Charles A. Ross

Sea-level changes: An integrated approach

Sea-Level Changes: An Integrated Approach - In October 1985, SEPM sponsored a four-day conference entitled ?Sea-Level Changes ? An Integrated Approach.? The purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea-level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues. The conference was successful in bringing together scientists from industry, academia, and government, representing all of the major geosciences disciplines. Presentations of many new papers, plus significant releases of data that were previously held proprietary, provided fertile ground for discussion. This much-cited volume represents the best of the material presented at the conference. Includes the early ?Vail? chart.

Late Paleozoic depositional sequences are synchronous and worldwide

More than fifty transgressive-regressive depositional sequences are present in Carboniferous and Permian shallow marine successions on stable cratonic shelves worldwide. These were synchronous depositional events resulting from eustatic sea-level changes that generally ranged from 100 to 200 m. Each transgressive-regressive sequence is correlatable using current fossil knowledge. They average about 2 m.y. and range from 1.2 to 4 m.y. in length. The presence within these strata of numerous, synchronous unconformities of considerable duration and worldwide extent suggests that the fossil record is very incomplete and that we are studying a punctuated fossil record and not a punctuated evolution based on a highly irregular mutation rate. These late Paleozoic transgressive-regressive depositional sequences facilitate correlations because depositional histories of a rock succession can support interpretations of faunal assemblages and faunal similarities in evaluating age relationships.

Paleoclimate of the Kimmeridgian/Tithonian (Late Jurassic) world: I. Results using a general circulation model

Abstract The Kimmeridgian/Tithonian (154.7−145.6 Ma) (middle and late Late Jurassic) was a time of expanded continental rifting, increased sea-floor spreading, and a relatively high eustatic sea level stand. These processes collectively caused the fragmentation and flooding of the megacontinent Pangea as well as the alteration of the global paleoclimate. Using a version of the Community Climate Model (CCM) from the National Center for Atmospheric Research, we report two Kimmeridgian/Tithonian paleoclimate seasonal simulations, with geologically inferred paleotopography: one using a CO 2 concentration of 280 ppm (pre-industrial level) and the other 1120 ppm. Increasing the CO 2 four-fold warms virtually the entire planet. The greatest warming occurs over the higher latitude oceans and the least over the equatorial and subtropical regions. Simulation of a warmer planet with an elevated greenhouse effect fits the distribution of paleoclimatically sensitive faunas, floras, and sedimentary rocks. Model results indicate that sea ice was restricted to the high latitudes of the Boreal and Austral seas, making landfall only in restricted areas. The trade winds bring heavy rainfall in December/January/February to eastern Gondwana and in June/July/August to the Tethys Sea margins. A strong June/July/August monsoon occurs over southeast Asia. The distribution of coals correlates to precipitation sufficient to maintain gymnosperm forests and coastal areas where water saturated sediments are a result of eustatic high stands of sea level. Evaporites are localized to areas of negative precipitation-evaporation. Runoff is restricted to regions of intense precipitation. Overall, the 1120 ppm CO 2 simulation provides a reasonable paleoclimate for the Kimmeridgian/Tithonian and provides a standard until a CCM with oceanic heat transport, a coupled atmospheric/oceanic model, or one with a finer grid cell configuration is available. The results need further scrutiny in areas with more detailed geologic information.

Carboniferous and Early Permian biogeography

During the Carboniferous, changes in the biogeographical distribution of shelf-dwelling, benthic marine invertebrates were made in response to changes in physical paleogeography and climatic variations. Calcareous foraminifers and bryozoans are principal examples of the general trends during the Early Carboniferous, which show that Tournaisian and early and middle Visean faunas were broadly cosmopolitan in a circumequatorial belt and that latitudinal diversity gradients were relatively minor. During the later part of the Visean and early part of the Namurian, the Hercynian orogeny, caused by the collision of Euramerica with Gondwana, disrupted these cosmopolitan equatorial faunal patterns. This was also a time of progressively cooler temperatures throughout the world, of dramatic reduction in faunal diversity, and of high rates of extinction of both species and genera. During middle Carboniferous time, strongly provincial faunas were common. Spasmodic, but limited, dispersals gave a few widespread genera significantly different stratigraphic ranges in different provinces so that Tethyan and non-Tethyan distributions are recognizable. Faunal diversity gradually increased during the middle Carboniferous and then declined at the end of the epoch with additional high levels of extinction of species and genera. During late Carboniferous time, several new faunal lineages became well established. Some filled vacant ecological niches; however, others took advantage of the increase in the number of niches that became available because of gradually warming climates. Provinciality continued to be pronounced. Diversity gradually increased and continued to do so through the Early Permian.

