Distinct Unconformity Research Articles

Field And Well Evidence For Major Unconformities In North Sarawak, Compared To Southwest Sabah, Malaysia

A review of biostratigraphic and lithofacies data is used to show that there is no major unconformity in the stratigraphic record of north Sarawak or southwest Sabah near the end of the Early Miocene (approximately 17-15 Ma). The existence of such an unconformity has been cited in many papers in the past decade and used as a data point in the construction of regional geological hypotheses. Exploration well and outcrop data identifies two major unconformities in SW Sabah (the base and top of Stage III; BMU and DRU; roughly 24 and 13-12 Ma), and in offshore west Sarawak a third unconformity (MMU c. 16 Ma) which fades in effect towards onshore Sarawak. In recent years the names of these distinct unconformities have become conflated as workers had overlooked the origins and definitions of these features. This history is reviewed here in order to clarify future work.

The subduction-related Great Unconformity in the Tarim intracraton, NW China

The Great Unconformity across the Proterozoic-Phanerozoic boundary records a significant change of the Earth's continents, atmosphere, environment and life. Its origin has been assumed to be glacioeustatic mechanism in the cratonic interior. On the contrary, we provide evidences for the sub-Cambrian Great Unconformity that can be traced across the Tarim Craton (NW China) from recent geochronological and seismic data. In the interior basement uplift, a distinct unconformity over an area of 300, 000 km2 presents a stratigraphic gap from the Paleoproterozoic (ca. 1.9 Ga) to the end Ediacaran. There is significant denudation beneath the Cambrian with compressional uplift. Given that the ca. 590–580 Ma Hankalchough diamictite (correlated with the Gaskiers glaciation) is absent in most areas, as much as 40 m.y. of stratigraphic gap constrained maximum span of the latest sub-Cambrian unconformity. The Precambrian uplift and denudation, and the age patterns of detrital zircon grains, overlap ca. 560–540 Ma advancing subduction process along the convergent southern margin of the Tarim Craton. This Great Unconformity may be linked to subduction-related uplift along the convergent southern Tarim margin, as occurred across peri-Gondwana. These relationships suggest tectonic driver rather than glaciation could lead to Great Unconformity in the cratonic interior as well as the continental margin

Tectonostratigraphic framework in the eastern Korean continental margin, East Sea: Implication for evolution of the Hupo Basin

AbstractIntegrating seismic interpretation facilitates the discernment of tectonostratigraphic evolution in the eastern Korean continental margin, East Sea. The sedimentary succession of this margin is divided into three major seismic units based on distinct unconformities. These unconformities and their associated seismic characteristics indicate that the eastern Korean continental margin has experienced four evolutionary stages through extensional and subsequent two‐phase tectonic inversions. Early Miocene back‐arc opening of the East Sea triggered the extension of NNW–SSE and N–S trending rifts, resulting in non‐marine to deep‐marine deposition of typical rift‐related linked sedimentological systems in the eastern Korean continental margin. In the early Late Miocene, changes in plate motion and the subduction mode of the Japanese island (NW–SE compression) caused the positive inversion of extensional fault‐bounded half‐grabens in the eastern Korean continental margin, and the Hupo Basin was likely created by the regional flexural response. During this depositional period, hemipelagic sedimentation accompanied by episodic gravity‐controlled slope failures was predominant in the deep‐water environment. The subsidence of the Hupo Basin was enhanced by crustal shortening (E–W compression) that was induced by subduction initiation at the western margin of the Ulleung Basin during the late Early Pliocene (ca. 3.8 Ma). At that time, sedimentary environment gradually became shallower with time and coarse‐grained terrigenous input into the Hupo Basin began so that shallow‐ to deep‐marine sedimentation occurred. In the Quaternary period, the uplift of the Hupo Bank and coeval subsidence of the Hupo Basin were maintained by continued compressive neotectonics. The shallow‐ to deep‐marine deposition continued, but greater quantities of coarse‐grained terrestrial sediment were transported into the Hupo Basin due to combined effects on tectonics and eustasy. Based on the tectonostratigraphic reconstruction developed in this study, we propose that the Hupo Basin is likely a Late Miocene–Quaternary compression‐related basin induced by crustal or thrust loading.

