Flavio Jadoul

Sedimentary evidence of a cambro-ordovician orogenic event in the Northwestern Himalaya

Abstract The Late Proterozoic-Early Paleozoic sequence of the Tethys Himalaya provides evidence of an orogenic episode close to the Cambrian-Ordovician boundary (Late Pan-African event). Pre-Middle Cambrian sediments were deposited on a terrigenous tidal flat gradually passing upward to a peritidal carbonate platform. The succession shows an overall transgressive trend, which is reproduced at smaller scale by second- and third-order coarsening-upward cycles terminated by the landward shift of carbonate facies during highstands of sea-level. Sandstone petrography suggests a passive continental margin setting for this part of the sequence. In Middle Cambrian times, the carbonate platform underwent gradual drowning and outer shelf/slope shales with intercalated tuffaceous material erupted from a nearby immature volcanic arc were deposited. The shales are overlain by basinal turbidites attesting to the growth of a sandstone lobe enclosed in lobe-fringe siltstones. In the Middle Ordovician, unconformable alluvial fan conglomerates and braid-plain sandstones derived from uplifted sedimentary sequences indicate erosion of a newly formed orogenic belt. Comparison with Early Paleozoic and recent counterparts allows to interpret the Cambro-Ordovician sedimentary evolution in terms of foreland basin development. The thermal subsidence of a passive margin, temporarily reduced by the passage of a “peripheral bulge”, was enhanced strongly by flexural downwarping towards a subduction zone. Collision of a buoyant continental margin at the trench hampered subduction. Rapid uplift ensued, with formation of a foreland fold-thrust belt shedding sedimentary detritus into the adjacent “molasse” basins.

Nature and evolution of Middle Triassic carbonate buildups in the Dolomites (Italy)

During the Middle Triassic, an evolution from carbonate bank to buildup, highly elevated in respect of the adjacent basins, is observed in the Alps. The Dolomites in the Southern Alps, Italy, are here considered as an example. During the Anisian, subtidal banks of the Upper Serla Fm., with rare peritidal cycles show only a limited biologic density and diversity. With the Late Anisian (Contrin Fm.) a gradual development of binding and framework-building communities can be observed. The sinking of wide areas of the Contrin carbonate bank into anoxic conditions (Plattenkalke Fm.) allowed the birth of isolated buildups. Their increased carbonate production, due to a greater diversity and density of organisms, was able to keep up with the rate of subsidence. The Latemar buildup — with additional data for the slope taken from Mt. Coldai (Civetta) and Sass da Putia — is used to illustrate the main character of a Lower Ladinian buildup. The restricted platform sediments display peritidal cycles: prevalent in the subtidal portion is Dasycladacean packstone/grainstone, associated with gastropods, ammonoids and blue-green algae; in the supratidal portion, desiccation and tepee structures are present. Scleractinian and sphinctozoan frameworks are rare on the upper part of the slope which is bound predominantly by encrusting organisms, mainly blue-green algae, porifers and Tubiphytes. The prevailing sediments on the upper part of the slope are bindstones, whilst packstones and rudstones are in the lower part of the slope. The latter carbonates were cemented early under submarine conditions, mostly by isopachous-palisade cement. Polyphasic internal sediments with phreatic-vadose influence are present, and can in the middle-lower part of the slope replace most of the original sediment.

The Tethys Himalayan passive margin from Late Triassic to Early Cretaceous (South Tibet)