Paleozoic evolution of southern margin of Permian basin

Prior to Late Pennsylvanian time, the Permian basin of West Texas was a part of the southern margin of the North American craton and was the site of shallow, warm-water carbonate deposition. As the South American margin of Gondwana gradually moved northwestward during late Paleozoic time, the oceanic floor deposits and overlying turbidite fan deposits between North America and South America became parts of large accretionary wedges of semiconsolidated sediments. In a series of steps, these wedges were folded and thrust toward the northwest against, then finally onto, the southern margin of North America. These compressive steps caused the repeated piling up of the accretionary wedges to form rapidly eroding highlands on the cratonic margins. Repeated loading on the North American margin formed a series of elongate, deep-water, depositional troughs (fore-deeps) immediately north and west of the accretionary wedges, and these received thick accumulations of turbidites. The youngest of these deep basins include the northwestern part of the Val Verde basin to the north and the Marfa basin to the west which received basinal deposits from within Desmoinesian into early Late Permian time. Loading of the cratonic margin also resulted in renewed faulting along generally northwest- to north-trending older zones of weakness. These zones are apparently of Precambrian age. On the craton, the late Paleozoic faults mainly had high-angle to vertical fault planes and commonly had large components of horizontal displacement. This fault system outlines the major uplifts and basins in the Permian basin region. The joining together of Gondwana and Euramerica across the Marathon salient of the orogenic belt was essentially completed by mid-Wolfcampian time. After that time, the southern margin of the Permian basin, represented by the Dugout and Marathon allochthons and the Diablo Platform, became a relatively stable tectonic area. The Glass Mountains expose a series of north- and northwest-dipping cuestas of Permian carbonate shelf and shelf-edge deposits that accumulated on a platform constructed from these two allochthonous accretionary wedges. These Permian deposits prograded north and northwestward into the narrow Hovey Channel, which connected the Marfa basin with the southern end of the Delaware basin. Carbonate-reef growth finally restricted the inflow of marine water into the Delaware basin near the end of late Guadalupian time.

Permian Sequence Stratigraphy

The Permian System contains a great number and diversity of depositional sequences (Fig. 1) which illustrate sedimentary responses to a series of sealevel fluctuations. These sea-level fluctuations had many different amplitudes and durations, and were accompanied by a wide spectrum of rates of deposition (Fig. 2). The mid-continent and southwestern North American stable cratonic successions serve as the basis for our Permian sea-level interpretations (Ross and Ross 1987a, b, 1988); however, equally useful sections appear to be present in China, particularly South China. In the southern hemisphere, Western Australia has marine and glacial-marine depositional sequences that may eventually help tie sea-level events in high and middle latitudes of Gondwana with those of low latitudes of cratonic North America and the Tethys.

Late Early Silurian (Wenlockian) general circulation model-generated upwelling, graptolitic black shales, and organic-rich source rocks—An accident of plate tectonics?

Through an evolutionary process of plate tectonics, the Wenlockian physical world was composed of an oceanic northern hemisphere and a mainly continental southern hemisphere dominated by the giant continent of Gondwana. The high-latitude position of Gondwana placed much of its extensive margin in the middle latitudes. Laurentia and Baltica occupied an equatorial position; Siberia and Kazakh were to the north. The global paleoclimate was created and driven by the diverse paleogeography of the two hemispheres. In the model, the northern hemisphere is dominated by strong zonality in all seasons. In contrast, the continental southern hemisphere reacts to wide temperature ranges of summer heating and winter cooling of Gondwana. The extensively flooded, ∼28000 km margin of Gondwana occupied the zone of the southern hemisphere westerlies. These westerlies produced intense surface upwelling and, by extension of present analogies, high primary biologic productivity. Gondwanas northern margin extended far enough toward the equator that significant winter sea ice did not form in the southern hemisphere: this allowed a vertically stratified sea to develop, as well as probable dysaerobic and anoxic conditions. A high organic flux of marine plankton accumulated on the sea floor, where it was subsequently buried and preserved. A strong positive correlation exists between upwelling generated by a general circulation model (GCM) on Gondwanas northern margin and the distribution of southern hemisphere basins with source rocks containing types I and II kerogens.

