Late Cretaceous Sea Research Articles

Long-Term Sea-Level Fluctuations Driven by Ocean Basin Dynamics

Earth's long-term sea-level history is characterized by widespread continental flooding in the Cretaceous period (approximately 145 to 65 million years ago), followed by gradual regression of inland seas. However, published estimates of the Late Cretaceous sea-level high differ by half an order of magnitude, from approximately 40 to approximately 250 meters above the present level. The low estimate is based on the stratigraphy of the New Jersey margin. By assimilating marine geophysical data into reconstructions of ancient ocean basins, we model a Late Cretaceous sea level that is 170 (85 to 270) meters higher than it is today. We use a mantle convection model to suggest that New Jersey subsided by 105 to 180 meters in the past 70 million years because of North America's westward passage over the subducted Farallon plate. This mechanism reconciles New Jersey margin-based sea-level estimates with ocean basin reconstructions.

Late Mesozoic to Paleogene stratigraphy of the Salar de Atacama Basin, Antofagasta, Northern Chile: Implications for the tectonic evolution of the Central Andes

The Salar de Atacama basin, the largest “pre-Andean” basin in Northern Chile, was formed in the early Late Cretaceous as a consequence of the tectonic closure and inversion of the Jurassic–Early Cretaceous Tarapacá back arc basin. Inversion led to uplift of the Cordillera de Domeyko (CD), a thick-skinned basement range bounded by a system of reverse faults and blind thrusts with alternating vergence along strike. The almost 6000-m-thick, upper Cretaceous to lower Paleocene sequences (Purilactis Group) infilling the Salar de Atacama basin reflects rapid local subsidence to the east of the CD. Its oldest outcropping unit (Tonel Formation) comprises more than 1000 m of continental red sandstones and evaporites, which began to accumulate as syntectonic growth strata during the initial stages of CD uplift. Tonel strata are capped by almost 3000 m of sandstones and conglomerates of western provenance, representing the sedimentary response to renewed pulses of tectonic shortening, which were deposited in alluvial fan, fluvial and eolian settings together with minor lacustrine mudstone (Purilactis Formation). These are covered by 500 m of coarse, proximal alluvial fan conglomerates (Barros Arana Formation). The top of the Purilactis Group consists of Maastrichtian-Danian alkaline lava and minor welded tuffs and red beds (Cerro Totola Formation: 70–64 Ma K/Ar) deposited during an interval of tectonic quiescence when the El Molino–Yacoraite Late Cretaceous sea covered large tracts of the nearby Altiplano-Puna domain. Limestones interbedded with the Totola volcanics indicate that this marine incursion advanced westwards to reach the eastern CD slope. CD shortening in the Late Cretaceous was accompanied by volcanism and continental sedimentation in fault bounded basins associated to strike slip along the north Chilean magmatic arc to the west of the CD domain, indicating that oblique plate convergence prevailed during the Late Cretaceous. Oblique convergence seems to have been resolved into a highly partitioned strain system where margin-parallel displacements along the thermally weakened arc coexisted with margin-orthogonal shortening associated with syntectonic sedimentation in the Salar de Atacama basin. A regionally important Early Paleocene compressional event is echoed, in the Salar de Atacama basin by a, distinctive, angular unconformity which separates Paleocene continental sediments from Purilactis Group strata. The basin also records the Eocene–Early Oligocene Incaic transpressional episode, which produced, renewed uplift in the Cordillera de Domeyko and triggered the accumulation of a thick blanket of syntectonic gravels (Loma Amarilla Formation).

The Cretaceous–Palaeogene boundary at Stevns Klint, Denmark: inversion tectonics or sea-floor topography?