Geotechnical prerequisites for safe and efficient accessing and mining of thick coal seam in highlands

Bel-Alma deposit is located in the Batken region in Kyrgyzstan. Orographycally, the deposit occurs on the south shoulder of the Kichik–Alai ridge, 1–1.5 km away from the cognominal river. The local relief is high-mountain, rocky and heavily broken. Geologically, the deposit is composed of the Upper Paleozoic rocks, which underlay the stratified Meso-Cenozoic deposits, and amorphous loose overburden. The productive strata is the bottom range of the Sogul series of the Jurassic system. The total thickness of the series in the cross-section is 102.7 m. The Sogul series rocks feature erosion and distinct structural unconformity in the Paleozoic residuum. The overlying strata are represented by red conglomerates and sandstone of the Upper Cretaceous age. The geotechnical estimation of the properties of rocks composing the study area in Bel-Alma deposit is performed. The rock mass structure is determined using geological exploration cores. The parameters of stable pit wall and benches are preliminarily optimized and will be included in the ultimate pit limit design.

EARLY–MIDDLE ORDOVICIAN SEASCAPE-SCALE AGGREGATION PATTERN OF SPONGE-RICH REEFS ACROSS THE LAURENTIA PALEOCONTINENT

ABSTRACTDuring the late Cambrian–Early Ordovician interval the predominant non-microbial reef builders were sponges or sponge-like metazoans. The lithological and faunal composition of Cambro-Ordovician sponge-dominated reefs have previously been analyzed and reviewed. Here we take the relationship between reef aggregation pattern at reef to seascape scale into account, and look for changes during the Early–Middle Ordovician interval, in which metazoans became dominant reef builders. In a comparison of sponge-rich reefs from eight sites of the Laurentia paleocontinent three different seascape level reef growth patterns can be distinguished: (1) mosaic mode of reef growth, where reefs form a complex spatial mosaic dependent on hard substrate; (2) episodic mode, where patch reefs grew exclusively in distinct unconformity bounded horizons within non-reefal lithological units that have a much larger thickness; and (3) belt-and-bank mode, where reefs and reef complexes grew vertically and laterally as dispersed patches largely independent from truncation surfaces. The distinct modes of growth likely represent specific reef forming paleocommunities, because they differ in content and abundance of skeletal metazoan framebuilders, bioturbation intensity of non-skeletal reef sediment matrix, and in association of reef growth with underlying hard substrate. We suggest, based on a review of Laurentian reef occurrences, that the mosaic mode dominated in Early Ordovician strata and that the dominance shifted toward the belt and bank mode from Middle Ordovician strata onward.

Late Cenozoic seismic stratigraphy of the Andaman Forearc Basin, Indian Ocean

Interpretation of new multichannel seismic reflection data from the Andaman Forearc Basin (AFB) in the northern Indian Ocean is presented here. The high-quality multichannel seismic data from the Andaman Forearc region enable us to examine the seismic characters and to demarcate seismic sequences bounded by distinct unconformities. Ages of marked seismic horizons have been calibrated with available litholog data from nearby industry boreholes. Seismic interpretation of new data shows that the AFB is filled with ~ 4.5-s-two way travel time (TWT) thick Neogene to Recent sediments. The entire basin assemblage exhibits two distinct major sequences pertaining to the Neogene and Quaternary times. A large part of the basin is filled with intermittent mass transport deposits (MTD). We infer that the episodic uplift of the Invisible Bank, protuberance of the outerarc and regular deformation through reactivation of preexisting normal faults since the Pleistocene could be attributed as causal mechanisms for the MTDs. Strong bottom simulating reflectors are identified in the Late Miocene and younger sediments of the outerarc and AFB at a depth of ~ 0.6 s TWT and correspond to the presence of gas hydrates in this region. Our interpretations have significant implications for geodynamic as well as resource exploration in the AFB.

