Full text: rdcu.be/tIj5.
The end of the Pliocene marked the beginning of a period of great climatic variability and sea-level oscillations. Here, based on a new analysis of the fossil record, we identify a previously unrecognized extinction event among marine megafauna (mammals, seabirds, turtles and sharks) during this time, with extinction rates three times higher than in the rest of the Cenozoic, and with 36% of Pliocene genera failing to survive into the Pleistocene. To gauge the potential consequences of this event for ecosystem functioning, we evaluate its impacts on functional diversity, focusing on the 86% of the megafauna genera that are associated with coastal habitats. Seven (14%) coastal functional entities (unique trait combinations) disappeared, along with 17% of functional richness (volume of the functional space). The origination of new genera during the Pleistocene created new functional entities and contributed to a functional shift of 21%, but minimally compensated for the functional space lost. Reconstructions show that from the late Pliocene onwards, the global area of the neritic zone significantly diminished and exhibited amplified fluctuations. We hypothesize that the abrupt loss of productive coastal habitats, potentially acting alongside oceanographic alterations, was a key extinction driver. The importance of area loss is supported by model analyses showing that animals with high energy requirements (homeotherms) were more susceptible to extinction. The extinction event we uncover here demonstrates that marine megafauna were more vulnerable to global environmental changes in the recent geological past than previously thought.
Polygonal fault systems (PFS) have been interpreted worldwide using seismic data imaging sedimentary strata. Normal faults initiate over a large area in fine-grained subaqueous strata soon after deposition. As they continue to grow laterally and vertically, their fault traces intersect to form polygons in plan view. These polygons are difficult to image without three-dimensional (3-D) seismic data. The faulting is initiated during sediment dewatering and mud particle consolidation that can be independent of external stresses. In the past 20 years, hundreds of basins worldwide have been interpreted to contain polygonal faults. This paper presents a PFS interpretation for fine-grained sediments deposited in the Late Cretaceous Western Interior Seaway of the Great Plains of North America. The faulted strata have been observed as a PFS at depths ranging from ~2750 m subsurface to outcrop. Seismic dataset interpretations and borehole analyses corroborate previously published outcrop analyses and seismic interpretations. The larger observed faults are mesoscale in size, with throws up to 80 m, and strike lengths up to ~1.5 km. Potentially encompassing over 2,000,000 km², observational averages imply 10⁷ or more mesoscale-size faults, with an order of magnitude greater number of smaller faults. At shallow depth and outcrop, the PFS model of extensive normal faulting could help to explain Late Cretaceous shale faulting attributed to other causes such as deeper sediment dissolution or glacial processes. In the subsurface, faulting and fracturing consistent with a PFS model can help to explain fault geometries observed in well control and 3-D seismic data.
The lower Pierre Shale consists primarily of the Sharon Springs Formation, which has been correlated regionally throughout Kansas, Colorado, Nebraska, Wyoming, South Dakota, and North Dakota. The unit represents distal sedimentation in a tectonically active foreland basin. Correlation of the lower Pierre Shale Group is complicated by the application of a single name, the Sharon Springs, to a wide range of facies. Bentonite correlation provides an independent framework for verifying the age equivalence of various facies of the Sharon Springs Formation. Bentonite correlation involves using a variety of unique chemical characteristics to differentiate individual beds. A combination of whole rock rare-earth-element geochemistry, phenocryst composition, biotite geochemistry, and stratigraphic position has been used to correlate bentonites of the lower Pierre Shale and equivalent units across the Western Interior of the United States.
Detrital zircons (DZ) from fluvial sandstones of the Western Canada Sedimentary Basin
and the U.S. Gulf of Mexico (GoM) passive margin indicate mid-Cretaceous through Paleocene
continental-scale drainage reorganization. DZ populations from the Early Cretaceous
Mannville Group of Alberta represent a continental-scale system that routed sediment from
the Appalachian Mountains and the eastern three-quarters of North America to the Boreal
Sea. In contrast, DZ populations from the GoM coastal plain show that only the southern
United States and Appalachian-Ouachita orogen contributed sediment to the GoM through
the Late Cretaceous, whereas by the Paleocene, southern North America, from the Western
Cordillera to the Appalachian Mountains, had been routed to the GoM. This continental-scale
drainage reorganization reflects the culmination of an ~300 m.y. trajectory that began with
Paleozoic Appalachian assembly, and broad east to west sediment routing, followed by assembly
of the Mesozoic Western Cordillera, which resulted in west-derived rivers in the United
States draining to the GoM in Texas, or to an ancestral Mississippi River in the Mississippi
embayment, setting up the template for sediment routing that persists today.