The Mesozoic succession of South Tibet, both in lithologies and stratigraphic thicknesses, compares much more closely with that of central Nepal than has been reported in the literature. Facies distribution, from relatively proximal environments in the south to more distal settings in the north, reflects the paleogeography of the Tethys Himalaya passive margin, representing the southern edge of Neotethys. As in central Nepal and NW Himalaya, accumulation rates increased greatly in the latest Triassic, when very thick shelfal siliciclastics and locally volcaniclastics (Tarap Formation) were followed by coastal sandstones (Zhamure Formation). Contrary to reports in the literature, this latter unit reaches well into the Liassic at least in proximal southern sectors. The base of the overlying Kioto Group, yielding rich faunal associations characterized by benthic foraminifers and Lithiotis, similar to those of western Tethys carbonate platforms, is dated as middle Liassic (most probably late Pliensbachian). The monotonous growth of carbonate ramps during the middle Liassic to early Dogger was interrupted by two siliciclastic episodes related to major paleogeographic changes in the Toarcian (middle part of the Kioto Group) and in the Bajocian–Bathonian (Laptal Formation). An early Callovian flooding event (Ferruginous Oolite Formation)—widespread all along the Tethys Himalaya—was locally followed by deposition of a markedly lenticular pelagic unit, previously reported only from the Thakkhola Graben (Dangar Formation). The overlying Spiti Shale is reduced to only 20 m in southern sectors, where it is not younger than the Oxfordian, whereas in northern sectors the unit is several tens of m thick and mainly ascribed to the Tithonian. Volcaniclastic deposition (Wolong Formation)—reported herein for the first time—probably began as early as the Kimmeridgian/early Tithonian and lasted until Aptian times, when it was replaced by sedimentation of relatively deep-water marlstones. This magmatic episode, related to tectonic extension associated with the detachment of India from Gondwanaland, took place earlier in South Tibet and progressively later in central Nepal and Northern India; as in Nepal volcanic products changed with time from predominantly basaltic composition to intermediate and bimodal character. The Early Cretaceous sedimentary evolution of South Tibet matches that of offshore NW Australia.

The north Karakorum side of the Central Asia geopuzzle

An Italian geological team visited a remote and in part never studied area in the northern Hunza region (Pakistan), which represents the link between the Karakorum and Pamir Ranges. The north Karakorum sequence commences in the Permian with terrigenous sediments, followed by shallow- to deep-marine carbonates deposited on a newly formed passive margin. Deep-water sedimentation continued till the end of the Middle Triassic, when carbonate platform conditions resumed. An episode of deltaic red sandstones with orogenic provenance is interbedded in the Liassic, and it is transgressed by a Middle to ?Upper Jurassic shallow water marine unit. Eventually, all of the sequence was faulted and folded, with weak metamorphic imprint, before fluviatile red polygenic conglomerates sealed the succession,in a spectacular unconformity. The north Karakorum provides an example of a microplate that rifted away from Gondwana in the Permian, reached deep-marine conditions in the Early Triassic, and marginally recorded compressive movements in the Liassic. A subsequent orogenic episode points to a reorganization of the southern Asian margin possibly around middle Cretaceous time. Finally, the north Karakorum was affected by strong fold-thrust deformation and low- to very low-grade metamorphism in the Cainozoic, related to the India-Asia collision.

How cold were the Early Permian glacial tropics? Testing sea-surface temperature using the oxygen isotope composition of rigorously screened brachiopod shells

Abstract: Brachiopod carbonate from Early Permian brachiopod shells from low palaeolatitude north Iran and higher palaeolatitude Pakistan Karakorum were screened for diagenesis and analysed for oxygen isotope ratios to derive seawater palaeotemperatures. Screening techniques employed included SEM ultrastructural analysis, cathodoluminescence (CL), image analysis of CL images, trace-element (Sr, Mn, Fe) determinations, and carbon and oxygen stable-isotope determinations. The Karakorum shells were found to be diagenetically altered, but those from north Iran were judged to be pristine. Using data from pristine material, two distinct time slices were analysed: the early and middle Asselian. Two contrasting δ18O values for seawater (0‰ and +1.0‰ V-SMOW) were used to account for different extensions of the Gondwanan ice caps. The δ18O data from north Iran indicate a range of seawater temperatures from +24.3 to +30.3 °C (for δ18Oseawater = 0‰ V-SMOW) or from +30.3 to +35.4 °C (for δ18Oseawater = +1.0‰ V-SMOW) for the early Asselian. Results for δ18O from the middle Asselian indicate seawater temperatures of +24.4 to +28.0 °C (for δ18Oseawater = 0‰) or +29.2 to +32.8 °C (for δ18Oseawater = +1.0‰). The maximum calculated temperatures in the middle Asselian are about 2 °C lower than those for the early Asselian. The average temperature for both time slices is similar to modern tropical sea-surface temperatures, indicating that low-latitude Early Permian ocean waters in Iran did not undergo significant cooling during the final Glacial III episode of Gondwanan glaciation. This confirms other evidence based on biotic provinces, which suggests that during the Permo-Carboniferous glaciation, the low-latitude warm belt became narrower and confined to the western Tethys and Cathaysian provinces, and was not subject to a reduction in temperature, but rather a reduction in size.

