Lateral Changes In Thickness Research Articles

Using Ground-Penetrating Radar (GPR) to Investigate the Exceptionally Thick Deposits from the Storegga Tsunami in Northeastern Scotland

A submarine landslide on the edge of the Norwegian shelf that occurred around 8150 ± 30 cal. years BP triggered a major ocean-wide tsunami, the deposits of which are recorded around the North Atlantic, including Scotland. Ground-penetrating radar (GPR) was used here to investigate tsunami sediments within estuaries on the coast of northeastern Scotland where the tsunami waves were funnelled inland. Around the Dornoch Firth, the tsunami deposits are up to 1.6 m thickness, which is exceptionally thick for tsunami deposits and about twice the thickness of the 2004 IOT or 2011 Tohoku-oki tsunami deposits. The exceptional thickness is attributed to a high sediment supply within the Dornoch Firth. At Ardmore, the tsunami appears to have overtopped a beach ridge with a thick sand layer deposited inland at Dounie and partly infilled a valley. Later, fluvial activity eroded the tsunami sediments locally, removing the sand layer. At Creich, on the north side of the Dornoch Firth, the sand layer varies in thickness; mapping of the sand layer with GPR shows lateral thickness changes of over 1 m attributed to a combination of infilling an underlying topography, differential compaction, and later reworking by tidal inlets. Interpretation of the GPR profiles at Wick suggests that there has been a miscorrelation of Holocene stratigraphy based on boreholes. Changes in the stratigraphy of spits at Ardmore are attributed to the balance between sediment supply and sea-level change with washovers dominating a spit formed during the early Holocene transgression, while spits formed during the subsequent mid-Holocene high-stand are dominated by progradation.

Lateral and vertical characteristics of floodplain aggradation cycles in the lower Eocene Willwood Formation, Bighorn Basin, Wyoming, USA

Sedimentation on river floodplains is a complex process that involves overbank flooding, crevasse splaying, and river avulsion. The resulting floodplain stratigraphy often exhibits floodplain aggradation cycles with alternating fine-grained overbank flooding deposits that underwent significant petrogenesis, and coarser-grained, avulsion-belt deposits largely devoid of pedogenic impact. These cycles are linked to lateral migration and avulsion of channels driven by internal dynamics, external factors, or a combination of both. To better understand the spatial and vertical variability of such floodplain aggradation cycles, we map these in three dimensions using a photogrammetric model of the lower Eocene Willwood Formation in the northern Bighorn Basin, Wyoming, USA. This allows identifying 44 floodplain aggradation cycles in ∼300 m of strata with an average thickness of 6.8 m and a standard deviation of 2.0 m. All the cycles are traceable over the entire model, pointing to their spatial consistency over the 10 km2 study area. At the same time, rapid lateral thickness changes of the floodplain aggradation cycles occur with changes up to 4 m over a lateral distance of 400 m. Variogram analyses of both field and numerical-model results reveal stronger consistency of floodplain aggradation cycle thicknesses along the paleoflow direction compared to perpendicular to paleoflow. Strong compensational stacking occurs at the vertical scale of 2−3 floodplain aggradation cycles (14−20 m), while full compensational stacking occurs at larger scales of more than six floodplain aggradation cycles (>41 m). The lateral and vertical thickness variability of the floodplain aggradation cycles, as well as their compensational stacking behavior, are interpreted to be dominantly driven by autogenic processes such as crevasse splaying and avulsing that preferentially fill topographic lows. External climate forcing may have interacted with these autogenic processes, producing the laterally persistent and vertically repetitive floodplain aggradation cycles. The spatial variability of floodplain aggradation cycles demonstrated in this study highlights again the need for three-dimensional data collection in alluvial floodplain settings rather than depending on one-dimensional records.

