Unconformity Research Articles

Hutton's Unconformity

Hutton's Unconformity

Contact relationships of Tertiary volcanic units from the Andes of central Chile (33°S): a reinterpretation based on 40Ar/39Ar dating.

Se dan a conocer ocho edades 40Ar/39Ar de rocas de dos sectores de la Cordillera de los Andes cerca de los 33°S donde se presentan unidades volcanicas separadas por discordancias angulares. Estos sectores corresponden a la Cuesta de Chacabuco en la continuacion norte del Valle Central y al Cerro Las Ollas en la parte oeste de la Cordillera de los Andes. En Cuesta de Chacabuco se reconocio la unidad Chacabuco, formada principalmente por basaltos, andesitas basalticas y rocas sedimentarias clasticas continentales. Esta unidad esta cubierta, con discordancia angular, por flujos piroclasticos de la unidad Algarrobo, un complejo volcanico erosionado. Una edad plateau 40Ar/39Ar (roca total, basalto) de 28,8 ± 0,3 Ma fue interpretada como la mejor estimacion de la edad del volcanismo para la unidad Chacabuco, mientras que las edades plateau obtenidas sobre anfibolas de una lava de andesita basaltica y de un dique andesitico para la unidad Algarrobo, fueron 19,6 ± 0,3 y 18,6±0,4 Ma, respectivamente. Se detecto asi un hiatus del orden de 8,6 Ma entre estas dos unidades. El Cerro Las Ollas ha sido un lugar clasico en la discusion acerca de la naturaleza del contacto entre unidades que han sido atribuidas a las formaciones Abanico y Farellones el que ha sido interpretado como discordancia de plegamiento y erosion o como contacto tectonico. Las nuevas edades obtenidas sobre biotita en tobas muestran una continuidad temporal para rocas situadas bajo y sobre el plano de contacto, evidenciando la pertenencia de estas tobas a un mismo evento volcanico. La mejor estimacion de edad para este evento es la edad media ponderada de 20,1 ± 0,1 Ma. En consecuencia, la relacion discordante observada en el cerro Las Ollas puede ser interpretada como intraformacional.

Hutton's Unconformity

Hutton's Unconformity

MALACOLOGICAL RESPONSE TO PLEISTOCENE SEA-LEVEL CHANGE IN THE NORTHERN PO PLAIN, N. ITALY: DETAILED PALAEOENVIRONMENTAL RECONSTRUCTIONS FROM TWO LOMBARDIAN CORES

The interdisciplinary study of two deep cores drilled in Pleistocene basin fill at Northern margin of Po Plain, has been integrated with qualitative and quantitative malacological analysis. The potential of quantitative malacological analysis, to refine results obtained from interdisciplinary studies, is here highlighted. The evolution of malacological assemblages has been recorded and correlated to the general regressive trend recognized all over the Po Basin. Lower Pleistocene marine deposits, found at core base (Jaramillo Subchron and older), were gradually replaced by transitional and continental deposits since latest early Pleistocene. Area was eventually covered by continental conglomerate deposits (“Ceppo” facies) during middle-late Pleistocene. Within this general trend, regional significance of a major unconformity (“r” surface), related to onset of Pleistocene glacial cycle, is confirmed. However, as evidenced by malacology, the roughly synchronous onset of coarse clastic progradation did not result in a synchronous shift from marine to transitional and continental settings all over the study area, as an effect of inherited topography and other local factors. During marine sedimentation, fossil record allowed us to recognize a transgressive event, reliably correlated to Marine Isotope Stage 35.

Stratigraphy of Paleogene Strata within the Mesihovina - Rakitno Area (Western Herzegovina)

The paper presents the stratigraphy of Paleogene strata from the central part of carbonate Dinarides. The carbonate and the clastic rock units were stu died. Their contact with underlying Senonian limes tone is marked by unconformity as a result of Laramian tectonic phase. The unconformity is denoted by the abundance of bauxite deposits. The abundance of calcareous nanoplancton species as well as ben tic foraminifera points at the sedimentation of Paleogene strata during the Eocene period. The succession of sediments with easily recognizeable transgressive, inundation and regres sive features has been established. Lithostratigraphical characteristics enabled the distinguishing of larger lithofacies groups. The simultaneousness of carbonate rocks with lower part of clastic succession as well as the transitionallitho· facies have been approved. The simultaneous overburden of bauxite deposits regardless of its lithology (carbonate of clastic) speaks in favour of the unite bauxitebearing phase.

Estratigrafia de sequências do Subgrupo Itararé (Neopaleozóico) no leste da Bacia do Paraná, nas regiões sul do Paraná e norte de Santa Catarina, Brasil

Seven type l third order sequences have been recognized, in which the facies, facies associaticms, lowstand, transgressive, highstand and glacio-isostatic regressive systems tracts, and bounding and internai surfaces had been identified. These sequences correspond to fining- to coarsening-upward successions. The base ofthe cycles, which attain 50 to 180 m in thickness, are limited by an unconformity marked by striated bedrock, intraformatiorial striated surfaces and/or glaciotectonized basement. Each erosional surface, which is considered as representative of one advance of a grounded glacier, is overlain by a subglacial tillite. Higher up the glaciogenic section shows glaciterrestrial, glacimarine and marine deposits, controlled by the retreat ofthe glacier front. The shales are considered as depqsited during a post-glacial transgression, while the upper sandstones are interpreted as related to the glacio-isostatic rebound that followed the retreat of the ice margin.