MARINE GEOLOGY OF THE BAHAMA ESCARPMENT

Abstract A 2 km thick stratigraphic section of the Bahama Escarpment off Cat Island in the northwestern Bahamas has been measured and sampled by submersible. The escarpment, from 3940 m upwards, consists of lower and middle Cretaceous peritidal, lagoonal, patch-reef and backreef limestones unconformably overlain by an intermittent cover of Maastrichtian and Eocene pelagic limestone. Data from additional dive traverses and dredging elsewhere on the Bahama Escarpment indicate that shallow-water platform limestones similar in age and lithofacies to the Cat Island sequence occur along the entire 800 km of the escarpment as far as Navidad Bank. Comparison of our section to stratigraphic data from coeval Mesozoic carbonate platforms surrounding the Gulf of Mexico and extending up the east coast of North America as well as from other ancient and modern carbonate platforms reveals that the Bahama sequences have the greatest similarity to deposits characteristic of the marginal zones (1–10 km wide) of these platforms. In particular, the Bahama sequence is most similar to the “bank-interior” and “backreef” portions of the marginal zone and not to the “reef” portion. It is clear from both modern and ancient examples that a purely constructive carbonate platform margin should consist of a reef and associated forereef deposits, and not only of “bank-interior” peritidal and lagoonal strata, as exposed along the Bahama Escarpment. Therefore, the reef and forereef debris which constituted the original seaward edge of the Bahama must have been removed by erosion. The amount of erosion is estimated as 1–5 km and is effected at present by spallation of joint blocks. Lithostatic pressure at 4–5 km depth may exceed the strength of the limestone comprising the platform, resulting in the pervasive jointing observed in the lower portion of the escarpment. Subsidence of the Cat Island Escarpment has been exponential, decreasing from 60 m/m.y. in the Early Cretaceous to 40 m/m.y. in the Middle Cretaceous. Extrapolated values for the Late Cretaceous and Tertiary are 20 m/m.y.

Norian serpulid and microbial bioconstructions: Implication for the platform evolution in the Lombardy Basin (Southern Alps, Italy)

SummaryThe development of peculiar margin facies and abundant talus breccias within the Dolomia Principale inner platform is commonly observed in the Lombardy Basin during the Norian. The organisms building these margins are mainly serpulids, benthic microbes, subordinate porostomata and other encrusting forms; typical margin organisms, as sponges or corals, are extremely rare or absent. The build-ups form narrow rims along the borders of tectonic-controlled intraplatform basins. Regional back-stepping and progradation of the margin facies on the talus breccias produced by the erosion of the reef is commonly observed in the uppermost Dolomia Principale depositional system. Widespread occurrence of serpulids and microbial margins in middle-late Norian times is indicative of stressed environmental conditions—fluctuation of salinity and temperature on the inner platform and in the intraplatform basins—controlled by palaeogeographic setting. Physical characteristics allowed the bloom of forms able to develop in a wide range of environmental conditions, such as serpulids.In the Late Norian, major input of fine-grained clastics is recorded; close to the Norian-Rhaetian boundary, carbonate ramps were regionally restored. Locally, small serpulid and microbial bioconstructions still persist in the lowermost part of the shaly succession, even if they are less abundant with respect to the Dolomia Principale. Patch-reefs generally do not build a platform margin, but represent isolated mounds within shaly deposits. These build-ups occur on the edge of former structural highs; the communities survived the environmental change responsible for the siliciclastic input and locally managed to produce mounds during the deposition of the lower part of the upper depositional system (Riva di Solto Shale).