The Cretaceous–Palaeogene boundary interval is exposed over 12 km in the coastal cliff, Stevns Klint, Denmark. An important lowermost Danian hardground has been interpreted as an originally horizontal marine abrasion surface. Its present elevation varies, from a few metres below, to about 35 m above sea level. This relief has traditionally been considered as resulting from late or post-Danian Laramide folding. New seismic profiles offshore Stevns Klint show, however, that the Base-Chalk reflector is not folded, is remarkably planar and has a gentle northward dip. Thus, the folding hypothesis cannot be upheld. Seismic stratigraphic analysis of the Chalk Group necessitates a fundamental revision of general ideas of chalk deposition. A highly irregular sea-floor topography was formed at many levels, and includes broad valleys, ridges, channels, drifts and mounds. A system of major WNW–ESE-oriented valleys and ridges can be traced into the succession exposed in Stevns Klint and further inland where it corresponds to the relief of the Cretaceous–Palaeogene boundary. The marked topographic elements of the chalk sea floor are elongate, with a WNW orientation roughly parallel to the axis of the Danish Basin and to the Sorgenfrei–Tornquist Zone forming the NE border of the basin. The sea-floor relief undoubtedly reflects the influence of strong contour-parallel bottom currents. The Cretaceous–Palaeogene boundary succession at Stevns Klint was thus developed on an underlying sea-floor topographic relief of about 40 m. Recognition of the highly irregular, current-influenced topography of the late Cretaceous sea floor stands in marked contrast to the conventional picture of quiet pelagic deposition of the chalk.

New Zealand's Geological Foundations

New Zealand is a fragment of Gondwana that, before Late Cretaceous sea floor spreading, was contiguous with Australia and Antarctica. Only about 10% of the area of continental crust in the wider New Zealand region (Zealandia) is emergent above sea level as the North and South Islands. No Precambrian cratonic core is exposed in onland New Zealand. The Cambrian to Early Cretaceous basement can be described in terms of nine major volcano-sedimentary terranes, three composite regional batholiths, and three regional metamorphic-tectonic belts that overprint the terranes and batholiths. The terranes (from west to east) are: Buller, Takaka, Brook Street, Murihiku, Maitai, Caples, Bay of Islands (part of former Waipapa), Rakaia (older Torlesse) and Pahau (younger Torlesse). The western terranes are intruded by three composite batholith (>100 km 2) sized belts of plutons: Karamea-Paparoa, Hohonu and Median, as well as by numerous smaller plutons. Median Batholith (including the Median Tectonic Zone) is a recently-recognised Cordilleran batholith that represents the site of subduction-related magmatism from ca. 375–110 Ma. Parts of the terranes and batholiths are variably metamorphosed and deformed: Devonian and Cretaceous amphibolite-granulite facies gneisses are present in Buller, Takaka, Median and Karamea-Paparoa units; Jurassic-Cretaceous subgreenschist-amphibolite facies Haast Schist overprints the Caples, Bay of Islands and Rakaia Terranes; Cretaceous subgreenschist facies Esk Head and Whakatane Mélanges bound the Pahau Terrane. In the South Island, small areas (<5 km 2 total) of Devonian, Permian, Triassic and Jurassic Gondwana sequences have been identified. In the North Island a widespread Late Jurassic overlap sequence, Waipa Supergroup (part of former Waipapa Terrane), has recently been proposed.

