Major Hiatus Research Articles

Sequence stratigraphy of the Jurassic of the Danish Central Graben

A sequence stratigraphic framework is established for the Jurassic of the Danish Central Graben based primarily on petrophysical log data, core sedimentology and biostratigraphic data from about 50 wells. Regional seismic lines are used to assist in the correlation of some wells and in the construction of isochore maps. In the Lower Jurassic (Hettangian–Pliensbachian) succession, five sequences have been identified. The Middle Jurassic is subdivided into four sequences that together span the uppermost Aalenian/lowermost Bajocian to the Callovian. In the Upper Jurassic, better well coverage permits greater stratigraphic resolution, and 11 sequences are identified and mapped. On the basis of the sequence stratigraphic correlation and the construction of isochore maps for individual sequences, the Jurassic basin history of the Danish Central Graben can be subdivided into seven discrete phases: (1) Shallow marine and offshore sediments deposited in a prerift basin extending from the North Sea to the Fennoscandian Border Zone (Hettangian–Pliensbachian). (2) Uplift and erosion in association with a Toarcian–Aalenian North Sea doming event. A major hiatus represents this phase in the study area. (3) Terrestrial and marginal marine sedimentation during initial rifting (latest Aalenian/earliest Bajocian – Late Callovian). (4) Early Oxfordian – Early Kimmeridgian transgression during and after a rift pulse. The sedimentary environment changed from coastal plain and marginal marine to fully marine. (5) Regression associated with a cessation or slowing of subsidence during a structural rearrangement that took place in the Late Kimmeridgian during a break in the main rift climax. Shallow to marginal marine sandstones were deposited above an erosion surface of regional extent. (6) Deep-water mudstones deposited in a composite graben with high subsidence rates related to rift pulses (latest Late Kimmeridgian – middle Middle Volgian). (7) Deposition of organic-rich mudstones and turbidite sandstones during the late Middle Volgian – Early Ryazanian. The main basin shallowed, became more symmetrical and experienced a decreasing rate of subsidence, recording the onset of the post-rift stage. A relative sea-level curve is constructed for the Middle–Late Jurassic. It shows close similarity to published eustatic (global) and relative (North Atlantic area) sea-level curves in the latest Bathonian – late Early Kimmeridgian, but differs in the Late Kimmeridgian – Middle Volgian interval, probably due to the high rate of subsidence in the study area.

Late Paleozoic depositional history of the Tarim basin, northwest China: An integration of biostratigraphic and lithostratigraphic constraints

This study provides the first detailed lithostratigraphic and biostratigraphic constraints for improving stratigraphic resolution for hydrocarbon prospecting and exploration in the Tarim basin. A total of 49 stratigraphic units (38 formations and 11 members), ranging in age from the latest Devonian to Permian, are reviewed or redefined in terms of nomenclatures, lithology, age constraints, and lateral distributions based on the detailed field works or newly published data. Of these, the Piqiang Formation (new formation) is proposed to include the reefal carbonates of Asselian–Sakmarian age from the northern Tarim. The subsurface upper Paleozoic stratigraphic framework of the desert areas of the basin is also established for the first time. The high-resolution, basinwide stratigraphic correlations reveal that the sedimentation of the basin in the late Paleozoic was extremely uneven. Of these, the Famennian to Changhsingian successions are completely recorded in the southwestern margin areas of the basin. Here, five eustatic sedimentary cycles are well recognizable, suggesting the sedimentation was more eustatically controlled and little affected by local tectonism. The late Paleozoic successions of both Kalpin and Taklimakan regions are commonly interrupted by major hiatuses at various horizons, suggesting that the sedimentation was apparently modified by local tectonism. Of these, the northward movement of the Tarim block and its subsequent collision with the Yili microcontinent (part of the Kazakhstan plate) may be principally accountable for the discrepancy in the sedimentation of the various regions in the basin in the late Paleozoic.

FORAMINIFERAL TIMING OF CARBONATE DEPOSITION ON THE LATE DEVONIAN (FAMENNIAN)-MIDDLE PENNSYLVANIAN (BASHKIRIAN) TENGIZ PLATFORM, KAZAKHSTAN

Calcareous foraminifers provide a time-stratigraphic framework to chronicle the development of the Tengiz carbonate platform that thrived from the latest Devonian (late Famennian) into the Middle Pennsylvanian (late Bashkirian). Correlative zones, based on documented foraminiferal assemblages and expressed primarily in terms of Russian horizons, confirm that the platform grew in a complex pattern of progradation and backstepping from the Tournaisian to late Visean and then underwent a major progradation that was terminated by sea level fall at the end of the Serpukhovian. Favorable conditions for carbonate sedimentation returned in the early Bashkirian and persisted into the late Bashkirian until the platform either was exposed and eroded or buried by siliciclastic deposition. Breaks in the foraminiferal succession point to major depositional hiatuses along the top of the Bashkirian platform, at the Mississippian-Pennsylvanian boundary, and possibly in the late Tournaisian although previous investigations support continuous deposition throughout the latter interval at Tengiz.