The transition between the Proterozoic and Phanerozoic eons, beginning 542 million years (Myr) ago, is distinguished by the diversification of multicellular animals and by their acquisition of mineralized skeletons during the Cambrian period. Considerable progress has been made in documenting and more precisely correlating biotic patterns in the Neoproterozoic-Cambrian fossil record with geochemical and physical environmental perturbations, but the mechanisms responsible for those perturbations remain uncertain. Here we use new stratigraphic and geochemical data to show that early Palaeozoic marine sediments deposited approximately 540-480 Myr ago record both an expansion in the area of shallow epicontinental seas and anomalous patterns of chemical sedimentation that are indicative of increased oceanic alkalinity and enhanced chemical weathering of continental crust. These geochemical conditions were caused by a protracted period of widespread continental denudation during the Neoproterozoic followed by extensive physical reworking of soil, regolith and basement rock during the first continental-scale marine transgression of the Phanerozoic. The resultant globally occurring stratigraphic surface, which in most regions separates continental crystalline basement rock from much younger Cambrian shallow marine sedimentary deposits, is known as the Great Unconformity. Although Darwin and others have interpreted this widespread hiatus in sedimentation on the continents as a failure of the geologic record, this palaeogeomorphic surface represents a unique physical environmental boundary condition that affected seawater chemistry during a time of profound expansion of shallow marine habitats. Thus, the formation of the Great Unconformity may have been an environmental trigger for the evolution of biomineralization and the 'Cambrian explosion' of ecologic and taxonomic diversity following the Neoproterozoic emergence of animals.
Most interpretations of the stratigraphic record are founded on the premise that the depositional environments that produced it either have not changed appreciably through time, or else have changed very slowly. Paradoxically, some of the most important transitions in the sedimentary archive are those interpreted to reflect relatively rapid, comprehensive paleoenvironmental change. Recognition of the anomalous nature of such transitions is vital to accurately understanding their significance but is not systematically incorporated in current stratigraphic models. The new term "xenoconformity" is therefore proposed, and defined as a stratigraphic surface or gradational interval that records a fundamental, abrupt, and persistent change in sedimentary facies across basinal to global scales. Xenoconformities may mark major paleoenvironmental tipping points and signal transformations in how paleoenvironmental signals were transferred into the stratigraphic record.
The COSUNA project of the American Association of Petroleum Geologists (AAPG) has printed newly constructed correlation charts. On those charts, 570 complete geological columns are shown representing the geologic provinces of the United States (including Alaska) and using both surface and subsurface data. However, the correlations depicted on these new charts should not be construed as final and fixed. They represent only the authors' best statements of the present stage of knowledge. This paper discusses the history of the project, COSUNA coordinators, research and data accumulation, and other aspects of the subject. Refs.
To make the COSUNA stratigraphic correlation charts more useful, all charts include a common reference chronostratigraphic scale and a common geochronometric (numerical time) scale in millions of years. This makes possible the comparison not only between the stratigraphic units of the various geologic provinces of the United States, but also between the American units and those of other parts of the world. The chronostratigraphic and geochronometric scales for the Paleozoic, Mesozoic, and Cenozoic used in the COSUNA stratigraphic correlation charts are shown. The numerical time scale is linear for all three, but a different spacing has been used for each. In the COSUNA stratigraphic correlation charts, it was necessary to use a flexible vertical spacing to fit best the stratigraphy of the area and to make the charts most legible. Refs.
Recent discoveries offshore Tanzania and Mozambique highlight East Africa as an emerging world-class pet-roleum province. Oil and gas estimates for this province total 12.5 BBO and 250 TCFG (Brownfield et al., 2012) as yet undiscovered. Play-opening reservoir systems have been verified in Paleocene, Eocene, and at least two Oligocene deep-water submarine fan and intraslope channel complexes (Law, 2011). In deep-water Tanzania, there have been seven gas discoveries (of eight attempts) since 2010, with recent announcements putting total gas reserves in Tanzania at 24-26 TCFG. In neighboring Mozambique, 19 wells were completed by Anadarko and ENI in the Ruvuma Basin from 2009 to June 2012, only two of which were not announced as commercial discoveries. With the additional drilling, the increase of reported reserves now approaches or exceeds 100 TCFG. Evidence continues to mount that suggests the Late Cretaceous section contains deposits from similar depositional settings (Tanzania Petroleum Development Corporation, 2003). There are also indications that the petroleum system may contain oil as well as the established gas.