Upper Cretaceous sequences and sea-level history, New Jersey Coastal Plain

We developed a Late Cretaceous sea- level estimate from Upper Cretaceous sequences at Bass River and Ancora, New Jersey (ODP [Ocean Drilling Program] Leg 174AX). We dated 11–14 sequences by integrating Sr isotope and biostratigraphy (age resolution ±0.5 m.y.) and then estimated paleoenvironmental changes within the sequences from lithofacies and biofacies analyses. Sequences generally shallow up-section from middle-neritic to inner-neritic paleodepths, as shown by the transition from thin basal glauconite shelf sands (transgressive systems tracts [TST]), to medial-prodelta silty clays (highstand systems tracts [HST]), and finally to upper–delta-front quartz sands (HST). Sea-level estimates obtained by backstripping (accounting for paleodepth variations, sediment loading, compaction, and basin subsidence) indicate that large (>25 m) and rapid (≪1 m.y.) sea-level variations occurred during the Late Cretaceous greenhouse world. The fact that the timing of Upper Cretaceous sequence boundaries in New Jersey is similar to the sea-level lowering records of Exxon Production Research Company (EPR), northwest European sections, and Russian platform outcrops points to a global cause. Because backstripping, seismicity, seismic stratigraphic data, and sediment-distribution patterns all indicate minimal tectonic effects on the New Jersey Coastal Plain, we interpret that we have isolated a eustatic signature. The only known mechanism that can explain such global changes— glacio-eustasy—is consistent with foraminiferal δ 18 O data. Either continental ice sheets paced sea-level changes during the Late Cretaceous, or our understanding of causal mechanisms for global sea-level change is fundamentally flawed. Comparison of our eustatic history with published ice-sheet models and Milankovitch predictions suggests that small (5–10 × 10 6 km 3 ), ephemeral, and areally restricted Antarctic ice sheets paced the Late Cretaceous global sea-level change. New Jersey and Russian eustatic estimates are typically one-half of the EPR amplitudes, though this difference varies through time, yielding markedly different eustatic curves. We conclude that New Jersey provides the best available estimate for Late Cretaceous sea-level variations.

Sedimentary Structures of the La Luna, Navay and Querecual Formations, Upper Cretaceous of Venezuela

Other| October 01, 2003 Sedimentary Structures of the La Luna, Navay and Querecual Formations, Upper Cretaceous of Venezuela OLIVER MACSOTAY; OLIVER MACSOTAY 1Urbanización El Trigal Norte, Ave. Atlántico, no. 155-61B, Valencia, Venezuela Search for other works by this author on: GSW Google Scholar ROBERT N. ERLICH; ROBERT N. ERLICH 2Burlington Resources International, 400 North Sam Houston Parkway East, Suite 1200, Houston, TX 77060, rerlich@br-inc.com * Corresponding author. Search for other works by this author on: GSW Google Scholar TULIO PERAZA TULIO PERAZA 3Intevep-Laboratorios Generales, Sur 1, nivel 2, oficina 222, Los Teques, Edo. Miranda, Venezuela Search for other works by this author on: GSW Google Scholar Author and Article Information OLIVER MACSOTAY 1Urbanización El Trigal Norte, Ave. Atlántico, no. 155-61B, Valencia, Venezuela ROBERT N. ERLICH * Corresponding author. 2Burlington Resources International, 400 North Sam Houston Parkway East, Suite 1200, Houston, TX 77060, rerlich@br-inc.com TULIO PERAZA 3Intevep-Laboratorios Generales, Sur 1, nivel 2, oficina 222, Los Teques, Edo. Miranda, Venezuela Publisher: SEPM Society for Sedimentary Geology Accepted: 01 Jun 2003 First Online: 03 Mar 2017 Online Issn: 1938-5323 Print Issn: 0883-1351 Society for Sedimentary Geology PALAIOS (2003) 18 (4-5): 334–348. https://doi.org/10.1669/0883-1351(2003)018<0334:SSOTLL>2.0.CO;2 Article history Accepted: 01 Jun 2003 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation OLIVER MACSOTAY, ROBERT N. ERLICH, TULIO PERAZA; Sedimentary Structures of the La Luna, Navay and Querecual Formations, Upper Cretaceous of Venezuela. PALAIOS 2003;; 18 (4-5): 334–348. doi: https://doi.org/10.1669/0883-1351(2003)018<0334:SSOTLL>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 SocietyPALAIOS Search Advanced Search Abstract The Late Cretaceous seas of Venezuela were characterized by deposition of organic carbon-rich limestones, cherts, and shales in the Maracaibo and Barinas/Apure Basins of western Venezuela (La Luna and Navay Formations) and in the Maturín and Guarico Sub-basins of eastern Venezuela (Querecual, Tigre, and San Antonio Formations). Deposition of these units typically occurred under low-energy, low-oxygen conditions that were punctuated by the episodic activity of bottom currents, debris flows, turbidites, syndepositional faulting, and by intermittent oxygenation. These events produced a characteristic set of sedimentary structures that can be used to provide more detailed interpretations of each depositional system. Four main types of sedimentary structures noted in this study are: (1) hydrodynamic—primarily produced through the actions of currents; (2) rheotropic—structures produced during loading and dewatering of unconsolidated sediments; (3) biogenic—formed through the actions of living organisms, such as trace fossils; and (4) diagenetic—structures and textures formed through near surface and burial diagenetic processes.Identification of these structures can provide new insights on the hydrodynamic and biologic settings of Venezuelan Cretaceous strata. Utilizing observations made in this study along with previously published works, bottom-water conditions in the La Luna depositional system are interpreted to have been maintained consistently in a state of anoxia or very low dissolved oxygen content. This contrasts with bottom waters in the Querecual depositional system, during which strong periods of anoxia or low dissolved oxygen content were punctuated by periods of intermittent oxygenation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The North American obligate cave fauna: regional patterns