A new scenario for the Domerian - Toarcian transition

Abstract In contrast to the majority of recently published hypotheses, we believe that the main trigger for early Toarcian anoxia is neither increased primary productivity during the Tenuicostatum and Falciferum Zones nor sudden methane hydrate degassing close to the transition between these two zones. In our opinion, this peculiar paleoceanographic episode is linked to a major, though short-lived, regression at the end of Upper Domerian. Sea-level fall resulted from sudden cooling due to increased volcanic activity. This generated global thermal insulation and subsequent glaciation. The regression is responsible for a major hiatus over NW-European epicontinental seas and is later followed by the well-known Lower Toarcian transgression. The interval corresponding to this hiatus allowed vegetation to colonise vast newly emerged surfaces. The leaching and drowning of the accumulated organohumic matter then triggered the anoxic cycle at the transgressive maximum, concomitant with a global warming.

Magnetostratigraphy of the Plio‐Pleistocene carbonate section of the Great Australian Bight

ODP Leg 182 drilled two north‐south transects in the western Great Australian Bight close to longitude 128°E. These transects penetrated a remarkably thick section of uppermost Pliocene and Pleistocene carbonate sediments, which are separated from Miocene sections by a major hiatus. In the eastern transect, at sites 1129, 1131 and 1127, the Brunhes‐Matuyama boundary (onset of C1n) was found at 343 m, between 280 and 300 m, and at 343 m below the sea floor, respectively. In the western transect, at sites 1132 and 1130, it was found between 170 and 181 m, and at 200 m, respectively. Within the Brunhes chron, inclination and intensity fluctuations and correlations between susceptibility and standard δ18O records were used to give age‐depth relations. These age relations from the palaeomagnetics are broadly consistent with the biostratigraphy and the δ18O results from Leg 182, but do not provide an entirely independent dataset. Rock magnetism stratigraphy at site 1131 revealed the principal bryozoan buildup to be at the time of the last glacial lowstand, as suggested by previous workers. At all sites the sedimentation rate increases from the basal unconformity up into the thick Upper Pleistocene section. In the western transect there appears to be a partial record of most of the Late Pliocene and Early Pleistocene chron boundaries (C1r1n to C2An3n) in a condensed section, but in the eastern transect only the Jaramillo (C1r1n) is observed.

Luminescence chronology of loess-paleosol sequences from southern South Island, New Zealand

The loess-paleosol sequences in New Zealand are among the thickest in the southern hemisphere, and preserve an important terrestrial record of paleoclimatic changes. Unlike loess-paleosol sequences in North Island, most of those in South Island lack discrete chronostratigraphic marker ash beds, excepting perhaps the widespread ∼24 ka Kawakawa Tephra. Therefore, any specific correlation to marine isotope stage (MIS) proxy records or to loess-paleosol sequences in the northern hemisphere has been unknown. Hence, we applied luminescence sediment dating to four key loess-paleosol sequences from southern South Island. Although these pioneering results suggest that further sampling and luminescence dating are needed, they do provide: (1) the first numeric age constraint (44±3 ka on overlying loess) for the end of deposition of the geomorphically and paleoclimatically important Edendale Terrace gravels; (2) the first numeric evidence that the youngest loess-paleosol unit (L1) at the near-coastal Romahapa and inland Stewarts Claim sites began to be preserved much earlier (at ∼60 ka) than at the inland Kingston Crossing site (∼35 ka); (3) the first direct evidence that the 6 m Romahapa sequence extends at least to ∼350 ka, the oldest yet TL-dated such sequence in South Island. Furthermore, the stratigraphically self-consistent (with one exception) luminescence ages between ∼60 and ∼350 ka conflict with a previous indirect chronostratigraphy inferred from trace concentrations of glass shards. This trace-shard chronology implies a major hiatus in loess-paleosol preservation (missing at least MIS 6–7) not recognized by luminescence dating.