Planetary analogue study using microseismic analysis for near-surface lava tube detection and exploration

The paper presents new results from microseismic data analysis for the detection and delineation of near-surface lava tubes. Single-station, free-field seismic noise data were collected at the Corona Volcano area (Lanzarote, Canary Islands) as part of the PANGAEA-X 2017 European Space Agency (ESA) astronaut training campaign. The site was selected as it represents a suitable analogue for lunar maria and provided the opportunity to cross-validate and ground truth seismic and other geophysical studies. In this paper, we present our observations on the distribution of frequencies and amplitudes of standing waves generated by microtremors in the space between the ground surface and underground cavities by averaging amplitude spectra of microseismic records.This study shows that the frequency of the vertical component peaks is a relatively good indicator of presence, lateral extent and approximate dip of the cavities. However, only the vertical component peaks with a frequency either equal to or close to central to high amplitude (3.1–8.5) horizontal-to-vertical (H/V) peaks are related to lava tubes. If the P-wave velocities in the rocks overlying the lava tubes are known, then these frequencies can also be used to estimate the depth of such cavities.This study also identified some important pitfalls that may limit the use and accuracy of our approach. These include layering of rocks overlying the lava tubes, lateral changes in thickness, lateral variations of the P-wave velocity, and the presence of shallow void-rich zones.Overall, the results of this study confirmed that microseismic data are suitable for cavity detection. It is also, to our knowledge, one of the first studies dealing with microseismic data from near-surface lava tubes. With the recent increase in planetary exploration, this method can be easily adapted to support the geophysical exploration of volcanic terrains on the Moon and Mars.

Fluvial architecture and sequence stratigraphy of the Burro Canyon Formation, southwestern Piceance Basin, Colorado

The Lower Cretaceous Burro Canyon Formation in the southwestern Piceance Basin, Colorado, is composed of deposits that represent a braided fluvial system with high net to gross that transitions stratigraphically upward into a low net-to-gross, low-sinuosity, meandering fluvial system. The fluvial deposits are composed of multiple upward-fining, conglomeratic-to-sandstone successions forming bars and bar sets that exhibit inclined heterolithic strata that we have interpreted to have formed by oblique and downstream accretion. We used well-exposed outcrops, detailed measured sections, and unmanned aerial system-based imagery to describe the fluvial architecture of the Late Cretaceous formation using a hierarchical approach. We described the Burro Canyon Formation as comprising sandstone-rich amalgamated channel complexes (ACC) overlain by non- to semiamalgamated channel complexes. The lower interval of the formation is composed of ACC that contain channel-fill elements with cross-stratification and numerous truncated contacts. These stacked channel-fill elements exhibit an apparent width range of 137–1300 ft (40–420 m) and a thickness range of 5–60 ft (1.5–18 m). The upper interval of the Burro Canyon Formation comprises mudstone-prone intervals of the nonamalgamated channel complex with isolated channel-fill elements interbedded with floodplain mudstones that represent a period of relatively high base level. Associate channel fill elements range in apparent width from 200 to 1000 ft (60 to 300 m) and thickness from 20 to 30 ft (6 to 18 m). The characteristics and spatial distribution of architectural elements of the Burro Canyon Formation correspond to one depositional sequence. The erosional basal surface of the formation, as well as lateral changes in thickness and net to gross, suggest that the Burro Canyon Formation within this study area was deposited as an incised valley fill. Fluvial deposits of the Burro Canyon Formation serve as outcrop analogs for subsurface interpretations in similar reservoirs.

Jurassic break-up of the Peri-Gondwanan margin in northern Colombia: Basin formation and implications for terrane transfer