U–Pb LA-ICP-MS detrital zircon ages from the Cambrian of Al Qarqaf Arch, central-western Libya: Provenance of the West Gondwanan sand sea at the dawn of the early Palaeozoic

Detrital zircons from various stratigraphic levels of the sandstone-dominated Cambrian Hasawnah Formation of the Al Qarqaf Arch type area (central-western Libya, Saharan Metacraton area) were geochronologically investigated for the first time by LA-ICP-MS techniques for U, Th, and Pb isotopes. Of 720 analyzed grains, 329 were concordant. Of the total, about 60% of the U–Pb zircon ages are Neoproterozoic and earliest Cambrian and cluster at c. 700–680, 670–650, 615–610, 590, 570–560, and c. 540–525Ma. These zircon populations are interpreted as detrital material derived from the Pan-African and possibly to a smaller proportion from the Cadomian orogen situated marginal to northwestern Gondwana. A few slightly older Neoproterozoic ages (c. 950–750Ma) point to rifting events related to the dispersal of the Rodinia supercontinent. A minority of zircons became formed during the configuration of Rodinia and cluster around the Mesoproterozoic–Neoproterozoic boundary (1039±11, 1006±12 and 993±13Ma). Further, some early Mesoproterozoic zircon ages had been found (1592±39 and 1475±20Ma). The potential source area for the Mesoproterozoic zircons is interpreted to have been far distant from the Al Qarqaf Arch, probably concealed within the Arabian–Nubian Shield or situated in Chad, or in the Congo and Tanzania cratons. There is still no evidence for the existence of massive Mesoproterozoic crust in the Saharan Metacraton area. A considerable proportion (28%) of zircons represents Palaeoproterozoic populations at c. 2.4–2.3Ga, and c. 2.2–1.6Ga. Less than 5% of all zircons are Archaean in age (c. 3.4–3.25Ga, c. 2.95–2.7Ga, c. 2.6–2.5Ga). A potential source area for Palaeoproterozoic and Archaean zircon grains is the West African Craton and the western part of the Saharan Metacraton. The best candidates for the main source region for the sandstones of the Hasawnah Formation in the Al Qarqaf Arch type area are the Neoproterozoic–early Cambrian orogens of the Pan-African cycle in the Trans-Saharan Belt (Pharussian and Dahomeyean belts) and of the peri-Gondwanan terranes (Cadomia). This conclusion is in accordance with published data from the Hoggar (Tassilis, Algeria) and from southwestern (eastern Murzuq Basin) and southeastern Libya (Al Kufrah Basin). In comparison to the strong input of Neoproterozoic zircon grains, input from the Palaeoproterozoic and Archaean sources of the cratonic basement (Saharan Metacraton and West African craton) is relatively limited. The exact source of the exotic Mesoproterozoic zircons remains problematic. The presented data lead to the conclusion that the centre of early Palaeozoic thermal subsidence in central-northern Africa has to be located in the region of the Saharan Metacraton. The distinct unconformity at the base of the Cambrian Hasawnah Formation indicates major uplift and considerable denudation in the latest Neoproterozoic to early Cambrian time interval. Because of the conspicuous maturity of the Hasawnah Formation siliciclastic deposits, a coeval intense chemical weathering under warm to humid climatic conditions in low to moderate southern latitudes and the formation of a Gondwanan peneplain is indicated.

Seismic stratigraphy and tectonic structure from a composite multi-channel seismic profile across the entire Dangerous Grounds, South China Sea

