Effects Of Eustasy Research Articles

Tectono‐stratigraphic signature of multiphased rifting on divergent margins (deep‐offshore southwest Iberia, North Atlantic)

Regional 2D multichannel seismic, borehole, dredge and outcrop data, together with burial models for strata in southwest Iberia, are used to investigate the tectono‐stratigraphic signature of multiphased rifting on divergent margins. Our burial model reveals that Mesozoic extension occurred during three main phases, each comprising distinct subsidence pulses separated by short‐lived periods of crustal uplift. The importance of the three phases varies across discrete sectors of the margin, each one revealing similar depositional architectures and associated tectonic systems tracts: 1) the Rift Initiation phase, characterized by incipient subsidence and overall aggradation/progradation over a basal unconformity, 2) the Rift Climax phase, which marks maxima of tectonic subsidence and is characterized by retrogradation‐progradation, and 3) the Late Rift phase, recording the progradational infill of the basin and the effects of eustasy. The Rift Initiation systems tracts comprise Sinemurian and late Callovian‐early Oxfordian strata. Marine units in the Pliensbachian and Late Oxfordian‐Kimmeridgian represent the Rift Climax phase, a period marked by the development of Maximum Flooding Surfaces. Late Rift deposits were identified in the Rhaetian‐Hettangian, Toarcian‐Bathonian and Kimmeridgian‐Berriasian. The results of this work are important to the economic exploration of deep‐offshore rift basins, as they reveal that sequence stratigraphy can be used to predict sedimentary facies distribution in more distal segments of such basins. Significantly, this work recognizes that multiple tectonic‐stratigraphic (rift) cycles can occur on deep‐offshore rift basins, from the onset of rift‐related extension until continental break‐up, a character that contrast to what is known from deep‐sea drilling data from the distal margin of Northwest Iberia.

Importance of high-frequency tectonic sequences during greenhouse times of Earth history

The relative importance of tectonism in the stratigraphic record should be more clearly expressed during greenhouse times of Earth history due to the lack of overmasking effects of high-frequency and high-amplitude sea-level changes typical for icehouse periods. Establishment of the importance of tectonism, especially at temporal scales comparable to durations of typical transgressive-regressive cycles, has been plagued by poor temporal resolution. Our outcrop and subsurface-based study of a Cenomanian shallow-marine siliciclastic interval, constrained by bentonite-based geochronology and detailed biostratigraphy, examines this problem. In a 2.2 m.y. interval, we identified four tectonically driven erosional surfaces that dominate preserved stratigraphy. Biostratigraphic correlation to a sea-level curve for the Cenomanian—where coeval high-frequency lowamplitude eustatic cyclicity has been demonstrated—allows the first direct comparison of the effects of eustasy and tectonics at temporal scales of hundreds of thousands of years during a global greenhouse time. We suggest that minor tectonic pulses locally overshadow the effects of eustasy and exert the dominant control over preserved stratigraphy. While subtle tectonic control on sedimentation has been documented throughout the Cretaceous Western Interior, the results of this study suggest that much of that deformation occurred at sub-million-year frequencies and at time scales comparable to eustatic transgressiveregressive cycles.

Sequence stratigraphy as a scientific enterprise: the evolution and persistence of conflicting paradigms

In the 1970s, seismic stratigraphy represented a new paradigm in geological thought. The development of new techniques for analyzing seismic-reflection data constituted a “crisis,” as conceptualized by T.S. Kuhn, and stimulated a revolution in stratigraphy. We analyze here a specific subset of the new ideas, that pertaining to the concept of global-eustasy and the global cycle chart published by Vail et al. [Vail, P.R., Mitchum, R.M., Jr., Todd, R.G., Widmier, J.M., Thompson, S., III, Sangree, J.B., Bubb, J.N., Hatlelid, W.G., 1977. Seismic stratigraphy and global changes of sea-level. In: Payton, C.E. (Ed.), Seismic Stratigraphy—Applications to Hydrocarbon Exploration, Am. Assoc. Pet. Geol. Mem. 26, pp. 49–212.] The global-eustasy model posed two challenges to the “normal science” of stratigraphy then underway: (1) that sequence stratigraphy, as exemplified by the global cycle chart, constitutes a superior standard of geologic time to that assembled from conventional chronostratigraphic evidence, and (2) that stratigraphic processes are dominated by the effects of eustasy, to the exclusion of other allogenic mechanisms, including tectonism. While many stratigraphers now doubt the universal validity of the model of global-eustasy, what we term the global-eustasy paradigm, a group of sequence researchers led by Vail still adheres to it, and the two conceptual approaches have evolved into two conflicting paradigms. Those who assert that there are multiple processes generating stratigraphic sequences (possibly including eustatic processes) are adherents of what we term the complexity paradigm. Followers of this paradigm argue that tests of the global cycle chart amount to little more than circular reasoning. A new body of work documenting the European sequence record was published in 1998 by de Graciansky et al. These workers largely follow the global-eustasy paradigm. Citation and textual analysis of this work indicates that they have not responded to any of the scientific problems identified by the opposing group. These researchers have developed their own descriptive and interpretive language that is largely self-referential. Through the use of philosophical and sociological assumptions about the nature of human activity, and in particular the work of Thomas Kuhn, we have attempted to illustrate (1) how the preconceptions of geologists shape their observations in nature; (2) how the working environment can contribute to the consensus that develops around a theoretical approach with a concomitant disregard for anomalous data that may arise; (3) how a theoretical argument can be accepted by the geological community in the absence of “proofs” such as documentation and primary data; (4) how the definition of a situation and the use or non-use of geological language “texts” can direct geological interpretive processes in one direction or another; and (5) how citation patterns and clusters of interrelated “invisible colleges” of geologists can extend or thwart the advancement of geological knowledge.