Maastrichtian to Paleocene depositional environment of the Dakhla Formation, Western Desert, Egypt: sedimentology, mineralogy, and integrated micro- and macrofossil biostratigraphies

Integrated sedimentology, mineralogy, geochemistry, and microfossil and macrofossil biostratigraphies of the Maastrichtian–early Paleocene Dakhla Formation of the Western Desert, Egypt, provide improved age resolution, information on the cyclic nature of sediment deposition, and the reconstruction of depositional environments. Age control based on integrated biostratigraphies of planktic foraminifera, calcareous nannofossils and macrofossils yields the following ages for stratigraphic and lithologic sequences. The contact between the Duwi and Dakhla formations marks the Campanian/Maastrichtian boundary (zone CF8a/b boundary) and is dated at about 71Ma. The age of the Dakhla Formation is estimated to span from 71Ma at the base to about 63Ma at the top (zones CF8a–Plc). The Cretaceous/Tertiary (K/T) boundary is within the upper unit of the Kharga Shale Member and marked by a hiatus that spans from 64.5Ma in the lower Paleocene (base Plc) to at least 65.5Ma (base CF2, base M. prinsii zones) in the upper Maastrichtian at Gebel Gifata, the type locality of the Dakhla Formation. As a result, the Bir Abu Minqar horizon, deposited between about 64.2 and 64.5Ma (Plc(l) zone), directly overlies the K/T boundary hiatus. Major hiatuses also span the late Maastrichtian–early Paleocene in sections to the northwest (c.61.2–65.5Ma at North El Qasr, c.61.2–69Ma at Bir Abu Minqar and c.61.2–65.5Ma at Farafra), and reflect increased tectonic activity.During the Maastrichtian–early Paleocene a shallow sea covered the Western Desert of Egypt and the clastic sediment source was derived primarily from tectonic activity of the Gilf El Kebir spur to the southwest of Dakhla and the Bahariya arch. Uplift in the region resulted in major hiatuses in the late Maastrichtian–early Paleocene with increased erosion to the southwest. The area was located near the palaeoequator and experienced warm, wet, tropical to subtropical conditions characterized by low seasonality contrasts and predominantly chemical weathering (high kaolinite and smectite). A change towards perennially more humid conditions with enhanced runoff (increased kaolinite) occurred towards the end of the Maastrichtian and in the early Paleocene with shallow seas fringed by Nypa palm mangroves. Sediment deposition was predominantly cyclic, consisting of alternating sandstone/shale cycles with unfossiliferous shales deposited during sea-level highstands in inner neritic to lagoonal environments characterized by euryhaline, dysaerobic or low oxygen conditions. Fossiliferous calcareous sandstone layers were deposited in well-oxygenated shallow waters during sea-level lowstand periods.

Holocene environmental change and the accumulation–erosion balance of sheltered river-mouth sediments, Göteborg, SW Sweden

Abstract Holocene sedimentological change was investigated in 15 sediment piston cores (250 cm long) from the mouth of the Gota alv River, outside of Goteborg, which is sheltered by an archipelago. The main objective was to interpret recent, natural and human-induced influences upon the accumulation–erosion balance and the sedimentary environment from Holocene sediment units. The five main units are: consolidated glaciomarine clay (Facies gC) lowermost, consolidated estuarine clay (Facies dC), soft sub-recent sediment (Facies omC and Facies C) uppermost and dumped sediment (Facies D) in some parts of the study area. The sediment facies are divided by two major hiatuses: (a) the early Holocene hiatus between Facies gC and dC involves a time gap of at least 7000 years, ending at ca. 4000 BP and (b) the late Holocene hiatus between the consolidated clays (Facies gC and dC) and Facies omC spans 1000 years and ends ca. 50 BP (i.e. 100 years ago). Both hiatuses probably relate to the effects of isostatic land uplift. Surface sediments consist mainly of an organic-matter-rich clay facies, suggesting changes in the trophic state of the estuary during the last 100 years. Together with human-induced increased river erosion and loss of accommodation space in the estuary, this caused the onset of sedimentation in the estuary. The recent sediments are contaminated with heavy metals (Cu, Zn, Pb, Hg), with highest concentrations in the lower part of Facies omC.