The obligate cave faunas of nine regions of the United States –Florida Lime Sinks, Appalachians, Interior Low Plateaus, Ozarks, Driftless Area,Edwards Aquifer/Balcones Escarpment, Guadalupe Mountains, Black Hills, andMother Lode – are described and compared. The number of aquatic(stygobitic) species ranged from zero (Black Hills) to 82 (Appalachians), andthe number of terrestrial (troglobitic) species ranged from zero (Florida LimeSinks) to 256 (Interior Low Plateau). Even at the level of genus, overlapbetween regions is low. Several predictor variables (karst area, number ofcaves, number of long caves, number of deep caves, distance from the Pleistoceneice margin, distance from the late Cretaceous Sea, and vegetation type – asurrogate for productivity) were assessed using rank order statistics,especially rank order multiple regression with a backward elimination procedure.For both stygobites and troglobites, only number of caves was a significantpredictor. The absence of a karst area effect suggested that the degree of karstdevelopment is better described by the number of caves rather than area ofkarst. There was no evidence that distance to Pleistocene glacial boundaries wasimportant, but there was some support for the importance of distance from late Cretaceous sea margins, a potential source of aquatic subterranean colonists. Finally,there was no indication that surface productivity had an effect on speciesrichness. Analysis was complicated by correlations among predictor variables.

Low-magnesium calcite produced by coralline algae in seawater of Late Cretaceous composition.

Shifts in the MgCa ratio of seawater driven by changes in midocean ridge spreading rates have produced oscillations in the mineralogy of nonskeletal carbonate precipitates from seawater on time scales of 10(8) years. Since Cambrian time, skeletal mineralogies of anatomically simple organisms functioning as major reef builders or producers of shallow marine limestones have generally corresponded in mineral composition to nonskeletal precipitates. Here we report on experiments showing that the ambient MgCa ratio actually governs the skeletal mineralogy of some simple organisms. In modern seas, coralline algae produce skeletons of high-Mg calcite (>4 mol % MgCO(3)). We grew three species of these algae in artificial seawaters having three different MgCa ratios. All of the species incorporated amounts of Mg into their skeletons in proportion to the ambient MgCa ratio, mimicking the pattern for nonskeletal precipitation. Thus, the algae calcified as if they were simply inducing precipitation from seawater through their consumption of CO(2) for photosynthesis; presumably organic templates specify the calcite crystal structure of their skeletons. In artificial seawater with the low MgCa ratio of Late Cretaceous seas, the algae in our experiments produced low-Mg calcite (<4 mol % MgCO(3)), the carbonate mineral formed by nonskeletal precipitation in those ancient seas. Our results suggest that many taxa that produce high-Mg calcite today produced low-Mg calcite in Late Cretaceous seas.