We interpret a more than 500km long composite multi-channel seismic reflection profile across the entire Dangerous Grounds, the South China Sea. Five tectonostratigraphic units are determined, together with seven sequence boundaries. Detailed analysis of extensional features, based on measurements of fault heaves, revealed two major phases of extension, separated by a distinct unconformity, which likely correspond with the beginning of sea-floor spreading in the South China Sea. Early extension occurred during continental rifting (Late Cretaceous–Early Oligocene), and resulted in formation of half-grabens and rotated blocks, controlled by a deeply rooted detachment system. Extension continued in our study area during the drifting phase of the East Subbasin of the South China Sea (Late Oligocene–Early Miocene) until the subsequent opening of the Southwest Subbasin at about 25Ma, but its intensity decreased markedly thereafter. Deeply rooted detachment systems evolved possibly during this second phase of extension at the continent–ocean transition area. From a coincidence of the lowermost reflections and Moho depth as derived from gravity inversion we tentatively interpret the flattened part of this detachment system as the crust–mantle boundary. It is suggested that the Dangerous Grounds reflect a magma-poor rift system at the initial stage of mantle unroofing. We suggest that a widespread carbonate platform developed across the Dangerous Grounds, concurrent with the period of seafloor spreading in the Southwest Subbasin of the South China Sea. Fault-related stretching factors (βf) along the line show discrepancies with the whole crustal stretching factors (βc). The continental crust of the Dangerous Grounds may have experienced nonuniform thinning.

Variations in sediment transport at the central Argentine continental margin during the Cenozoic

The construction of the sedimentary cover at most passive continental margins includes gravitational downslope transport and along‐slope contourite deposition, which are controlled by tectonics, climate and oceanography. At the eastern continental margin of Argentina the history of deposition and erosion is intimately linked to the evolution of the South Atlantic and its water masses. Here we present a detailed seismic investigation of the mixed depositional system located between 41°S and 45°S. The study provides a northward complement to prior investigations from the southern Argentine margin and together with these may be used as background information for future ocean drilling in the region. Prominent features in our seismic cross sections are submarine canyons, mass wasting deposits, contourite channels, and sediment drifts. Four major seismic units above regional reflector PLe (∼65 Ma) are separated by distinct unconformities of regional extent. Using a dense grid of reflection seismic profiles, we mapped the depocenter geometries of the seismic units and derived a chronology of the depositional processes during the Cenozoic. While the Paleocene/Eocene (∼65–34 Ma) is characterized by hemipelagic sedimentation under relatively sluggish bottom water conditions, strong Antarctic bottom water (AABW) circulation led to widespread erosion on the slope and growth of a detached sediment drift during the Oligocene and early Miocene (∼34–17 Ma). After deposition of an aggradational seismic unit interpreted to represent the Mid‐Miocene climatic optimum (∼17–14 Ma), gravitational downslope sediment transport increased during the middle to late Miocene (∼14–6 Ma) possibly related to tectonic uplift in South America. The Pliocene to Holocene unit (<∼6 Ma) is very heterogeneous and formed by interactions of downslope and along‐slope sediment transport processes as indicated by the evolution of canyons, slope plastered drifts and channels.

Seismic Stratigraphy, Tectonic Structure and Extension Factors Across the Dangerous Grounds: Evidence from Two Regional Multi‐Channel Seismic Profiles

AbstractTwo recently acquired regional multi‐channel seismic profiles across the Dangerous Grounds, the South China Sea (SCS), are interpreted. One line crosses the entire Dangerous Grounds from the deep oceanic Southwest Subbasin of the South China Sea, along the Spratly Islands to close to Borneo, and the other one runs east of the Reed Bank. Based on the interpretations of these profiles, we worked out the stratigraphic sequences, tectonic structures and extension factors. Five tectonic‐stratigraphic units are determined, together with 7 sequence boundaries. Detailed analyses on the extension factors based on the measurement of fault heaves reveal two episodes of continent extension separated by a distinct unconformity. The main extension occurred during the continental rifting (Early Tertiary), and resulted in the formation of half‐grabens and rotated blocks, controlled by deeply rooted detachment system. During the second extension phase (drifting period, Late Oligocene‐Early Miocene) intensity reduced sharply and focused on the continental‐oceanic transition area. A widespread carbonate platform developed across the Dangerous Grounds concurrent with the spreading period of the SCS. Fault‐related stretching factors (βf ) to the west and east of Reed Bank are found to be comparable and show discrepancy with the whole crust stretching factors (βc) in the study area as derived from gravity modeling. Thus we conclude that the continental crust of the Dangerous Grounds experienced depth‐dependent extension.