From the Great Bahama Bank into the Straits of Florida: A margin architecture controlled by sea-level fluctuations and ocean currents

High-resolution, multichannel seismic data collected across the Great Bahama Bank margin and the adjacent Straits of Florida indicate that the deposition of Neogene–Quaternary strata in this transect are controlled by two sedimentation mechanisms: (1) west-dipping layers of the platform margin, which are a product of sea-level–controlled, platform-derived downslope sedimentation; and (2) east- or north-dipping drift deposits in the basinal areas, which are deposited by ocean currents. These two sediment systems are active simultaneously and interfinger at the toe-of-slope. The prograding system consists of sigmoidal clinoforms that advanced the margin some 25 km into the Straits of Florida. The foresets of the clinoforms are approximately 600 m high with variable slope angles that steepen significantly in the Pleistocene section. The seismic facies of the prograding clinoforms on the slope is characterized by dominant, partly chaotic, cut-and-fill geometries caused by submarine canyons that are oriented downslope. In the basin axis, seismic geometries and facies document deposition from and by currents. Most impressive is an 800-m-thick drift deposit at the confluence of the Santaren Channel and the Straits of Florida. This “Santaren Drift” is slightly asymmetric, thinning to the north. The drift displays a highly coherent seismic facies characterized by a continuous succession of reflections, indicating very regular sedimentation. Leg 166 of the Ocean Drilling Program (ODP) drilled a transect of five deep holes between 2 and 30 km from the modern platform margin and retrieved the sediments from both the slope and basin systems. The Neogene slope sediments consist of peri-platform oozes intercalated with turbidites, whereas the basinal drift deposits consist of more homogeneous, fine-grained carbonates that were deposited without major hiatuses by the Florida Current starting at approximately 12.4 Ma. Sea-level fluctuations, which controlled the carbonate production on Great Bahama Bank by repeated exposure of the platform top, controlled lithologic alternations and hiatuses in sedimentation across the transect. Both sedimentary systems are contained in 17 seismic sequences that were identified in the Neogene–Quaternary section. Seismic sequence boundaries were identified based on geometric unconformities beneath the Great Bahama Bank. All the sequence boundaries could be traced across the entire transect into the Straits of Florida. Biostratigraphic age determinations of seismic reflections indicate that the seismic reflections of sequence boundaries have chronostratigraphic significance across both depositional environments.

From the Great Bahama Bank into the Straits of Florida: A margin architecture controlled by sea-level fluctuations and ocean currents

Research Article| June 01, 2000 From the Great Bahama Bank into the Straits of Florida: A margin architecture controlled by sea-level fluctuations and ocean currents Flavio S. Anselmetti; Flavio S. Anselmetti 1Comparative Sedimentology Laboratory, Rosenstiel School of Marine and Atmospheric Science, Division of Marine Geology and Geophysics, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149, USA Search for other works by this author on: GSW Google Scholar Gregor P. Eberli; Gregor P. Eberli 1Comparative Sedimentology Laboratory, Rosenstiel School of Marine and Atmospheric Science, Division of Marine Geology and Geophysics, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149, USA Search for other works by this author on: GSW Google Scholar Zan-Dong Ding Zan-Dong Ding 1Comparative Sedimentology Laboratory, Rosenstiel School of Marine and Atmospheric Science, Division of Marine Geology and Geophysics, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2000) 112 (6): 829–844. https://doi.org/10.1130/0016-7606(2000)112<829:FTGBBI>2.0.CO;2 Article history received: 05 Oct 1998 rev-recd: 06 Aug 1999 accepted: 30 Aug 1999 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Flavio S. Anselmetti, Gregor P. Eberli, Zan-Dong Ding; From the Great Bahama Bank into the Straits of Florida: A margin architecture controlled by sea-level fluctuations and ocean currents. GSA Bulletin 2000;; 112 (6): 829–844. doi: https://doi.org/10.1130/0016-7606(2000)112<829:FTGBBI>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract High-resolution, multichannel seismic data collected across the Great Bahama Bank margin and the adjacent Straits of Florida indicate that the deposition of Neogene–Quaternary strata in this transect are controlled by two sedimentation mechanisms: (1) west-dipping layers of the platform margin, which are a product of sea-level–controlled, platform-derived downslope sedimentation; and (2) east- or north-dipping drift deposits in the basinal areas, which are deposited by ocean currents. These two sediment systems are active simultaneously and interfinger at the toe-of-slope. The prograding system consists of sigmoidal clinoforms that advanced the margin some 25 km into the Straits of Florida. The foresets of the clinoforms are approximately 600 m high with variable slope angles that steepen significantly in the Pleistocene section. The seismic facies of the prograding clinoforms on the slope is characterized by dominant, partly chaotic, cut-and-fill geometries caused by submarine canyons that are oriented downslope. In the basin axis, seismic geometries and facies document deposition from and by currents. Most impressive is an 800-m-thick drift deposit at the confluence of the Santaren Channel and the Straits of Florida. This “Santaren Drift” is slightly asymmetric, thinning to the north. The drift displays a highly coherent seismic facies characterized by a continuous succession of reflections, indicating very regular sedimentation.Leg 166 of the Ocean Drilling Program (ODP) drilled a transect of five deep holes between 2 and 30 km from the modern platform margin and retrieved the sediments from both the slope and basin systems. The Neogene slope sediments consist of peri-platform oozes intercalated with turbidites, whereas the basinal drift deposits consist of more homogeneous, fine-grained carbonates that were deposited without major hiatuses by the Florida Current starting at approximately 12.4 Ma. Sea-level fluctuations, which controlled the carbonate production on Great Bahama Bank by repeated exposure of the platform top, controlled lithologic alternations and hiatuses in sedimentation across the transect. Both sedimentary systems are contained in 17 seismic sequences that were identified in the Neogene–Quaternary section. Seismic sequence boundaries were identified based on geometric unconformities beneath the Great Bahama Bank. All the sequence boundaries could be traced across the entire transect into the Straits of Florida. Biostratigraphic age determinations of seismic reflections indicate that the seismic reflections of sequence boundaries have chronostratigraphic significance across both depositional environments. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Permian brachiopods from the SelongXishan section, Xizang (Tibet), China Part 1: Stratigraphy, Strophomenida, Productida and Rhynchonellida