Strontium isotope stratigraphy of Turonian–Campanian Gosau-type rudist formations in the Northern Calcareous and Central Alps (Austria and Germany)

Shells of hippuritid rudist bivalves from 16 localities of Upper Cretaceous rudist formations in Gosau-type sedimentary basins of the Eastern Alps (Northern Calcareous Alps and Austroalpine Units) have been analysed for87Sr/86Sr values. The extent of diagenetic alteration of original isotopic compositions was assessed by analyses of concentrations of Sr, Mg, Fe and Mn, and proved to be minor in the samples investigated. A comparison of87Sr/86Sr values of rudist shells with previously published variations of87Sr/86Sr in Late Cretaceous sea water allows correlation with biostratigraphical standard zonations. Considerable stratigraphic revisions are the result for localities where previous biostratigraphical evidence was scarce or imprecise, while existing stratigraphical concepts are confirmed for many other localities where such evidence exists. A Late Turonian to earliest Coniacian age of several shells of Vaccinites inaequicostatus at localities in the environs of Lake St. Wolfgang is confirmed by similar isotopic ratios of shells of the same species in Greece and northern Italy. Previous identifications of hippuritids at the localities studied are discussed, and four characteristic associations are identified.

40Ar/39Ar age of the Manson impact structure, Iowa, and correlative impact ejecta in the Crow Creek Member of the Pierre Shale (Upper Cretaceous), South Dakota and Nebraska

A set of 34 laser total-fusion 40 Ar/ 39 Ar analyses of sanidine from a melt layer in crater-fill deposits of the Manson impact structure in Iowa has a weighted-mean age of 74.1 ± 0.1 Ma. This age is about 9.0 m.y. older than 40 Ar/ 39 Ar ages of shocked microcline from the Manson impact structure reported previously by others. The 74.1 Ma age of the sanidine, which is a melt product of Precambrian microcline clasts, indicates that the Manson impact structure played no part in the Cretaceous-Tertiary (K-T) mass extinction at 64.5 Ma. Moreover, incremental-heating 40 Ar/ 39 Ar ages of the sanidine show that it is essentially free of excess 40 Ar and has not been influenced by postcrystallization heating or alteration. An age spectrum of the matrix of the melt layer shows effects of 39 Ar recoil, including older ages in the low-temperature increments and younger ages in the high-temperature increments. At 17 places in eastern South Dakota and Nebraska, shocked quartz and feldspar grains are concentrated in the lower part of the Crow Creek Member of the Pierre Shale (Upper Cretaceous). The grains are largest (3.2 mm) in southeastern South Dakota and decrease in size (0.45 mm) to the northwest, consistent with the idea that the Manson impact structure was their source. The ubiquitous presence of shocked grains concentrated in a thin calcarenite at the base of the Crow Creek Member suggests it is an event bed recording an instant of geologic time. Ammonites below and above the Crow Creek Member limit its age to the zone of Didymoceras nebrascense of earliest late Campanian age. Plagioclase from a bentonite bed in this zone in Colorado has a 40 Ar/ 39 Ar age of 74.1 ± 0.1 Ma commensurate with our sanidine age of 74.1 Ma for the Manson impact structure. 40 Ar/ 39 Ar ages of bentonite beds below and above the Crow Creek are consistent with our 74.1 ± 0.1 Ma age for the Manson impact structure and limit its age to the interval ±74.5 0.1 to 73.8 ± 0.1 Ma. Recently, two origins for the Crow Creek have been proposed—eastward transgression of the Late Cretaceous sea and a Manson impact-triggered tsunami. We conclude that most data are in accord with an impact origin for the Crow Creek Member and are at odds with the marine transgression hypothesis.

Paleoecological relationships of the Late Cretaceous lamniform shark, Cretoxyrhina mantelli (Agassiz)

Several specimens of the Late Cretaceous lamniform shark, Cretoxyrhina mantelli (Agassiz), from the Niobrara Chalk of Kansas suggest that the shark fed on teleosts, mosasaurs, and possibly plesiosaurs. These animals are active vertebrates, so C. mantelli probably occupied the apex of the food chain in the Late Cretaceous seas. This top predator, however, was probably scavenged frequently by anacoracid sharks. Carcharodon carcharias (great white shark) and carcharhinid sharks are considered as the modern guild counterparts for Cretoxyrhina mantelli and anacoracids, respectively.