Jurassic extensional basins developed along the northwestern margin of South America during the break-up of Pangea. Presently, these basins are dispersed in several tectonic blocks of the northern Andes and Mexico, hindering reconstruction of western equatorial Pangea before break-up. This is the case of the Cosinas Basin (Guajira block) and the Machiques Basin (Perijá Range), in northern Colombia, which are filled by Jurassic sedimentary and volcano-sedimentary successions. Autochthonous and para-autochthonous hypotheses on the origin of this basins have been proposed. The purpose of this research is to document the sedimentological evolution, depositional age (Sr-isotope + U-Pb geochronology), sediment provenance and paleogeography of the Cosinas and Machiques basins in order to constrain whether these basins formed within a single extensional margin or they formed as extensional basins in different tectonic blocks. Volcanic detrital zircon U-Pb ages documented in La Quinta Formation in the Machiques Basin and at the base of Rancho Grande Formation in the Cosinas Basin suggest that extensional basins were active in Early Jurassic time. However, a significant difference exists in their subsequent history. Whereas in the Machiques Basin dominates the accumulation of Lower and Middle Jurassic volcanoclastic deposits with abrupt lateral thickness changes, accumulation in the Cosinas Basin is dominantly of siliciclastic strata, with the record of two major marine incursions in Late Jurassic time. Integration of provenance results indicates that the Santander Massif supplied sediments to the Machiques Basin. In contrast, Middle to Upper Jurassic sandstones of the Cosinas Basin document unroofing of basement blocks that include metamorphic, sedimentary and plutonic rocks from the Guajira and Maya blocks. The similarity in age and composition of pre-Jurassic rocks in northwestern South America and the so-called peri-Gondwana blocks in the Mexican subcontinent (i.e., Maya and Oaxaquia blocks) challenge the use of detrital zircon population as an indicator of the autochthonous or para-autochthonous origin of the Guajira block. Large uncertainty of paleomagnetic results, and the lack of constraints for the time magnetization acquisition preclude estimating paleolatitudes for the Guajira block in Jurassic time but support previous interpretation of ca. 70°-90° clockwise rotation of the Guajira block relative to stable South America craton.Our preferred paleogeography considers that the Cosinas and Machiques basins were close to each other along the western continental margin of Pangea during the onset of extension in Early Jurassic time. The change from continental to marine depositional environments in Middle to Late Jurassic time along the Cosinas Basin, which have not been identified in the Machiques Basin or other autochthonous Jurassic basins in northwestern South America, allow us to propose that these blocks were separated during the Callovian - Tithonian interval, with the Cosinas Basin remaining closer to a conjugate Mexican margin, that we interpret as the Maya block. Collision of the Guajira block with the South American margin occurred near the Jurassic-Cretaceous boundary, as documented by deformation of Jurassic units previous to deposition of Berriasian strata in the Guajira block.

Facies, depositional environments, and sequence stratigraphy analysis of the upper Barremian-lower Aptian carbonates in the northeast Kelardasht, N Iran

The carbonates accumulated within a time interval spanning the late Barremian-early Aptian, comprise the Member 2 of the Chalous Formation. The member cropped out in the northern Central Alborz Mountains Range, northern Iran. Detailed field surveys, lithofacies and facies analyses was carried out to investigate the depositional environments and sequence stratigraphy analysis at five outcrop sections. Based on petrographic investigation and texture, sixteen facies and microfacies were defined and assigned to the back-barrier lagoon, shelf margin/shoal, and shallow open marine facies belts. Vertical and lateral facies changes, lack of several geological evidence including siliciclastics, bioherms, calciturbidites, clinoforms, together with lateral thickness changes as well as comparison to recent and ancient carbonate platforms, suggest deposition on a non-rimmed isolated carbonate platform. The upper Barremian to lower Aptian deposits of the study areas comprises three third-order depositional sequences.We infer that relative sea-level changes and climate played a deeply influential role in the platform evolution and its geometry on the base of facies analysis, vertical transition of fauna, diagenetic overprints, accompanied by stacking patterns and geometries, and correlation of the depositional sequences.

Sedimentology, diagenesis, clay mineralogy and sequential analysis model of Upper Paleocene evaporite-carbonate ramp succession from Tamerza area (Gafsa Basin: Southern Tunisia)