Jurassic and Cretaceous orogenic events in the North Karakoram: age constraints from sedimentary rocks

Abstract At least two major episodes of tectonic deformation are recorded in the Mesozoic sedimentary succession of the North Karakoram. The older event took place between the Liassic and the early part of the Mid-Jurassic, as documented by tens to hundreds of metres thickness of dark grey litharenites (Ashtigar Fm.). The latter are sharply overlain by 35 m to 150 m thick red sandstones, interbedded with shales and siltstones (Yashkuk Fm.). Terrigenous bodies increase in thickness from west to east, and from north to south. Petrographic composition indicates the orogenic character of these quartzo-lithic sandstones, characterized by mafic volcanic and serpentinite detritus, being replaced upwards by sedimentary and subordinate metasedimentary clasts. The Ashtigar Fm. may have been deposited in a collisional basin close to a newly-formed suture belt. Contribution from allochthonous oceanic rocks was significant in the first stages of collision, whereas detritus was subsequently derived mainly from uplifted sedimentary successions. Terrigenous units were conformably transgressed by shallow-water limestones in the Aalenian, and later on marine conditions persisted until the earliest Cretaceous. The second event of tectonic deformation occurred between the earliest and the Late Cretaceous, when the fan delta Tupop Conglomerate, mostly derived from uplifted carbonate and clastic rocks, sealed the strongly folded and faulted underlying succession. Mid-Cretaceous granitoid plutons are also widespread in the North Karakoram. This orogenic event is interpreted as related to the final welding of the Kohistan arc and Karakoram microplate to Asia. The highly irregular topography of the Tupop Conglomerate is onlapped unconformably by reddish to grey nodular limestones, some hundred metres thick and containing phosphatized inoceramids and nannofossils of Campanian age (Darband Fm.). These are the youngest marine sediments detected in the Karakoram to date.

Northern England Serpukhovian (Early Namurian) farfield responses to southern hemisphere glaciation

Abstract: During the Serpukhovian (early Namurian) icehouse conditions were initiated in the southern hemisphere. However, nearfield evidence is inconsistent: glaciation appears to have started in limited areas of eastern Australia in the earliest Serpukhovian, followed by a long interglacial, whereas data from South America and Tibet suggest glaciation throughout the Serpukhovian. New farfield data from the Woodland, Throckley and Rowlands Gill boreholes in northern England allow this inconsistency to be addressed. δ18O from well-preserved late Serpukhovian (late Pendleian to early Arnsbergian) Woodland brachiopods vary between −3.4 and −6.3‰, and δ13C varies between −2.0 and +3.2‰, suggesting a δ18O seawater (w) value of around −1.8‰ VSMOW, and therefore an absence of widespread ice-caps. The organic carbon δ13C upward increasing trend in the Throckley borehole (Serpukhovian to Bashkirian; c. −24 to c. −22‰) and the Rowlands Gill borehole (Serpukhovian; c. −24 to c. −23‰) suggests large-scale burial of organic material, probably in burgeoning lycophyte-dominated coal forest, implying a fall in pCO2. pCO2 reduction appears not to have caused large-scale glaciation until the early Bashkirian, but a scenario of coalescing upland ice-caps through the Serpukhovian with a background of decreasing pCO2 appears to be similar to the process that initiated Cenozoic Antarctic glaciation.

Stratigraphy of Rhaetian to lower Sinemurian carbonate platforms in western Lombardy (Southern Alps, Italy) : paleogeographic implications

A stratigraphical revision of the Upper Triassic-Lower Jurassic succession of the western Southern Alps (Varese area) leads to the introduction of one new lithostratigraphical unit: the ?upper Hettangian-lower Sinemurian Alpe Perino Limestone and the recognition, also in the western Lombardy, of the Rhaetian Zu Limestone, consisting of subtidal cycles, with inner platform facies at the base (Campo dei Fiori Dolomite). These formations represent lagoonal-peritidal to subtidal carbonate depositional systems. A locally angular unconformity between the Rhaetian (or the lower Hettangian) and the upper Hettangian-Sinemurian formations, characterised by ‘terra rossa’ paleosols and pedogenetic carbonate breccias, can be correlated through all of the investigated profiles. The Lower Jurassic paleogeography of the western Lombardy Basin was characterised by an emerged area from Hettangian until earliest Sinemurian times in a warm humid paleoclimate. The Alpe Perino Limestone represents a small peritidal to subtidal carbonate platform developed only in a shallow-water gulf of the M. Nudo area. The platform was surrounded by a wide emerged area (island or peninsula ?), in the west to southeast, and by open subtidal and basinal environments, in the east to northeast (M. Generoso basin).