Sequence stratigraphy of Miocene-Pliocene carbonate-siliciclastic shelf deposits in the eastern Mediterranean margin (Israel): effects of eustasy and tectonics

Mio-Pliocene marine sediments comprise a discontinuous succession of carbonates and mixed carbonate siliciclastics on the Neogene shelf, which coincides with the Judean foothills area. On the slope (present Mediterranean coastal plain) the succession forms a wedge up to 1500 m thick of mostly fine-grained siliciclastics. The facies and architecture of the succession were influenced both by sea-level fluctuation and by the geodynamic change from thermally subsiding passive margins of the Arabo-African plate to a complex pattern of vertical movements that were associated with the breakup of the plate along the Red Sea Rift and Dead Sea Transform and the convergence of the African-Arabian and Eurasian plates. The Miocene eustatic succession is highly interrupted by vertical uplift tectonics, while the Pliocene eustatic succession is well represented despite the limited subsidence rates. During most of the Early and Middle Miocene the shelf was tectonically uplifted above sea level, except for a short period of flooding in early-Middle Miocene times (N8, N9), when sea-level rise outstripped the uplift trend and a carbonate platform formed. The combination of tectonic uplift and sea-level fall prior to Late Miocene times (about 11 m.y. ago) resulted in the incision. of a deep Beer Sheva Canyon, about 280 m below the shelf plane. Late Miocene sandy marlstones of the Bet Eshel cycle filled the Beer Sheva Canyon and spilled over it. They display a shallowing-upward trend, resulting in progradational clinoform stacking of the highstand systems tract, and channelled debris flows of the forced-regression systems tract. The Messinian desiccation event left the shelf emerged while depositing evaporites on the slope and in the basin. The degree of erosion during the Messinian was relatively minor because of arid conditions and limited runoff. The Pliocene shelf shows a very low subsidence rate and compared to the Miocene succession is less affected by vertical uplifts. The Pliocene deposits are divided into six sequences. Their relative onlap curve largely corresponds to the sequence stratigraphic scheme in Sicily and the Gulf of Mexico. The two lowermost sequences (I and II) are composed of two sandy units; the lower forms an incised valley fill of shoreface deposits in the Beer Sheva area, and offshore sand deposits in distal-shelf areas (Besor). Although largely eroded, sequences I and II display maximum coastal onlap of all Pliocene cycles in the Beer Sheva Canyon area. Sequence III consists of mixed carbonate-siliciclastic grainstones of shoreface/foreshore deposits in the proximal area, and lower shoreface deposits in more distal areas. Sequences IV and V may be considered as a single 3rd-order cycle, where IV represents regressive lowstand (shale and sand) deposits and V represents prograding highstand (sandy grainstone) deposits. Sequence VI consists of marginal-marine to coastal plain (marlstones and sands) deposits and is largely truncated by the Ahuzam Conglomerate, representing a major uplift phase of latest Pliocene to Early Pleistocene age and marking the permanent emergence of the shelf.