Abstract A brachiopod fauna containing some 42 brachiopod species of 30 identified genera and 2 unidentifiable genera is recorded from the Selong Group of the Selong Xishan section in southern Xizang (Tibet). Various species of Strophomenida, Productida and Rhynchonellida from this fauna are described and revised in this paper. New taxa include Neochonetes (Sommeriella) regularis nov. sp., Taeniothaerus densipustulatus nov. sp., Krotovia inflata nov. sp., Retimarginifera xizangensis nov. sp., Costatumulus tazawai nov. sp. and Neopsilocamara laevis nov. gen. et sp. Brachiopods from the Selong Group and the basal 0.07 m of the Kangshare Formation indicate an age ranging from Capitanian to Changhsingian in terms of the lates three-fold Permian timescale. Three brachiopod assemblages at the Selong Xishan section are recognised. The Marginalosia-Composita Assemblage in the lower part of the Selong Group is considered to be of Capitanian to Wuchiapingian age. The brachiopods in the Chonetella nasuta Assemblage suggest a Wuchiapingian to possibly early Changhsingian correlation. The Martinia-Waagenites Assemblage in the basal part of the Kangshare Formation is late Changhsingian in age. The brachiopod succession of the Selong Xishan section indicates that there is no major hiatus present within the Selong Group and the Permian-Triassic boundary beds.

Upper Cambrian carbonate platform evolution, Elvinia and Taenicephalus zones (pterocephaliid-ptychaspid biomere boundary), northwestern Wyoming

ABSTRACT The Pterocephaliid-Ptychaspid biomere boundary is an Upper Cambrian extinction horizon that can be used to correlate mixed carbonate-siliciclastic strata in northwestern Wyoming. The biomere boundary is defined by the change between the Elvinia and Taenicephalus Zones of the standard North American trilobite zonation. It is also signaled chemostratigraphically by a positive trend in carbon-isotope ratios. Thus, both biostratigraphic horizons and carbon-isotope data are used to constrain the position of the biomere boundary in northwestern Wyoming. Study of 21 stratigraphic sections reveals the presence of distinct facies belts for the middle and upper Elvinia Zone and lower Taenicephalus Zone. These are interpreted to reflect deposition in three distinct paleoenvironments: (1) oolitic shoal, (2) intrashelf basin, and (3) inner platform-margin. Following a major hiatus in sedimentation in Wyoming during much of Pterocephaliid biomere time, a healthy shallow-water carbonate factory was reestablished during Elvinia Zone time. This produced abundant deposits of oolitic and skeletal packstone in the shoal environment and an areally extensive thrombolite boundstone facies in the intrashelf basin setting. Approximately coincident with the biomere boundary extinction event (Irvingella major time), ooid production was terminated and the thrombolites were drowned. The correlation of this major change in the pattern of sedimentation allows for the recognition of parts of two depositional sequences in the study area. This drowning event (sequence boundary) may also be coeval with the accumulation of transported shallow-water carbonate deposits in an inner-platform-margin environment. During early Taenicephalus Zone time, sedimentation rates were apparently reduced and the entire study area is marked by thin concentrations of trilobite and brachiopod shells. The nonskeletal carbonate factory was never reestablished in the former shoal environment, where thick successions of homogeneous lime mud accumulated during the rest of Taenicephalus Zone time. Flat-pebble conglomerate and shale accumulated in the storm-influenced intrashelf basin during this time. Sea-level change and changes in seawater chemistry likely both played roles in the drowning event across the Elvinia-Taenicephalus boundary. A positive trend in carbon-isotope values across the Pterocephaliid-Ptychaspid biomere boundary is observed in the six sections analyzed. This is consistent with previous studies that indicated that the mass extinction occurs within a regional shift towards positive values, and this may reflect the development of low-oxygen conditions on the shelf. The global significance of these isotopic trends, changes in relative sea level, and the mass extinction has not been confirmed, although recent study of the extinction event at the base of the Pterocephaliid biomere has revealed a similar trend towards positive carbon-isotope values that is worldwide.