The Geology of Goa Group: Revisited

Abstract The supracrustals that constitute the Goa Group of Gokul et al. (1985) can be divided into two lithostratigraphic sequences namely the Barcem Group and the Ponda Group. The former comprises predominantly greenstones (metabasalts) and rests on a basement of the 3300-3400 Ma Anmode Ghat trodhjemite gneiss with a crudely developed quartz-pebble conglomerate at the base, and shows lithological similarities with the lower part of the Bababudan Group. The younger sequence is dominated by clastics, and is assigned to a new stratigraphic group formally termed the Ponda Group which is equivalent to the Chitradurga Group of the Dharwar Supergroup. This group rests on a basement of the 2700-2900 Ma Chandranath granite gneiss with a distinct unconformity marked by a polymict, granite-clast metaconglomerate. The conglomerate displays many similarities with the Talya conglomerate that occurs at the base of the Chitradurga Group. It is overlain by a psamolitic sequence which is followed in ascending order by the chemogenic sediments that host the BIF and by the deep water turbidite sequence (argillite-graywacke association) with intercalations of mafic volcanics. The supracrustal sequence is intruded by the Bondla layered mafic-ultramafic complex along a major shear zone (NW-SE) that largely controls the course of the northwesterly flowing tributary of River Mandovi. The late intrusive, Canacona potassic granite marks the culmination of the sedimentation in the Shimoga-Goa basin.

The First Dating of Cave Ice from the Tatra Mountains, Poland and its Implication to Palaeoclimate Reconstructions

Lodowa Cave in Ciemniak, which belongs to the dynamic ice cave type, contains the biggest perennial block of cave-ice in the Tatra Mountains. The ice represents congelation type, since it originates from freezing of water which infiltrates the cave. Two generations of ice have been recognized in this cave. They are divided by the distinct unconformity. The ice building both generations is layered. Two moths which were found in the younger generations were sampled and dated by <sup>14</sup>C method yielding 195 ± 30 and 125 ± 30 years. Bearing in mind the position in the section and the fact that the cave ice has waned since the 20s of the last century, the age is 1720-1820 AD and 1660-1790 AD respectively. It proves that the ice was formed during the Little Ice Age. Hence, the erosion boundary which underlies this generation records the degradation of ice before the Little Ice Age most probably during the Medieval Warm Period. The ice volume in the cave was substantially smaller before the Little Ice Age than it is today, despite the clear tendency to melting, which has been recognized since 20s of the last century. The older generation of ice is supposed to have its origins in a cold stage between the Atlantic period and the Medieval Warm Period.

Observations and comments on the classification schemes proposed for the Voltaian Basin sediments on the West African craton by various Geologists

Subsurface data indicate over 6000 metres thick of sedimentary infill in the Voltaian Basin on the West African craton made up of three major lithofacies (groups) separated by distinct unconformities. These are: Lower Voltaian Group composed of over 2000 metres thick of indurated quartzitic sandstones with subordinate shales at the base, overlain by dense, greenish, poorly sorted and graded bedded sandstones and siltstones with subordinate shales of about 2000 metres thick; Middle Voltaian Group consisting of the triad: conglomerate – dolomitic limestone – bedded chert with intercalations of sandstone, siltstone, mudstone and shales of over 700 metres thick, and overlain by about 550 metres thick of greenish, flysch-type sediments. The Upper Voltaian Group is composed of distinctly continental facies (molasses) of reddish brown, chocolatebrown, brown coloured, poorly sorted sandstones, conglomerates, siltstone and mudstones of about 400 metres thick, and a sequence of thinly bedded, micaceous sandstones with clay galls of about 120 metres thick capped by massive cross-bedded arkosic sandstone of about 300 metres thick. Reviews of the classification schemes proposed by various geologists are basically similar. However, general discrepancies, inconsistencies and contradictions in the stratigraphic positions of some of the rock units have been observed, as well as terminologies to describe rock units which are inconsistent with stratigraphic concepts and principles.