Late Cretaceous sea level from a paleoshoreline

The contemporary elevation of an ancient strandline provides a measure of eustasy at a single time in the geologic past. If the elevation of a paleoshoreline has not changed since deposition (or if it has changed by a known amount), then the measured (or corrected) elevation equals the sea level at that time. The ideal location for measuring an ancient sea level is a paleoshoreline deposited high on the shoulder of a stable craton where postdepositional elevation changes through tectonic movement, lithospheric compensation to loads, and sediment compaction are minimized. The contemporary elevation of a Late Cenomanian (≈93 Ma) shoreline was determined at five localities along the tectonically stable, eastern margin of the Cretaceous Western Interior Seaway, North America. This shoreline, represented by marine‐to‐nonmarine facies transitions in strata of the Greenhorn sequence (UZA‐2 cycle of Haq et al. (1987)), was identified from outcrop and borehole data. Biostratigraphic zonations constrained the geologic age at each locality. Sequence stratigraphic correlations, based on identifying discrete progradational units and the surfaces that separate them, were used to refine age correlations to better than 100 kyr between localities. A single Cenomanian shoreline was correlated within a single progradational unit, and its elevation was determined at five localities. This paleostrandline occurs 265–286 m above present‐day sea level, at an average elevation of 276 m. Isostatic and flexural corrections were applied to remove the effects of postdepositional vertical movement, including sediment compaction by loading, uplift due to erosion, and glacial loading and rebound. Errors inherent in each measurement and each correction were estimated. Corrections and their cumulative error estimates yield a Late Cenomanian elevation of 269±87 m above present sea level. The corrected elevation approximates sea level at 93 Ma and provides a measure of Late Cenomanian eustasy prior to the Early Turonian highstand. Establishing the absolute value for eustasy at a single point in geologic time provides a frame of reference for calibrating relative sea level curves, as well as constraining the magnitudes of tectonic subsidence, sediment flux, and othervariables that controlled water depth and relative sea level.

Absolute paleobathymetry of Upper Cretaceous chalks based on ostracodes-Evidence from the Demopolis Chalk (Campanian and Maastrichtian) of the northern Gulf Coastal Plain

The presence of abundant and diverse sighted ostracodes in chalk and marl of the Demopolis Chalk (Campanian and Maastrichtian) in Alabama and Mississippi strongly suggests that the Late Cretaceous sea floor was within the photic zone. The maximum depth of deposition is calculated from an equation based on eye morphology and efficiency and estimates of the vertical light attenuation. In this equation, K , the vertical light attenuation coefficient, is the most critical variable because it is the divisor for the rest of the equation. Rates of accumulation of coccoliths during the Cretaceous are estimated and are on the same order as those in modern areas of high phytoplankton production, suggesting similar pigment and coccolith concentrations in the water column. Values of K are known for a wide range of water masses and pigment concentrations, including areas of high phytoplankton production; thus light attenuation through the Cretaceous seas can be estimated reliably. Waters in which attenuation is due only to biogenic matter-conditions that result in deposition of relatively pure chalk-have values of K ranging between 0.2 and 0.3. Waters rich in phytoplankton and mud-conditions that result in deposition of marl-have K values as great as 0.5. Substituting these values for K results in a depth range of 65 to 90 m for deposition of chalk and a depth of 35m for deposition of marl. These depth values suggest that deposition of many Cretaceous chalks and marls around the world were deposited under relatively shallow conditions.

?Tethyaster albertensis, a Late Cretaceous (Turonian) sea star from the Cardium Formation, Alberta, Canada

Although many of the surviving lineages of sea stars appeared during an early Mesozoic radiation of the class and have undergone limited change since then, they have left a very poor fossil record, particularly in the Mesozoic of North America (Blake, 1981). This record from the Late Cretaceous of Alberta is made more significant by the fact that it is apparently only the second occurrence of a member of the family Astropectinidae in the Cretaceous of North America; Lophidiaster silentiensis was described by McLearn (1944) from the Lower Cretaceous (Albian) Hasler Formation, from a now-submerged locality on the Peace River in northern Alberta. All previously recorded fossil sea stars from the North American Cretaceous are representatives of the family Goniasteridae.