Integrated sedimentological studies, diagenesis, sequential analysis and clay mineralogy on the Upper Paleocene rocks in Tamerza area provide important information on the reconstruction of the depositional basin, cyclicity, and paleoclimatic contexts. Facies analysis and petrographic studies have led to the recognition of nine facies that were deposited in three facies belts: Sebkha, inner ramp and outer ramp summarized in a carbonate ramp model: Homoclinal ramp under an arid climate.The upward and lateral changes in thickness and composition show a general regressive trend that records a transition from an outer ramp to Sebkha, creating different types of confinement. The facies stacking patterns constitute several kinds of meter-scale, shallowing-upward cycles. Nine different types of depositional cycles and several models of Sebkha sequences were defined. These different types of facies, characterized within the Thelja Formation, compose seven depositional sequences, mainly made of carbonates, marls and evaporates. Detailed multi approach analysis provides important information on evaporitic sequence stratigraphy.In carbonates beds, the diagenetic analysis provides an overview and chronology of diagenetic processes. A particular attention was paid to early stage cementation which enables us to characterize better the depositional environments. In addition to cementation, other features define the diagenetic history.X-ray diffraction reveals the presence of smectite suggesting an arid climate. Moreover, the clinoptilolite and the frequency of primary dolomite indicate different degrees of confinement.The seven depositional sequences showing a hierarchical organization of many cycles, as described above, suggested that eustatic sea level oscillations caused by cyclic perturbations of the Earth's orbit play a fundamental role in determining the formation of hierarchical cyclic rhythmicity.

Pitfalls and limitations in seismic attribute interpretation of tectonic features

Seismic attributes are routinely used to accelerate and quantify the interpretation of tectonic features in 3D seismic data. Coherence (or variance) cubes delineate the edges of megablocks and faulted strata, curvature delineates folds and flexures, while spectral components delineate lateral changes in thickness and lithology. Seismic attributes are at their best in extracting subtle and easy to overlook features on high-quality seismic data. However, seismic attributes can also exacerbate otherwise subtle effects such as acquisition footprint and velocity pull-up/push-down, as well as small processing and velocity errors in seismic imaging. As a result, the chance that an interpreter will suffer a pitfall is inversely proportional to his or her experience. Interpreters with a history of making conventional maps from vertical seismic sections will have previously encountered problems associated with acquisition, processing, and imaging. Because they know that attributes are a direct measure of the seismic amplitude data, they are not surprised that such attributes “accurately” represent these familiar errors. Less experienced interpreters may encounter these errors for the first time. Regardless of their level of experience, all interpreters are faced with increasingly larger seismic data volumes in which seismic attributes become valuable tools that aid in mapping and communicating geologic features of interest to their colleagues. In terms of attributes, structural pitfalls fall into two general categories: false structures due to seismic noise and processing errors including velocity pull-up/push-down due to lateral variations in the overburden and errors made in attribute computation by not accounting for structural dip. We evaluate these errors using 3D data volumes and find areas where present-day attributes do not provide the images we want.

Architecture and evolution of an Early Permian carbonate complex on a tectonically active island in east-central California

The newly named Upland Valley Limestone represents a carbonate complex that developed on and adjacent to a tectonically active island in east-central California during a brief interval of Early Permian (late Artinskian) time. This lithologically unique, relatively thin limestone unit lies within a thick sequence of predominantly siliciclastic rocks and is characterized by its high concentration of crinoidal debris, pronounced lateral changes in thickness and lithofacies, and a largely endemic fusulinid fauna. Most outcrops represent a carbonate platform and debris derived from it and shed downslope, but another group of outcrops represents one or possibly more isolated carbonate buildups that developed offshore from the platform. Tectonic activity in the area occurred before, probably during, and after deposition of this short-lived carbonate complex.

Sedimentological thickness variations within Silurian mudstone-dominated turbidite deposits and the effects on cleavage fanning (Anglo-Brabant Deformation Belt, Belgium)

The effect of vertical changes in lithology on cleavage refraction and cleavage fanning is relatively well understood. In contrast, the control that lateral changes in bed thickness and related multilayer characteristics have on cleavage fanning has not been widely documented. Mudstone-dominated Wenlock-age turbidites of the Anglo-Brabant Deformation Belt, Belgium, exhibit pronounced lateral thickness changes, which we attribute to intraslope flow ponding during foreland basin development. The mudstone-dominated nature of the turbidites is considered to reflect a particularly fine-grained source area, rather than a distal origin. Formation boundaries and lateral changes in lithofacies unit thickness are reflected in the amount of cleavage fanning. The degree of convergent cleavage fanning increases with an increase in thickness of the less competent units and with a decrease in thickness and number of the more competent units. As such, a detailed analysis of changes in cleavage–bedding angle aids in the distinction of different lithostratigraphic units of similar appearance, the location of their boundaries and identification of subtle lateral sedimentological changes. The utilization of cleavage and bedding data to recognize and describe vertical and lateral changes in a semi-quantitative way allows the linkage of subtle changes in multilayer rheology to finite strain trajectories. Supplementary materials: Details of position of lithological profiles are available at www.geolsoc.org.uk/SUP18708 .