石灰質ナンノ化石層序の最近の知見とその応用 とくに常磐海域坑井の新第三系・第四系層序について

This paper deals with a recent summary of calcareous nannofossil biostratigraphy and an example of biostratigraphic application. According to recent studies of nannofossil biostratigraphy, a total of 60 calcareous nannofossil datums and bioevents can be available for the Neogene and Quaternary. Newly proposed bioevents based on size variations of Reticulofenestra coccoliths are introduced.Calcareous nannofossils in the exploratory wells in offshore Joban were examined and two major hiatuses were identified in the Miocene sequences, indicating relative sea level drops during the Neogene.

Aspects of the structural evolution of the Lusitanian Basin in Portugal and the shelf and slope area offshore Portugal

The study provides a regional seismic interpretation and mapping of the Mesozoic and Cenozoic succession of the Lusitanian Basin and the shelf and slope area off Portugal. The seismic study is compared with previous studies of the Lusitanian Basin. From the Late Triassic to the Cretaceous the study area experienced four rift phases and intermittent periods of tectonic quiescence. The Triassic rifting was concentrated in the central part of the Lusitanian Basin and in the southernmost part of the study area, both as symmetrical grabens and half-grabens. The evolution of half-grabens was particularly prominent in the south. The Triassic fault-controlled subsidence ceased during the latest Late Triassic and was succeeded by regional subsidence during the early Early Jurassic (Hettangian) when deposition of evaporites took place. A second rift phase was initiated in the Early Jurassic, most likely during the Sinemurian–Pliensbachian. This resulted in minor salt movements along the most prominent faults. The second phase was concentrated to the area south of the Nazare Fault Zone and resulted here in the accumulation of a thick Sinemurian–Callovian succession. Following a major hiatus, probably as a result of the opening of the Central Atlantic, resumed deposition occurred during the Late Jurassic. Evidence for Late Jurassic fault-controlled subsidence is widespread over the whole basin. The pattern of Late Jurassic subsidence appears to change across the Nazare Fault Zone. North of the Nazare Fault, fault-controlled subsidence occurred mainly along NNW–SSE-trending faults and to the south of this fault zone a NNE–SSW fault pattern seems to dominate. The Oxfordian rift phase is testified in onlapping of the Oxfordian succession on salt pillows which formed in association with fault activity. The fourth and final rift phase was in the latest Late Jurassic or earliest Early Cretaceous. The Jurassic extensional tectonism resulted in triggering of salt movement and the development of salt structures along fault zones. However, only salt pillow development can be demonstrated. The extensional tectonics ceased during the Early Cretaceous. During most of the Cretaceous, regional subsidence occurred, resulting in the deposition of a uniform Lower and Upper Cretaceous succession. Marked inversion of former normal faults, particularly along NE–SW-trending faults, and development of salt diapirs occurred during the Middle Miocene, probably followed by tectonic pulses during the Late Miocene to present. The inversion was most prominent in the central and southern parts of the study area. In between these two areas affected by structural inversion, fault-controlled subsidence resulted in the formation of the Cenozoic Lower Tagus Basin. Northwest of the Nazare Fault Zone the effect of the compressional tectonic regime quickly dies out and extensional tectonic environment seems to have prevailed. The Miocene compressional stress was mainly oriented NW–SE shifting to more N–S in the southern part.