It has been claimed that the geological column as a faunel succession is not just a hypothetical concept, but a reality, because all Phanerozoic systems exist superposed at a number of locations on the earth. Close examination reveals, however, that even at locations where all ten systems are superposed, the column, as represented by sedimentary-thickness, is mostly missing. In fact, the thickest local accumulation of rock is only a tiny fraction of the inferred 600-million year's worth of depositions. The global 'stack' of index fossils exists nowhere on earth, and most index fossils do not usually overlie each other at the same locality. So, even in those places where all Phanerozoic systems have been assigned, the column is still hypothetical. Locally, many of the systems have not been assigned by the index fossils contained in the strata but by indirect methods that take the column for granted — clearly circular reasoning. Thus the geologic column does not exist and so does not need to be explained by Flood geology. Only each local succession requires an explanation and Flood geology is wholly adequate for this task. Does the geologic column exist? If so, to what extent? With geological periods and epochs extending for hundreds of millions of years the column clearly contradicts the biblical time scale. Thus for many people, the geological column is an obstacle to their accepting a recent Creation and a world-wide Flood as recorded in Scripture. Creationists have shown that the geological column presents no problem to Flood geology. It is nothing more than a hypothetical classification scheme based on selected rock outcrops in Europe, and used flexibly to classify rocks around the world. 12 Anti-creationists have responded that the column is valid, having been built up in a thoroughly logical way long before the theory of evolution was invented, and that many of those who contributed to its building were creationists. 3 One unanswerable argument for the hypothetical character of the column is that nowhere in the world does the complete column exist. The majority of the geological periods are missing in the field. Although anti-creationists usually have not disputed that the column is mostly missing, they have argued that we should not expect the entire column to exist in the field. Erosion, they argue, is why the complete column is never found. 3 Hence they claim that rocks deposited during one period would be eroded away during a later period. So, while those defending the column have invented ad hoc reasons to explain the missing geologic periods, they did not deny the hypothetical nature of the column. Recently however, there have been a number of recurrent claims that the geological column is more than a hypothetical concept and that it actually exists. 4 Some of these claims have been made on the Internet and, as an active creationist scientist, I don't have the time to fan the windmills of debate on this totally unregulated, unrefereed medium. Anyone can say anything on it, no matter how untrue. However, the claims made on this medium should not be ignored completely. We must provide responses from time to time so the critics and their readers don't think their claims are unanswerable. It is on the Internet that a number of geographical localities have been nominated where it has been asserted that the entire column is actually superposed period upon period in the one place. 5 This is one of the few intellectual-sounding arguments on the anti-creationist sites that some people may mistakenly take seriously. Thus I address the bogus arguments of some of these articles relating to the geologic column. I want to examine these claims closely, first correcting common misrepresentations of creationist literature on this subject, then delving into the geologic issues involved.
The age and significance of sequence boundaries on Jurassic to Early Cretaceous rifted continental margins in three ocean basins have been documented. The margins are the Santos basin in the South Atlantic, the Grand Banks in the North Atlantic, and the Beaufort Sea in the Arctic Ocean. Large industry data bases were used for the interpretation of each area. Megasequence boundaries separate the major phases of basin evolution, for example syn-rift and post-rift. Boundaries developed with an average periodicity of 49 m.y. Sequence boundaries define the component parts of each megasequence and developed with a modal periodicity of 10-15 m.y. Out of 27 total boundary ages, most (16) are developed on just one margin. Only two possible age ranges overlap on all three margins. 80% of the megasequence boundaries and 50% of the sequence boundaries show a direct causal connection with coveal faulting and/or folding. The rest of the boundaries appear as unstructured surfaces separating transgressive and/or regressive sedimentary wedges and are interpreted to result from changes in the rate of basin subsidence, sediment input, and long-term eustatic sea level. These data do not support theories advocating synchronous worldwide boundary development resulting from periodic, short-term falls in global eustatic sea level. Only in like basins of the same age, with identical subsidence and sediment input rates, are boundaries likely to develop synchronously. Hence, the concept of global synchroneity of sequence boundary development may well be an illusion created by the similarity in age of the majority of basins studied. As a result of this study, it seems wise to discard the global approach to basin analysis. 13 figures, 1 table.