Traces In the Dark--Sedimentary Processes and Facies Gradients In the Upper Shale Member of the Upper Devonian-Lower Mississippian Bakken Formation, Williston Basin, North Dakota, U.S.A

Abstract Black, organic-rich rocks of the upper shale member of the Upper Devonian–Lower Mississippian Bakken Formation, a world-class petroleum source rock in the Williston Basin of the United States and Canada, contain a diverse suite of mudstone lithofacies that were deposited in distinct facies belts. The succession consists of three discrete facies associations (FAs). These comprise: 1) siliceous mudstones; 2) quartz- and carbonate-bearing, laminated mudstones; and 3) macrofossil-debris-bearing massive mudstones. These FAs were deposited in three facies belts that reflect proximal to distal relationships in this mudstone system. The macrofossil-debris-bearing massive mudstones (FA 3) occur in the proximal facies belt and contain erosion surfaces, some with overlying conodont and phosphate–lithoclast lag deposits, mudstones with abundant millimeter-scale siltstone laminae showing irregular lateral thickness changes, and shell debris. In the medial facies belt, quartz- and carbonate-bearing, laminated mudstones dominate, exhibiting sub-millimeter-thick siltstone layers with variable lateral thicknesses and localized mudstone ripples. In the distal siliceous mudstone facies belt, radiolarites, radiolarian-bearing mudstones, and quartz- and carbonate-bearing, laminated mudstones dominate. Overall, total organic carbon (TOC) contents range between about 3 and 10 wt %, with a general proximal to distal decrease in TOC content. Abundant evidence of bioturbation exists in all FAs, and the lithological and TOC variations are paralleled by changes in burrowing style and trace-fossil abundance. While two horizontal traces and two types of fecal strings are recognized in the proximal facies belt, only a single horizontal trace fossil and one type of fecal string characterize mudstones in the distal facies belt. Radiolarites intercalated into the most distal mudstones are devoid of traces and fecal strings. Bedload transport processes, likely caused by storm-induced turbidity currents, were active across all facies belts. Suspended sediment settling from near the ocean surface, however, most likely played a role in the deposition of some of the mudstones, and was probably responsible for deposition of the radiolarites. The distribution pattern of high-TOC sediments in proximal and lower-TOC deposits in some distal facies is interpreted as a function of higher accumulation rates during radiolarian depositional events leading to a decrease in suspension-derived organic carbon in radiolarite laminae. The presence of burrows in all FAs and nearly all facies in the upper Bakken shale member indicates that dysoxic conditions prevailed during its deposition. This study shows that in intracratonic high-TOC mudstone successions such as the upper Bakken shale member bed-load processes most likely dominated sedimentation, and conditions promoted a thriving infaunal benthic community. As such, deposition of the upper Bakken shale member through dynamic processes in an overall dysoxic environment represents an alternative to conventional anoxic depositional models for world-class source rocks.

Revealing deep structural influences on the Upper Cretaceous Chalk of East Anglia (UK) through inter-regional geophysical log correlations

New borehole geophysical log interpretations between Wiltshire and north Norfolk show detailed lateral changes in the spatial relationships of Chalk Group marker beds. They show how marker beds in the Turonian and Coniacian Chalk Group in East Anglia pass laterally into their correlatives further west, and reveal unusual lateral thickness changes affecting stratigraphical intervals in the East Anglian succession. Newly enhanced regional gravity and magnetic data indicate that these thickness changes are probably related to WNW to ESE trending structural lineaments in the Palaeozoic basement rocks of the buried Anglo-Brabant Massif.The later part of the Mid Turonian and early part of the Late Turonian succession across East Anglia is greatly thickened, and shows almost no lateral variability. These relatively soft, smooth-textured chalks equate with thinner, hard, nodular beds formed in both shallow marine and deeper basinal settings elsewhere in southern England. Since it seems unlikely that there was greater sediment accommodation space across East Anglia at this time compared to basinal areas, this thickening may reflect a localised coccoliths productivity pulse, or perhaps a sheltered palaeogeographical position that protected the area from sediment-winnowing marine currents.A residual gravity low across north Norfolk, previously interpreted as a granite pluton, may instead represent two elongated (fault-bounded) sedimentary basins.