Palynology and Chronostratigraphy of the Upper Cretaceous in the Subsurface of the Barinas Area, Western Venezuela

Palynological and well-log analyses of Cretaceous strata from southeastern Venezuela allow the recognition of third-order eustatic cycles and are the basis for a chronostratigraphic framework with a datum approximately every 0.8 m.y. This definition permits the correlation of events in the local geologic evolution with regionally important sedimentological and tectonic events. An Early Cretaceous marine transgression reached areas north of the Andes before 113 Ma; however, according to our framework, the transgression extended into the Barinas area shortly after 98 Ma. An interval of relatively low sea level, from 95.5 Ma to 91.5 Ma, is represented by oil-bearing sandstones and dolomitic limestones. A second marine transgression reached the area at approximately 91.5 Ma and resulted in the deposition of organic-rich clastics that are correlatable with source rocks from areas to the north. A major hiatus (Santonian-Campanian) encompasses approximately 11 m.y. and probably is related to the emplacement of oceanic rocks in western Colombia. A younger major hiatus (Maastrichtian-Eocene) encompasses approximately 26 m.y. and probably is related to the emplacement of the Lara nappes in northern Venezuela. Two other minor intra-Turonian and intra-Maastrichtian hiatuses are recognized and encompass approximately 1.5 m.y. each.

Late Quaternary environmental changes of the central part of the Basin of Mexico; correlation between Texcoco and Chalco basins

A 10.8 m core was collected from Lake Texcoco to investigate paleoenvironmental changes for the late Quaternary in the central Basin of Mexico. Results from pollen, magnetic susceptibility and loss-on-ignition analyses reveal important fluctuations. A time scale has been established from five AMS 14C dates and tephra correlation. A humid period is recorded from at least 34,000 to ca. 23,000 yr B.P. where mesophytic and wetland taxa are important. A trend of dry and cold conditions began during the late glacial maximum, reflected by an expansion of the pine forests and lowering of pollen diversity and concentration. This trend continued during the late glacial when grasslands developed, and culminated at ca. 14,000 yr B.P. Volcanic activity was particularly noteworthy during this time in association with this environmental change. A major hiatus marks the culmination of the dry period. Wetter conditions during the early (?) Holocene-mid-Holocene allowed Texcoco to be a persistent saline lake. Correlation between Chalco and Texcoco sub-basins based on 14C dated pollen assemblages suggests that the major environmental changes affecting both sites are of climatic origin. However, volcanic eruptions also have played an important role in modeling the landscape as well as the evolution of vegetation and the lacustrine systems within the basin, and some of the limnological changes in the lakes could be explained by volcanism rather than by climatic causes.

Morphologic change in the clawed lobsterHoploparia(Nephropidae) from the Cretaceous of Antarctica

AntarcticHoplopariaexhibit morphologic changes upsection in a stratigraphic record considered to be long (approximately 15 m.y.) and free of major hiatuses. Five characters exhibit change upsection, and the overall morphologies of the geologically oldest and youngest lobsters are different. The observed patterns could be the result of either phyletic evolution or gradual invasion of one or more species into the range of the original species. The most parsimonious interpretation of the data argues against the invasion hypothesis but supports the phyletic evolution hypothesis.

5616289 Substance and/or heat exchanging tower

Using stable isotope data obtained on multiple aliquots of diamonds from worldwide sources, it has been argued that carbon and nitrogen in diamond are decoupled. Here we re-investigate the carbon–nitrogen relationship based on the most comprehensive microbeam data set to date of stable isotopes and nitrogen concentrations in diamonds (n = 94) from a single locality. Our diamond samples, derived from two kimberlites in the Chidliak Field (NE Canada), show large variability in δ13C (− 28.4 ‰ to − 1.1‰, mode at − 5.8‰), δ15N (− 5.8 to + 18.8‰, mode at − 3.0‰) and nitrogen contents ([N]; 3800 to less than 1 at.ppm). In combination, cathodoluminescence imaging and microbeam analyses reveal that the diamonds grew from multiple fluid pulses, with at least one major hiatus documented in some samples that was associated with a resorption event and an abrupt change from low δ13C and [N] to mantle-like δ13C and high [N]. Overall, δ13C appears to be uncorrelated to δ15N and [N] on both the inter- and intra-diamond levels. Co-variations of δ15N–log[N], however, result in at least two parallel, negatively correlated linear arrays, which are also present on the level of the individual diamonds falling on these two trends. These arrays emerge from the two principal data clusters, are characterized by slightly negative and slightly positive δ15N (about − 3 and + 2‰, respectively) and variable but overall high [N].Using published values for the diamond-fluid nitrogen isotope fractionation factor and nitrogen partition coefficient, these trends are perfectly reproduced by a Rayleigh fractionation model.Overall, three key elements are identified in the formation of the diamond suite studied: (1.) a low δ13C and low [N] component that possibly is directly associated with an eclogitic diamond substrate or introduced during an early stage fluid event. (2.) Repeated influx of a variably nitrogen-rich mantle fluid (mildly negative δ13C and δ15N). (3.) In waning stages of influx, availability of the mantle-type fluid at the site of diamond growth became limited, leading to Rayleigh fractionation. These fractionation trends are clearly depicted by δ15N–[N] but are not detected when examining co-variation diagrams involving δ13C. Also on the level of individual diamonds, large (≥ 5‰) variations in δ15N are associated with δ13C values that typically are constant within analytical uncertainty. The much smaller isotope fractionation factor for carbon (considering carbonate- or methane-rich fluids as possible carbon sources) compared to nitrogen leads to an approximately one order of magnitude lower sensitivity of δ13C values to Rayleigh fractionation processes (i.e. during fractionation, a 1‰ change in δ13C is associated with a 10‰ change in δ15N). As a consequence, even minor heterogeneity in the primary isotopic composition of diamond forming carbon (e.g., due to addition of minor subducted carbon) will completely blur any possible co-variations with δ15N or [N]. We suggest this strong difference in isotope effects for C and N to be the likely cause of observations of an apparently decoupled behaviour of carbon and nitrogen isotopes in diamond.