Slavin Yar: a new Upper Cenozoic reference section in the Tunka rift valley (Southwestern Baikal region)

The biostratigraphic study of a new Upper Cenozoic reference section in the Tunka rift valley (southwestern Baikal region) accompanied by radiocarbon measurements made it possible to date its lithological units. It is established that the section is largely composed of Upper Pleistocene fluvial sediments resting with distinct angular unconformity uapon Pliocene conglomerates. The revealed structural features of the section confirm the views that the directed development of the Tunka depressions was complicated by local inversions, when the sedimentation area became reduced. The main sedimentation features during the Late Cenozoic and its stages are reconstructed for the studied area.

Stratigraphy of Late Cenozoic sediments of the western Chukchi Sea: New results from shallow drilling and seismic-reflection profiling

The Quaternary history of Beringia and of the Arctic–Pacific marine connection via the Bering Strait is poorly understood because of the fragmentary stratigraphic record from this region. We report new borehole and seismic-reflection data collected in 2006 in the southwestern Chukchi Sea. Sediment samples were analyzed for magnetic properties, grain size, heavy minerals, and biostratigraphic proxies (spores and pollen, foraminifers, ostracodes, diatoms, and aquatic palynomorphs). Two shallow boreholes drilled between the Chukotka Peninsula and the Wrangel Island recovered sediments of two principal stratigraphic units with a distinct unconformity between them. Based on predominantly reverse paleomagnetic polarity of the lower unit and pollen spectra indicative of forested coasts and climate warmer than present, the age of this unit is estimated as Pliocene to early Pleistocene (broadly between ca. 5 and 2 Ma). Attendant sedimentary environments were likely alluvial to nearshore marine. These deposits can be correlated to the seismic unit infilling valleys incised into sedimentary bedrock across much of the study area, and possibly deposited during a transgression following the opening of the Bering Strait. The upper unit from both boreholes contains Holocene 14C ages and is clearly related to the last, postglacial transgression. Holocene sediments in Borehole 2 indicate fast deposition at the early stages of flooding (between ca. 11 and 9 ka) to very low deposition, possibly related to expansive sea ice. Closer to shore, deposition at Borehole 1 resumed much later (ca. 2 ka), likely due to a change in the pattern of coastal erosional processes and/or the demise of a landbridge between the Chukotka Peninsula and the Wrangel Island inferred from studies on mammoth distribution.

Detrital-zircon fission-track ages from the Lower Cenozoic sediments, NW Himalayan foreland basin: Clues for exhumation and denudation of the Himalaya during the India-Asia collision

Detrital-zircon fission-track (FT) ages from the Lower Cenozoic Sub-Himalayan foreland basin reflect the progressive effects of crustal thickening and exhumation on the Himalayan source rocks as a consequence of the India-Asia collision. The oldest stratum, the transgressive marine Paleocene-Eocene Subathu Formation (57–41.5 Ma) contains ca. 50 Ma detrital-zircon P1 peak, which was derived from the Indus Tsangpo Suture Zone and the Ladakh Batholith of the Asian plate. A dominant 302.4 ± 21.9 Ma peak with a few 520 Ma grains in this formation has been derived by erosion of the zircon partial-annealing zone (ZPAZ) of 240–180 °C. As the first imprint of the collision, this zone affected the Himalayan Proterozoic basement and its Tethyan sedimentary cover. Since the detritus in the Subathu has been derived both from the Indian and Asian plates, the possible suturing of these plates took place during the Subathu sedimentation. A sudden change in the provenance is recorded in the detrital-zircon FT cooling ages in the Oligo-Miocene Dagshai and Kasauli Formations, which have dominant 30 and 25 Ma P1 peaks, respectively. We interpret a distinct unconformity spanning ~10 m.y. between the Subathu and Dagshai Formations. Since ca. 30 Ma, molassic sedimentation coincides with shifting of the source rocks to the Himalayan metamorphic belt. This belt has sequentially undergone three distinct cooling and exhumation pulses after the ultrahigh-pressure–high-pressure (UHP-HP) metamorphism (53–50 Ma) in the extreme north and two widespread M1 and M2 metamorphisms (40–30 and 25–15 Ma) in the middle parts. These events appear to be largely responsible for the deposition of the ca. 30 Ma zircon Himalayan peak and ca. 25 and 15 Ma young Himalayan peaks, respectively; the latter appears within the Lower Siwalik Subgroup (13–11 Ma). During the Lower Siwalik deposition, pre-Himalayan peaks gradually decrease with the intensification of the Himalayan events in source rocks. In spite of uninterrupted fluvial sedimentation in the Dagshai-Kasauli–Lower Siwalik sequences since 30 Ma, breaks of ~5–7 m.y. in the zircon FT ages reveal pulsative cooling and exhumation in the well-identified source areas. Although cooling and exhumation of the Himalayan source rocks remained almost uniform during the Eocene, source heterogeneity is reflected in fluvial sedimentation since 37 Ma from Pakistan to Nepal in response to the India-Asia collision.