Timing of basin inversion on the Isle of Wight: New evidence from geophysical log correlation, seismic sections and lateral facies change in the Palaeogene Headon Hill Formation

Paleogene thickness patterns across the Bouldnor Syncline and Porchfield Anticline in the northwestern Isle of Wight have been deduced using outcrop information, borehole correlation, gamma-ray logs and seismic reflection data. The thickness patterns provide evidence for an early phase of basin inversion at around the Bartonian–Priabonian boundary (Late Eocene) in the Isle of Wight. Paleogene strata older than the Becton Sand Formation show little evidence for significant lateral changes in thickness, even though the boreholes are located at various structural positions around the Bouldnor Syncline and Porchfield Anticline. In contrast, both seismic reflection and borehole data provide evidence for marked thinning of Paleogene strata onto the Porchfield Anticline at around the level of the Becton Sand Formation and basal Headon Hill Formation (Totland Bay Member) which probably results from an episode of basin inversion and growth folding. The inversion event was relatively minor and short-lived and continues to point toward the main phase of the basin inversion being late Oligocene or younger. However, it still has important implications for understanding structural control on sedimentation patterns in the Headon Hill Formation, with the migration of sandy channelised depositional systems into the axis of the Bouldnor Syncline, and the sequence stratigraphic significance of the important Bartonian–Priabonian regression event, which may related to tectonics rather than global sea-level change.

新潟堆積盆地，加茂地域における下部〜中部中新統の年代層序と堆積盆発達史

The Kamo area is situated in the eastern margin of the Niigata sedimentary basin, Northeast Japan Arc. The Lower to Middle Miocene successions in the Kamo area are divided into the Tonoiri, Otani, and Nanatani Formations in ascending order. In this study, we show newly determined fission-track ages, chronostratigraphy, and regional stratigraphic correlations of the Lower to Middle Miocene successions, and discuss basin development and tectonics in the Kamo area. The age of each formation is as follows—approximately 18-17 Ma, 17-15.2 Ma and 15.2-12.3 Ma for the Tonoiri, Otani, and Nanatani Formations, respectively. On the basis of the age framework of the successions studied, we can correlate the successions with stratigraphic successions in other areas of the northern Niigata sedimentary basin. In particular, a glauconite sandstone bed in the uppermost part of the Nanatani Formation can be correlated to that intercalated in the boundary horizon between the Nanatani and Teradomari Stages in the Niigata sedimentary basin and that in the boundary horizon between the Nishikurosawa and Onnagawa Stages in the Akita region. Lithofacies and paleobathymetrical data show rapid subsidence and transgressions during the deposition of the Tonoiri and Otani Formations. Furthermore, the Tonoiri and Otani Formations show abrupt lateral changes in lithologies and thickness, which suggests that differential subsidence occurred in the basin, probably in response to rifting of the Eastern Japan Sea Rift System during the Early Miocene between 18 and 15.2 Ma. In contrast, the overlying Nanatani Formation has almost a uniform lithology with minor lateral changes in thickness. This suggests that the rifting may have ceased at around 15.2 Ma and subsequent uniform subsidence seems to have occurred between 15.2 and 12.3 Ma. Consequently, the syn-rift stage sediments can be assigned to the Tonoiri and Otani Formations and the post-rift stage sediments can be assigned to the Nanatani Formation.

