial 5 orders 3 were lost at the end‐Frasnian Kellwasser crisis while only 1 from the remaining 2 orders survived the Devonian‐Carboniferous boundary Hangenberg event. In both cases extinction was preceded by a unidirectional evolutionary trend in eye reduction accompanied by impoverishment of lower rank taxa. This phenomenon is obviously a result of selective adaptation under constant long‐lasting environmental influences. Specialization to obligate epi‐ or even endo‐benthic life habit, however, led fatally to extinction when stable conditions became substantially perturbed. Sudden sea‐level changes with subsequent break in the REDOX‐equilibrium took place at the Kellwasser and Hangenberg events, which were most probably responsible for trilobite mass extinctions.

Tectonically Controlled Sedimentation in the Mesozoic Basins of the Antarctic Peninsula: ABSTRACT

The Antarctica Peninsula (AP) lies on a medium-size block of continental crust and is one of a mosaic of west Antarctica coastal block that underwent a complex tectonic evolution during Gondwana breakup. The peninsula represents the eroded roots of a microcontinental volcanic arc; this arc lay above the easterly subducting proto-Pacific plate, and was active throughout the Mesozoic. The exposed Mesozoic basins display a complex stratigraphy, reflecting local tectonic and volcanic events. There are a few general trends. Almost all basins are post-late Oxfordian, their fill is entirely clastic, and is largely derived from the Antarctica peninsula volcanic arc. Most basins were affected by a period of arc expansion in the latest Jurassic or earliest Cretaceous, which manifests itself as inputs of lava or coarse volcaniclastic sediment overlying mudstones with an open marine fauna. Barriasian and older mudstones are generally finer grained and darker than mudstones from post-Berriasian strata. However, it must be emphasized that these are only general trends. Deformation is variable, commonly progressive. No lithostratigraphic units can be correlated between any two basins, nor are there any interregional unconformities. No matching is possible with basins of equivalent age in formerly contiguous areas of Gondwana. There is evidence thatmore » some global eustatic events are recorded in the sedimentary records of at least two of the AP Mesozoic basins, but these have effect only in periods of local tectonic quiescence. The dominant control on sedimentation in this large segment of the Pacific rim was arc tectonics.« less

The Nature and Significance of Condensed Sections in Gulf Coast Late Neogene Sequence Stratigraphy: ABSTRACT

ABSTRACT Because it lends itself to predictable interpretations, sequence stratigraphy is gaining acceptance as a unifying concept in stratigraphy in general and in exploration in particular. Any regional seismic stratigraphic study must be predicated on a sound chronostratigraphic framework. Among the components of stratigraphic sequences and depositional systems, the condensed section is probably the most fundamental stratigraphic unit. Condensed sections are relatively thin, highly fossiliferous hemi-pelagic beds that form at the time of maximum transgression. Most reliable biostratigraphic datums occur within them, and, being regionally widespread, they are excellent chronostratigraphic markers. Since they are the only consistent tangible stratigraphic unit which ties deep water and shelf environments, they are of paramount importance in delineating sequence boundaries and systems tracts. This paper discusses various criteria for recognizing condensed sections, and documents 20 seismically resolvable condensed sections in the pre-Wisconsin Plio-Pleistocene to uppermost Miocene of the Gulf Coast. They are recognized by abundance and species diversity peaks, and are calibrated in terms of the foraminiferal and calcareous nannofossil datums within them. All of these condensed sections are associated with an increase in paleo-water depth. As indicated by the chronostratigraphic control provided by planktonic datums, they are not random events and can be shown to be regionally correlative. The scheme presented here has been developed in conjunction with seismic sequence studies and has been integrated with regional seismic grids. Sixteen of these condensed sections are maximum flooding events; four of them have been found to develop between lowstand slope fans and lowstand prograding wedges. The characteristics of condensed sections in various paleo-environmental settings are discussed, and the Miocene/Pliocene transition of the Gulf Coast is related to global eustatic events.

Batyrbay Section, South Kazakhstan, U.S.S.R. – potential stratotype for the Cambrian–Ordovician Boundary

AbstractThe base of the Cordylodus proavus conodont Zone is the most appropriate horizon for the Cambrian–Ordovician Boundary. At Batyrbay the horizon reflects a significant global eustatic event that is marked by distinctive geochemical changes and by drastic changes in the trilobite and conodont faunas. The palaeomagnetic polarities change from reversal to normal just below this horizon.