The Mid-Cimmerian tectonic event (Bajocian) in the Alborz Mountains, Northern Iran: evidence of the break-up unconformity of the South Caspian Basin

Abstract The Mid-Cimmerian tectonic event of Bajocian age can be documented all across the Iran Plate (Alborz Mountains of northern Iran, NE Iran, east-central Iran) and the southern Koppeh Dagh (northeastern Iran). In the Alborz area, the tectonic event consisted of two main pulses. A distinct unconformity (near the Lower–Upper Bajocian boundary) at or near the base of the Dansirit Formation is the sedimentary expression of rapid basin shallowing due to uplift and erosion. Another unconformity is developed in the early Upper Bajocian, close to or at the top of the Dansirit Formation. Locally, it is expressed as an angular unconformity due to block rotation and is overlain by a thin transgressive conglomerate followed by silty marls of the deep-marine Upper Bajocian–Callovian Dalichai Formation. This upper unconformity signals a rapid subsidence pulse. On the Tabas Block of east-central Iran, a single unconformity can be documented that is time-equivalent to those bounding the Dansirit Formation (i.e. ‘mid-Bajocian’). Local folding gives direct evidence of compressional tectonics, and conglomerates indicate subaerial denudation of older Mesozoic or Palaeozoic strata. After a stratigraphic gap, transgressive sediments of ?Late Bajocian–Bathonian age follow, suggesting a fusion of the lower and upper Mid-Cimmerian unconformities in east-central Iran. Along the southern margin of the Koppeh Dagh Mountains (NE Iran), a Late Bajocian subsidence pulse initiated the opening of the strongly subsiding Kashafrud Basin, an eastwards extension of the South Caspian Basin. In all of these areas, one phase of uplift and erosion took place followed by a pronounced pulse of subsidence running counter to trends of the eustatic sea-level curve. Thus, what is generally understood as the Mid-Cimmerian tectonic event is now thought to consist of a tectonic phase, confined to the Bajocian. This phase is explained as the expression of the onset of sea-floor spreading within the South Caspian Basin situated to the north of the present-day Alborz Mountains. This strongly subsiding basin developed close to the Palaeotethys suture during the Toarcian–Aalenian and went through a change from the rifting- to the spreading-stage during the Bajocian. The Mid-Cimmerian event therefore reflects the break-up unconformity of the South Caspian Basin.

A distinct unconformity in the Cango Caves Group of the Neoproterozoic to early Paleozoic Saldania Belt in South Africa: its regional significance