Development of carbonate platforms on an extensional (rifted) margin: the Valanginian–Albian record of the Prebetic of Alicante (SE Spain)

Within the upper Valanginian to upper Albian deposits of the easternmost part of the Prebetic Zone of the Betic Cordillera (Iberian Peninsula), seven lithostratigraphic formations made up of shallow-water carbonate and carbonate-siliciclastic sediments and of outer-platform hemipelagic sediments have been recognized. These formations were deposited in the most distal part of a platform that developed on the Southern Iberian Continental Palaeomargin. The geodynamic context was a margin affected by extensional or transtensional faults that produced tilted blocks. The interval studied records three major second-order transgressive-regressive facies cycles: (I) A late Valanginian to earliest Aptian cycle, mostly represented by hemipelagic and condensed sedimentation, with the development of a tectonically controlled high without sedimentation that separated two sectors with different sedimentary evolution and that ended with an episode of shallow-water carbonate platform development; (II) An earliest to latest Aptian cycle, with a transgressive phase represented by a retrogradational shallow-water carbonate platform capped by a drowning event leading to hemipelagic sedimentation, which was affected by an anoxic event (OAE 1a); the regressive phase is represented by progradation and aggradation of shallow-water carbonate deposits. Finally (III) a latest Aptian to early-late Albian cycle that records the expansion of mixed platform deposits in the entire area, ending with a phase of shallow-water carbonate platform development. Extensional tectonics leading to spatial and temporal changes in subsidence patterns is envisaged as the main control on sedimentation at a local scale, resulting in notable lateral changes in thickness as the main signature. Tectonics exerted a strong control on the distribution of sedimentary environments only during Cycle I. At a higher order, sea-level fluctuations are responsible for sequential organization, and environmental factors determined shallow-water carbonate platform development and demise, as well as oceanic anoxic events. The relevant continuity of the stratigraphic record in the distal part of the Prebetic platform has led to the recognition of events related to cycle boundaries, which result mainly from a combination of tectonics and sea-level changes.

Episodic growth of normal faults as recorded by syntectonic sediments, July oil field, Suez rift, Egypt

The July oil field is a major normal-fault–bounded structural block in the Suez rift basin, Egypt. It is adjacent to a major structural transfer zone, which has controlled sediment influx to the rift basin center for the past 20 m.y. The lower to middle Miocene Upper Rudeis Formation, part of the synrift stratigraphic sequence, records deformation of the July structural block. The formation contains abrupt lateral changes in thickness and facies, which record earlier phases of fault movement and deformation in the July field area that do not conform to the present-day structural configuration.The Upper Rudeis Formation was deposited as turbidites in a submarine-fan system sourced from the western rift shoulder. It was deposited over and around bathymetric highs created by coeval fault displacement in the July field area. By studying thickness and facies patterns, we have determined that the present-day main bounding fault to the July block consisted of a series of unlinked fault segments, which linked after Upper Rudeis deposition. A subsidiary fault west of the block exerted the most control on thickness patterns, not the present-day main bounding fault. Thus, commonly used models of deposition in active half grabens are difficult to apply at July field.

Upper Cretaceous tectonic phases and end Cretaceous inversion in the Chalk of the Anglo-Paris Basin

Three intra-Upper Cretaceous tectonic phases, Stille's Ilsede (Late Turonian-Early Coniacian), Wernigerode (Late Santonian-Early Campanian) and Riedel's Peine (latest Lower Campanian) are investigated in the Anglo-Paris Basin. Criteria for recognizing these events include field evidence of slumping, allochthonous chalks; lateral changes in thickness and lithology; seismic evidence for slump horizons; and lacunae closely related to tectonic axes. Further evidence from seismic sections indicates large-scale channel development during these phases but the spatial relationship with tectonic lineaments is more difficult to determine. Each phase rejuvenates the tectonic topography which is subsequently buried by post-tectonic facies changes, for example from Lewes to Seaford Chalk following the Ilsede tectonic phase and Culver to Portsdown Chalk, following the Peine tectonic phase. The tectonic phases are recognized along local tectonic lineaments in contrast to more widespread sea-level fluctuations. New information on the end Cretaceous inversion in the London and Anglo-Paris basins supports the recognition of a London axis of uplift where the chalk at subcrop was more deeply eroded than at outcrop. Maximum inversion in the Weald-Wessex area occurred in East Sussex along the southern margin of the Weald where the thickest chalks had previously been deposited.