Oxygen and carbon isotope compositions of Upper Cretaceous chalk from the Danish sub‐basin and the North Sea Central Graben

ABSTRACTThe oxygen and carbon isotopic compositions of Upper Cretaceous chalk have been studied from a large number of outcrops and boreholes in the Danish sub‐basin and the North Sea Central Graben. The carbon isotopic compositions vary from +0.50%o to +3.00%oδ13C which correspond to the expected carbon isotopic composition of carbonate precipitated from normal sea water. The oxygen isotopic compositions of samples from outcrops and near‐surface drilled sections fall in the range from —0.50%o to — 2.00%oδ18O, which is close to the expected values for carbonate produced by coccoliths in the Late Cretaceous sea. A geographic distribution exists in the Danish sub‐basin showing relatively heavy oxygen isotope values in the centre of the basin and slightly more negative values in the eastern part of the basin. Primarily the phenomenon is thought to reflect minor variations in the oceanographic parameters in the Late Cretaceous sea.The deep subsurface sections represent a depth interval of 0–3100 m. Mechanical compaction in the uppermost part of the sequences apparently has no influence on the isotopic composition. Chemical compaction dominates at greater depth, leading to temperature‐induced isotopic re‐equilibration in the water‐rock system. In these sequences the oxygen isotope values range from − 1.50%o to — 7.50%oδ18O and are significantly correlated with depth of burial, cementation and porosity. The data indicate that pressure‐dissolution, recrystallization, reprecipitation and ion‐exchange between solid carbonate phase and pore water, are all actively involved in the oxygen isotope re‐equilibration process. This process is believed to take place in a diagenetically closed system and is tentatively divided into two phases: 1) an early diagenetic phase which takes place at porosities down to approximately 20% in which the chalk possesses a pore‐water controlled isotopic re‐equilibration system and 2) a late diagenetic phase at porosities below 20% in which the re‐equilibration process increasingly becomes influenced by the rock‐introduced isotopic change in the composition of the formation water.

Late Cretaceous upwelling and anoxic sedimentation in northwestern South America

Black shales and bituminous limestones were widely deposited in northwestern South America during the maximum transgression of late Cretaceous seas. These Cenomanian to Santonian (Campanian) anoxic sediments are distributed in two main subprovinces. Thick chert and phosphorite accumulated in the Trough Subprovince, landward of a possibly partially emergent Cordillera Central (Colombia). Only minor intercalations of chert were deposited in the Platform Subprovince, lying northward in the Maracaibo Basin (northwestern Venezuela). Faunal evidence (depauperate benthic fauna, planktic foraminiferal assemblages, high-dominance clupeoid fish assemblages) and sedimentological evidence suggest anoxic condition prevailed throughout the region, probably the result of dynamic upwelling in the Trough Subprovince, but the reflection of a “global” anoxic event within the Platform Subprovince.

Carbonate Substrates of the Late Cretaceous Sea, Central Great Plains and Southern Rocky Mountains

Carbonate Substrates of the Late Cretaceous Sea, Central Great Plains and Southern Rocky Mountains