Late Paleozoic collision of North and South America

The geographic distribution of the Early Permian Midcontinent-Andean fusulinacean faunal realm and the regional histories with similar depositional and tectonic patterns indicate that the Ouachita-Marathon geosyncline was adjacent to Colombia and Venezuela during the collision of North and South America. The age relationships of folding, thrust faulting, and metamorphism show that this collision was completed from east to west during latest Pennsylvanian to Early Permian time.

Pennsylvanian and Early Permian Depositional History, Southeastern Arizona

In southeastern Arizona the Black Prince Limestone, Horquilla Limestone, Earp Formation, Colina Limestone, and Supai Formation are Pennsylvanian and Lower Permian strata with complex transgressive and regressive relations. Subdivision of these formations into easily recognizable lithologic members is possible only above the basal unconformities of the Black Prince and Horquilla Limestones where red clastic sediments intertongue with the predominantly limestone sequence. Lithologic facies changes in the upper part of the Horquilla Limestone pass laterally and vertically into the Earp Formation which intertongues upward with the overlying Colina Limestone and on the north with the lower part of the Supai Formation. Regional unconformities are common and 15 are recognized an traced as stratigraphic datum planes subdividing Pennsylvanian and Lower Permian strata into depositional units. These depositional units, dated by fusulinaceans, include Morrowan to early late Wolfcampian deposits. Only two regional hiatuses of long duration occur: one within the Derryan Series and the other at the base of the Missourian Series. Longer hiatuses occur locally where parts of the succession are missing because of lack of deposition or because of erosion associated with local structural adjustments. Four main tectonic features influenced sedimentary environments in the region: the Papago inner shelf in the southwest received shallow marine sediments; the San Pedro outer shelf in the central part of the region received slightly deeper water sediments; the Mogollon inner shelf on the north and northeast was frequently emergent and received principally clastic deltaic sediments; and the Pedregosa basin in the southeast was rimmed with carbonate banks and shoals and its center, particularly during Late Pennsylvanian and Early Permian time, was the site of thick clastic sedimentation. Contemporaneous faulting locally accompanied major changes in lithofacies on the Mogollon shelf. A complicated set of faulted blocks separated the San Pedro outer shelf from the Pedregosa basin and diffe ences in times of deposition and erosion, thicknesses, and lithologies suggest that each fault block had an independent history of structural adjustment and sedimentary conditions.

Foraminiferal zonation of late Paleozoic depositional sequences

From the later part of the Devonian through the Permian, calcareous foraminifers became abundant and evolved rapidly. This rapid evolution of taxa forms the basis of a detailed zonation through the Carboniferous and Permian. Comparison of this evolutionary history of foraminifers, their biostratigraphic zonation, and the depositional sequences in which they occur suggests that sea-level events in late Paleozoic depositional history contributed significantly in subdividing a fairly continuous evolutionary record into a succession of about 75 identifiable foraminiferal zones during a 100–125 Myr time span. Although variable in terms of duration and vertical occurrences, the more completely recorded high-stand intervals give brief histories of the foraminiferal evolutionary record and are sandwiched between the poorly recorded or unrecorded low-stand intervals. Many of the individual foraminiferal zones are confined to a single depositional sequence. The late Paleozoic carbonate foraminiferal fossil record, as with the rest of the fossil record, is strongly affected by sediment deposition-nondeposition as a result of major changes in sea level. This incomplete fossil record is the result of repeated depositional breaks because of the way that depositional sequences form. It is not possible to ascribe macromutations, ‘punctuated’ evolution or ‘punctuated gradualism’ as the cause of this evolutionary pattern of the shelf-carbonate fossil record. This pattern is distinctive and we refer to it as ‘sequence evolution’ and ‘sequence extinction’. In the later part of the Middle Permian and in the Late Permian, the fossil record clearly illustrates that a series of faunal losses through ‘sequence extinctions’ progressively exceeded faunal replacements and new species through ‘sequence evolution’, but not a ‘mass extinction’ as is commonly ascribed to the end of the Permian Period. Most Permian faunas became extinct in the interval of 8 to 4 million years before the end of the Late Permian.