Lithostratigraphy of a concealed caldera-related ignimbrite sequence within the Borrowdale Volcanic Group of west Cumbria

SUMMARY A sequence of volcaniclastic rocks, more than 1200m thick, is described from beneath Permo-Triassic and Carboniferous rocks in the Sellafield area of west Cumbria. The rocks are locally weakly cleaved and are considered to be part of the Ordovician Borrowdale Volcanic Group which crops out a few kilometres to the east. Seven formations are defined from six of the deep boreholes drilled by UK NIREX Ltd. The rocks are predominantly high-grade welded ignimbrites. Abrupt lateral thickness changes and intraformational collapse mesobreccias characterize the sequence. The ignimbrites range from andesite to high-SiO 2 rhyolite. Individual formations show distinctive vertical zonation profiles. Compositions fall on a common magmatic trend, indicative of a similar fractionation history. There is no overall compositional trend with time through the succession, though the high-SiO 2 rhyolites were probably erupted early in the cycle. Deposition is considered to have been within an actively subsiding basin, probably a caldera complex. This is considered to have had an episodic history, with periods of caldera collapse following eruptions during which an evolving magma chamber was tapped.

Eastern Venezuela Basin's Post-Jurassic Evolution as a Passive Transform Margin Basin

Passive transform margins are segments of rifted continental margins bounded by transform faults that are active during rifting and that become inactive during drifting. Examples include the northern coast of Brazil and its matching margin along the Liberia-Nigeria coast. We propose that the northern margin of the Eastern Venezuela Basin was dominantly a passive transform margin during the Cretaceous and early Paleogene, rather than a purely passive margin. Published microplate reconstructions of the southern Caribbean show Jurassic separation of the Bahamas platform from northern South America along a northwest-trending transform fault postulated to lie just northeast of Trinidad and the Guianas. We conjecture that the [open quotes]Deflexion de Barcelona[close quotes] (a northwest-trending zone of strike slip faults along the southwestern edge of the Serrania del Interior) is controlled by a basement geofracture that is the onshore expression of Jurassic transform fault southwest of and subparallel to the southwestern Bahamas transform. Implications of this conjecture for the Eastern Venezuela Basin include: (1) absence of McKenzie-type regional crustal stretching, Mesozoic thermal anomaly, and Mesozoic thermal-tectonic subsidence; (2) abrupt rather than gradual seaward changes in crustal thickness; (3)abrupt lateral changes in thickness and facies of Mesozoic sediments, as in the Piaui-Ceara basins ofmore » northern Brazil; (4) tendency for structural styles developed during Neogene compression to include more strike-slip faults and en enchelon fold sets (because of reactivation of Mesozoic transforms) than would be expected by structural inversion of a purely passive margin.« less

The Beaufort Sea Fold-and-Thrust Belt, Northwestern Canada: Implications for Thrust-Belt Evolution

The northeasternmost segment of the Cordilleran thrust belt of western North American underlies the Beaufort Sea continental margin. Folds and associated northesat-directed thrusts in this region formed synchronously with Tertiary sedimentation. As a result, the times of fold development can be determined from reflection seismic data by analyzing lateral thickness changes in stratigraphic sequences of known ages, and onlap and truncation relationships at unconformities. Thrust faulting occurred throughout the late Paleocene-Pliocene. The abundant temporal data indicate the deformational seuqence was significantly differet from the simple, steplike, foreland-propagating model formulated in other less well-dated thrust belts. Many thrusts were active simultaneously, especially during the late Eocnee, when the region of active thrusting had an across-strike width of greater than 200 km. This observation calls into question the popular concept that only one thrust moves at a time as a thrust belt develops. The thrust belt propagated along, as well as across, strike. During the late Paleocene-middle Eocene, the area of active thrusting was bounded on the southeast by poorly imaged zones of right-lateral strike-slip faults that apparently are the northern offshore continuation of the Rapid fault array. The change in the age of thrusting along strike results in no obvious geometricalmore » anomalies and could not be deduced without timing information. This has an important implication: temporal data cannot necessarily be projected along strike in a thrust belt.« less