Recent advances in understanding the southern North Sea Basin: a summary

Abstract After a short-lived but very successful phase of exploration in the UK North Sea, the need for extreme confidentiality ceased to exist for those fields that had already been discovered, including the gas fields of the southern North Sea; this led to a widespread dissemination of data from 1974 onwards, to the great benefit of the industry as a whole. Early difficulties in producing gas from reservoirs whose permeability had been reduced by the growth of diagenetic illite led to studies in depth- and facies-related diagenesis, which continues today in different parts of the Southern Permian Basin. A change from analogue to digital recording of seismic data coincided with improvements in computer technology, and transformed seismic from a tool with limited depth penetration, which required the skill of a dedicated specialist to interpret, to one that today is used universally by all exploration and production geologists; these advances led, in turn, to much improved stratigraphic and structural resolution. Recent studies, especially in the former East Germany, have led to a better definition of the Permian. In Central Germany, Lower Rotliegend volcanics are overlain by sedimentary sequences of the Müritz sub-group (Upper Rotliegend 1), which are coeval coveal part of the combined Altmark and Saalian Unconformity. In northern Germany, this major hiatus is overlain by the Havel and Elbe subgroups (Upper Rotliegend 2) which, together with the succeeding Zechstein sequences, were all deposited within the youngest stages of the Permian, covering a probable time span of no more than 16 Ma. Deposition of up to 4000 m or so of Late Permian strata in such a short time indicates structurally controlled subsidence in addition to any of a thermal nature.

Depositional Evolution and Sequence Stratigraphy of the Shelf and Slope Area Off South Gabon, West Africa

ABSTRACT The Tertiary succession off South Gabon is divided into stratigraphic sequences. The study includes interpretation of seismic data and data from seven wells within the study area, covering 26,000 km2. Four Paleogene sequences (P1-P4) are recognized within the Danian to earliest early Ypresian time interval. There was a major hiatus from the middle Eocene to the early Miocene in the study area. Six sequences (N1-N6) are identified in the Neogene, N1-N4 lower and middle Miocene and N5 and N6 probably upper Miocene to recent. The Danian and Thanetian sequences (P1-P3) are separated by Type 2 sequence boundaries, and the most widespread flooding of the area occurred at the Danian-Thanetian transition within sequence P2. The boundary between P3 and P4, close to the Thanetian-Ypr sian transition, is probably of Type 1. The Paleocene sequences consist of transgressive systems tracts and highstand systems tracts. The lower Miocene sequence (N1) was formed mainly by progradation of the shelf as a result of high sediment supply and had a tectonic origin. The middle Miocene sequence boundaries are characterized by deep erosion, e.g., canyon formation into the substratum, and thus are Type 1 sequence boundaries. The middle Miocene sequences (N2-N4) consist of lowstand systems tracts, transgressive systems tracts (early part), and late highstand systems tracts. The upper Miocene to Holocene interval (N5-N6) could not be subdivided because the deposits are relatively thin. The Tertiary mainly siliciclastic sediments are mainly fine-grained sediments, though sandstones are identified at sequence boundaries of Danian to Ypresian ages. Sandstones are also inferred to have been deposited during the early fill of canyons and basinward in the middle Miocene. The processes involved in the formation of these sequences is thought to be a combination of basin type (tectonics), sediment supply, and sea-level changes. Curves of relative sea level and eustatic sea level were constructed for the studied interval, and these curves are compared with earlier published sea-level curves for the Tertiary.