The discovery of deep karstic features, associated with a sharp contact between the carbonate-containing Kombuis Member of the Matjies River Formation and the overlying shaley (lower) part of the Groenefontein Formation of the Cango Caves Group (CCG) in the southwestern Cape in South Africa, indicates that it represents an unconformity which was formed sub-aerially. Previous chrono-, chemo- and biostratigraphic data are supported by genetic stratigraphy, which suggest that the unconformity, called the Groenefontein Unconformity, can be correlated with the Vingerbreek Unconformity occurring in the southern Nama Group, Namibia. This information enables more reliable correlation of the stratigraphic units of the Cango Caves and the Nama Groups and indicates modification of previously proposed correlations. A correlation between the Kombuis Member of the Matjies River Formation (CCG) with the Kuibis Subgroup (Nama Group) as well as the Groenefontein Formation (CCG) with the Vingerbreek Member of the lower Schwarzrand Subgroup (Nama Group) is proposed. The correlation of the Nooitgedagt Member (CCG) with units of the Port Nolloth and Nama Groups is less certain. The Nooitgedagt Member may be a correlate of the Holgat Formation of the Port Nolloth Group or of the lower part of the Kuibis Subgroup of the Nama Group in Namibia. It may also be possible to correlate the Nooitgedagt Member with an even older formation of the Port Nolloth Group. The Groenefontein and Vingerbreek unconformities were probably formed as a result of a drop in sea level caused by a circa 547 Ma minor glaciation which occurred during latest Ediacaran times in southwest Gondwana.

Integrated stratigraphy and palaeoecology of the Lower and Middle Miocene of the Porcupine Basin

AbstractA high-resolution palynological analysis and a detailed palaeomagnetic study of a marine sequence recovered during IODP Expedition Leg 307 in the Porcupine Basin southwest of Ireland provide new insights into the regional depositional history and palaeoenvironmental evolution during Early Neogene times. The Hole 1318B studied was drilled on the upper slope of the continental margin in a water depth of 409 m, upslope from a province of carbonate mounds (the Belgica mound province). The diverse and well-preserved dinoflagellate cyst associations consist typically of deep neritic and oceanic species, mixed with a neritic component transported from the shelf, reflecting the deep depositional setting at the continental margin. The palaeomagnetic record together with the ranges of key dinoflagellate cyst species constrain the age of the studied sequence between 16.7 Ma and 12.01 Ma, that is, between the late Burdigalian and late Serravallian. The distinct unconformity terminating the Miocene sequence correlates to the global sequence boundary Ser4/Tor1 dated at 10.5 Ma, and represents, according to previous extensive seismic studies, a basin-wide erosional event. The overlying sediments are of Middle Pleistocene or younger age. Downslope from IODP Site 1318, carbonate mounds root on the erosional surface. The dinoflagellate cyst associations from the Porcupine Basin distinctly mirror the global cooling phase following the Middle Miocene Climatic Optimum. Cooling phase Mi3, a short-lived glaciation, is particularly well expressed and here dated at 13.6 Ma. The palynomorph record furthermore indicates a reduction of the productivity and an increase of oceanic oligotrophic species after 14 Ma, suggesting a reduction or perhaps even a shutdown of the upwelling.

A seismo-stratigraphic analysis of glaciomarine deposits in the eastern Riiser-Larsen Sea (Antarctica)

Multichannel seismic data from the eastern parts of the Riiser-Larsen Sea have been analyzed with a sequence stratigraphic approach. The data set covers a wide bathymetric range from the lower continental slope to the abyssal plain. Four different sequences (termed RLS-A to RLS-D, from deepest to shallowest) are recognized within the sedimentary section. The RLS-A sequence encompasses the inferred pre-glacial part of the deposits. Initial phases of ice sheet arrival at the eastern Riiser-Larsen Sea margin resulted in the deposition of multiple debris flow units and/or slumps on the upper part of the continental rise (RLS-B). The nature and distribution of these deposits indicate sediment supply from a line or a multi-point source. The subsequent stage of downslope sediment transport activity was dominated by turbidity currents, depositing mainly as distal turbidite sheets on the lower rise/abyssal plain (RLS-C). We attribute this to margin progradation and/or a more focussed sediment delivery to the continental shelf edge. As the accommodation space on the lower rise/abyssal plain declined and the base level was raised, the turbidite channels started to backstep and develop large channel–levee complexes on the upper parts of the continental rise (RLS-D). The deposition of various drift deposits on the lower rise/abyssal plain and along the western margin of the Gunnerus Ridge indicates that the RLS-D sequence is also associated with increased activity of contour currents. The drift deposits overlie a distinct regional unconformity which is considered to reflect a major paleoceanographic event, probably related to a Middle Miocene intensification of the Antarctic Circumpolar Current.