Small inversion structures on seismic sections from the southern North Sea

Fig. 1. Structural elements of the southern North Sea, after Glennie and Bougner (1981) and Dunning (1985). Fig. 2 at about 0.9 to 1.0 sees TWT. The corresponding depths are about 800 to 900 metres, and there is virtually no structuring at or above this level. The Chalk extends to rather more than 1.4 sees TWT at the left hand side of Fig. 2. Seismic energy travels through carbonates at much higher velocities than through Tertiary shales, and the Chalk thickness at this point is about 600 metres. The strong reflection from its base is, however, severely distorted by geometrical effects and particularly by diffraction hyperbolae generated by discontinuities at the edges of fault blocks. The way in which these features are produced on seismic sections is well understood but where several intersect it is extremely difficult to 'see through' to the true geological structure. Fortunately the processing technique known as migration can be used to correct for such distortions (Fig. 3). A single seismic section can be successfully migrated only if the seismic line crosses the geological structures at right angles to their strike. This condition The inversion of sedimentary basins to form structural highs has long been recognised as a feature of the geology of southern England. The classic example is the Weald Basin of Sussex and Kent, which suffered inversion early in the Tertiary. More recently, inversion structures have been mapped in the course of exploration for oil and gas in the North Sea. In the southern North Sea the major inverted feature is the Sole Pit Basin, which occupies much of the subsea region between the UK and the Netherlands (Fig. 1; cf Glennie & Boegner, 1981). The Sole Pit inversion extends almost to the southern limit of North Sea basinal structures. This limit is marked by the northern flank of the London-Brabant Platform, which was a positive and intermittently emergent structural feature from Early Palaeozoic times until the Cretaceous. Numerous seismic surveys have covered the region between the Platform and the Sole Pit, many of the most recent having been shot in anticipation of the UK Ninth Round allocations in late 1984. The Sole Pit Basin formed in the Permian as a large half-graben, subsiding along the Dowsing Fault Zone which formed its southwest margin. Rotation took place about a horizontal axis along the Swart Bank Hinge to the NE (Fig. 1). The hinge converges with the Dowsing Fault near the UK-Netherlands median line off the coast of East Anglia . Associated faulting produced numerous small basins which are shown by seismic reflection surveys to have also suffered Cretaceous inversion. The small inverted basins are preserved beneath a thick pile of younger sediments. Late Cretaceous seas which transgressed across the London-Brabant platform deposited a rather uniform layer of Chalk in the region close to the present-day Norfolk coast . Further to the northeast, and crossing the Swart Bank Hinge , the West Netherlands Basin developed late in the Cretaceous and continued to subside and receive sediments throughout the Tertiary. The Tertiary and youngest Cretaceous rocks have suffered little or no deformation and are clearly imaged on 'Final Stack' seismic sections (cf Fig. 2). The strongest reflectors are the top of the Chalk and a thin high-velocity Eocene bed. Together these give rise to the complex of high-amplitude events , seen on o ! km 50 [ [ I r

Approach for Determining Drainage Boundary of Wells Producing from Low-Permeability Sandstone Reservoirs: ABSTRACT

A new methodology for determining the drainage boundary in low-permeability reservoirs without having to obtain extensive pressure buildup or drawdown data requires using a fairly long production history. Pictured Cliffs formation, a fairly large gas-producing reservoir in the San Juan basin, northwestern New Mexico, consists of interbedded clay and silty tight sandstone beds deposited during the final regression of the Late Cretaceous sea. Gas production from wells in this formation is from the low-permeability blanket-type sandstone intervals of uniform thickness that form the reservoir. Low-permeability reservoirs (in contrast to conventional permeable reservoirs) do not exhibit a consistent relationship between porosity and permeability. Thus, in such reservoirs, the pressure and flow regime of the wells do not follow conventional reservoir behavior. Low-permeability reservoirs belong to a class of reservoirs known as volumetric or bounded cylindrical reservoirs, where the wells are surrounded by impermeable boundaries and/or wells that are producing in adjoining areas. In conventional permeable reservoirs, when a single well is placed on production, a pressure drawdown is propagated into the reservoir. A drainage radius will increase and move outward from the well until its natural limit is reached or a barrier is encountered. In low-permeability reservoirs, the advancement of themore » drainage radius halts at the end of the transient period of the pressure decline versus time curve. Thus, the drainage radius can be calculated from this curve and volumetric equations.« less

Foraminiferids and their bearing on late Cretaceous sea levels in southern Sweden

"Foraminiferids and their bearing on late Cretaceous sea levels in southern Sweden." Geologiska Föreningen i Stockholm Förhandlingar, 106(4), pp. 386–387