During the Cenozoic, until the Middle Miocene, the Niger Delta grew through pulses of sedimentation over an oceanward-dipping continental basement into the Gulf of Guinea; thereafter progradation took place over a landward-dipping oceanic basement. A 12,000 m thick succession of overall regressive, offlapping sediments resulted that is composed of three diachronous siliciclastic units: the deep-marine pro-delta Akata Group, the shallow-marine delta-front Agbada Group and the continental, delta-top Benin Group. Regionally, sediment dispersal was controlled by marine transgressive/regressive cycles related to eustatic sea-level changes with varying duration. Differential subsidence locally influenced sediment accumulation. Collectively, these controls resulted in eleven chronostratigraphically confined delta-wide mega-sequences with considerable internal lithological variation. The various sea-level cycles were in or out of phase with each other and with local subsidence, and interfered with each other and thus influenced the depositional processes. At the high inflection points of the long-term eustatic sea-level curve, floodings took place that resulted in delta-wide shale markers. At the low inflection points, erosional channels were formed that are often associated, downdip, with turbidites in low-stand sediments (LSTs). The megasequences contain regional transgressive claystone units (TST) followed by a range of heterogeneous fine-to-coarse progradational or aggradational siliciclastic (para)sequence sets formed during sea-level high-stand (HST). An updated biostratigraphic scheme for the Niger Delta is presented. It also updates a sedimentation model that takes into consideration local and delta-wide effects of sea-level cyclicity and delta tectonics. Megasequences were formed over time intervals of ~5 Ma within individual accurate megastructures that laterally linked into depobelts. The megasequences form the time-stratigraphic frame of the delta and are the backbone for the new delta-wide lithostratigraphy proposed here. Such a new lithostratigraphy is badly needed, in particular because of the vigorous new activity in the offshore part of the Niger Delta (not covered in this contribution). There, as well as in the onshore part of the delta, the traditional lithostratigraphic subdivision of the Cenozoic Niger Delta section into three formations is insufficient for optimum stratigraphic application; moreover, the various informal subdivisions that have been proposed over time are inconsistent.
The geometry and petrology of sedimentary rocks preserved in the three intracratonic basins of the Brazilian craton (Paraná, Parnaiba, and Amazon) indicate the history and character of vertical movements of the cratonic area. Cyclic successions of erosional and depositional events are synchronous on the Brazilian craton and are correlated with cratons of other continents. The principal evolutionary stages of the Brazilian craton are interpreted as tectonic-sedimentary cycles, each represented by a stratigraphic record identified as a sequence, and each distinguished by its own special characteristics. Cambrian-Ordovician sedimentary rocks, representing deposition in paraplatform basins during a transitional stage at the close of the Brazilian orogenic cycle, are assigned to the Alpha sequence. The Beta (Ordovician and Silurian), Gamma (Devonian-early Carboniferous), and Delta (late Carboniferous-Late Permian sequences) and the Delta-A subsequence (Middle Triassic-Jurassic), corresponding to geotectonic cycles of the cratonic stabilization stage, indicate accumulation in large subsiding basins. The succession of facies in each of these sequences documents cyclical changes in the ratio of sediment supply to subsidence. Marine transgression and basin subsidence increased progressively through the Devonian phase of the Gamma sequence and declined thereafter, in a pattern similar to that of the North American craton and the Russian platform. The frequency of stratigraphic discordances in the three lower sequences suggests a higher degree of cratonic upwarping, progressively diminishing to a stage of maximum stabilization during Delta-A deposition, perhaps related to conditions immediately prior to rupture of the Gondwana plate. Breakup of the Gondwana plate, accompanied by volcanism and remobilization of cratonic areas, initiated the reactivation stage during which two geotectonic cycles are recognized. The first, represented by the Epsilon sequence (Cretaceous), began with local subsidence in isolated basins, followed by widespread continental sedimentation under platform conditions. The second reactivation cycle was responsible for accumulation of the Zeta sequence (Cenozoic), which was characterized by thin residual deposits on an extensive Tertiary peneplain and by accumulations in Quaternary basins, the latter of minor importance except where adjacent to the uplifted eastern continental margin.
The concept of major rock-stratigraphic units of interregional scope was introduced in 1948 (Longwell, 1949). It is now possible to restate the concept and to define more explicitly the sequences delimited by interregional unconformities in the continental interior of North America. The sedimentary record of the North American craton from late Precambrian to present is characterized by six major unconformities. These interregional unconformities subdivide the cratonic stratigraphic column into six sequences - major rock-stratigraphic units (of higher than group, megagroup, or supergroup rank) which can be identified, where preserved, in all cratonic interior areas. At the cratonic margins the bounding unconformities tend to disappear in continuous successions, and the cratonic sequences are replaced by others controlled by events in the marginal basins and eugeosynclinal borders. Although the time values of the unconformities vary widely because of differences in degree of nondeposition and amount of erosion, the approximate dates of the regressional maxima represented are: (1) very late Precambrian, (2) early Middle Ordovician, (3) early Middle Devonian, (4) "post-Elvira" Mississippian, (5) early Middle Jurassic, and (6) late Paleocene. A seventh major regression is now in progress.
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