Lower and lower Middle Pennsylvanian fluvial to estuarine deposition, central Appalachian basin: Effects of eustasy, tectonics, and climate

Interpretations of Pennsylvanian sedimentation and peat accumulation commonly use examples from the Appalachian basin because of the excellent outcrops and large reserve of coal (>100 billion metric tons) in the region. Particularly controversial is the origin of Lower and lower Middle Pennsylvanian quartzose sandstones; beach-barrier, marine-bar, tidal-strait, and fluvial models all have been applied to a series of sand bodies along the western outcrop margin of the basin. Interpretations of these sandstones and their inferred lateral relationships are critical for understanding the relative degree of eustatic, tectonic, and climatic controls on Early Pennsylvanian sedimentation. Cross sections utilizing >1000 subsurface records and detailed sedimentological analysis of the Livingston Conglomerate, Rockcastle Sandstone, Corbin Sandstone, and Pine Creek sandstone (an informal member) of the Breathitt Group were used to show that each of the principal quartzose sandstones on the margin of the central Appalachian basin contains both fluvial and marginal marine facies. The four sandstones are fluvially dominated and are inferred to represent successive bed-load trunk systems of the Appalachian foreland. Base-level rise and an associated decrease in extra-basinal sediment at the end of each fluvial episode led to the development of local estuaries and marine reworking of the tops of the sand belts. Each of the sand belts is capped locally by a coal, regardless of whether the upper surfaces of the sand belts are of fluvial or estuarine origin, suggesting allocyclic controls on deposition. Peats were controlled by a tropical ever-wet climate, which also influenced sandstone composition through weathering of stored sands in slowly aggrading braidplains. Recurrent stacking of thick, coarse-grained, fluvial deposits with extra-basinal quartz pebbles; dominance of bed-load fluvial–lowstand deposits over mixed-load, estuarine-transgressive deposits; thinning of sand belts around tectonic highs and along faults; cratonward shift and amalgamation of successive sand belts on the margin of the basin; and truncation of successive sand belts toward the fault-bound margin of the basin are interpreted as regional responses to Alleghenian tectonism, inferred to have been the dominant control on accommodation space and sediment flux in the Early Pennsylvanian basin.

Regional Cretaceous Stratigraphic Relationships across the Southern Segment of the Western Interior Basin, Arizona, New Mexico, and Southern Colorado

Cretaceous strata in Arizona, New Mexico, and southern Colorado were deposited in the southern part of the asymmetric Western Interior basin and consist of east- and northeastward-prograding clastic wedges of shoreface sandstones and coastal-plain deposits separated by thick tongues of west- and southwestward-extending marine shale and, on the eastern shelf, by limestone. The interlayering of the two primary facies (sandstone and shale) is the result of either regional subsidence, eustasy, sediment supply, or a combination of these. Constructing and comparing regional cross sections will aid in accomplishing one of the goals of the Western Interior Cretaceous (WIK) project of the International Union of Geological Science's Global Sedimentary Geology Program, which is to identify the global effects of eustasy. Preserved Cretaceous strata in this part of the Western Interior basin range from a few hundred feet thick on the east to about 7000 ft on the west. A cross section across this area reveals facies relations, major sequence boundaries, unconformities, and available biostratigraphic and chronostratigraphic data for the sections preserved in the present Laramide basins. Sequence boundaries occur in the middle late Albian, at the base of the late Turonian, and within the latest Turonian. Younger sequence boundaries, if present, aremore » less obvious, and their associated unconformities have not been identified.« less

The Combined Effects of Eustasy, Tectonism, and Clastic Influx on the Development of Pennsylvanian Cyclic Carbonates, Southern Sangre de Cristo Mountains, New Mexico

Pennsylvania cyclothems are well documented on stable continental shelves and the cyclicity has generally been attributed to glacio-eustasy. As a contrast, Atokan-Desmoinesian cyclic carbonates of the southern Sangre de Cristo Mountains developed in a tectonically active foreland basin, formed by thrusting along the Picuris-Pecos fault during early Pennsylvanian time. Strata exposed in two sections (Dalton Bluff, 260 m; Johnson Mesa, 340 m) are characterized by (1) shallowing-upward cycles, (2) cycles of variable thickness (5-20 m), (3) incomplete cycles, (4) cycles interrupted by terrigenous clastic input, and (5) noncyclic intervals. Allocyclic mechanisms alone cannot fully explain these observations; the authors herein propose that a complex interplay among eustasy, tectonism, and clastic sediment supply were responsible for the observed cycles. Lithofacies analysis indicates that location within the foreland basin played a significant role in cycle attributes. In the deeper portions of the basin (e.g., Dalton Bluff), an idealized cycle, from base to top consists of (1) shale/marl facies, (2) brachiopod wackestone facies, (3) phylloid algal facies, and (4) marine clastic facies. No evidence for subaerial exposure of cycle caps is noted. In contrast, in shallow portions of the basin near the forebulge (e.g., Johnson Mesa) the marine clastic facies is substituted bymore » crinoidal grainstone/packstone facies that is capped by subaerial exposure surface. Each of the two cycles displays an overall grand (lower order) shallowing-upward cycle. This grand cycle developed as sediments infilled the initially starved foreland basin.« less