Geology Of The Tertiary Rocks Around Kemah-Erzincan-Çayırlı Region and Their Source Rock Characteristics

Basement of the study area is comprised of Paleozoic rocks which are overlain by rather thick Triassic-Cretaceous Munzur limestone in carbonate facies. This unit is tectonically overlain by Cretaceous ophio- litic complex. In the region, these units are unconformably overlain by Tertiary clastic and carbonaceous deposits. The basement of the Tertiary is comprised of Paleocene clastic rocks which is conformably overlain by Eocene turbiditic deposits. In Erzincan and Cayýrlý region the Eocene rocks are unconformably overlain by rather thick clastic and carbonaceous Miocene sequence. The sequence in the basin ends up with Pliocene clastic deposits. In the region, in the north of the Pulk (Balýklý) village, live oil seeps are observed. In Eocene Gulandere formation and in Komur member of the Miocene Kemah formation, and also in different levels of the Balýklý formation bitu- men and asphaltic levels are observed. Results of geochemical analyses show that these rocks do not have source rock potential considering the amount of total organic matter, maturity and type of organic matter. Biomarker analyses of the oil seep indicate that the oil was derived from an early mature clastic source rock of marine origin.

THE TYRRHENIAN SECTION OF SAN GIOVANNI DI SINIS (SARDINIA):STRATIGRAPHIC RECORD OF AN IRREGULAR SINGLE HIGH STAND

A new analysis of the most representative Upper Pleistocene (Tyrrhenian, MIS 5e) section of San Giovanni di Sinis (Oristano, Sardinia) has provided a more detailed genetic stratigraphy of a low wave energy beach and temperate lagoon up to emerged peri-lagoonal facies deposits. These peri-lagoonal facies contain remains of fossil vertebrates, which, though few and fragmentary, bear witness to an at least temporary freshwater palaeoenvironment and the presence of deers and terrapins. Besides, the stratigraphy of this outcrop shows shoreface-backshore sandstones overlaying an erosion surface cut on the vertebrate-bearing layers. Facies analysis and sequence stratigraphy of the succession have provided support to a new eustatic interpretation significance. In fact, there appears to be evidence of one irregular single eustatic highstand, rather than two eustatic peaks as previously believed. The facies evolution and the local stratigraphic disconformities are interpreted as being associated with a lateral shift of the depositional environment within the same system formed during the MIS 5e sea level variations. As sea water level continued to rise so an erosional unconformity, caused by wave ravinement, formed between the low wave energy beach-lagoon sequence and the successive wave dominated beach facies sequence. This interpretation is supported by comparison with other sections of the Tyrrhenian in western Sardinia. The maximum sea level attained during the Tyrrhenian stage is a clear indication of a warm-temperate climate which can be correlated to the well known orbital interglacial configuration when the eustatic signal of Greenland's ice sheet melting occurred. SHORT NOTES

STRATIGRAPHY OF THE CENOZOIC SUBSURFACE SUCCESSION OF THEVENETIAN-FRIULIAN BASIN (NE ITALY): A REVIEW

The present paper reviews and improves the stratigraphy of the Cenozoic subsurface succession of the Venetian-Friulian Basin by means of a foraminiferal study, integrated by calcareous nannofossil analysis in some key stratigraphic intervals, of thirteen ENI wells drilled in the 60-70s and kept reserved since now.The Venetian Friulian Basin is a complex basin due to the superimposition of three overlapping foreland systems, during the Paleocene to Pleistocene time interval. These systems are related to the evolution of three collisional belts (Southern Alps, Apennines and Dinarides) which experienced structuring phases at different time intervals, with different belt orientation, vergence and topography. The peculiar location of the basin at the tectonic knot, between the Po Plain Basin and the Adriatic Basin that are the most important Italian sites of gas reservoirs, makes it as an interesting case-study both from a scientific and industrial point of view. Data of both foraminiferal and nannofossil assemblages were performed using, tentatively, the standard zonations proposed for the tropical-subtropical realm and the Mediterranean region. In the chronostratigraphic intervals where standard events were not recorded, other biohorizons from “regional” zonal schemes were applied. The data obtained allowed to check the applicability of the biohorizons used in the stated schemes to a subsurface succession analysed mainly from cuttings. Results allowed also to provide a rather precise biostratigraphic correlation of the thirteen studied wells which is a fundamental step to better describe the geometry of the sedimentary infill and to reconstruct the synoptic chronostratigraphic frame providing the timing of the sedimentary events and the main sedimentary or erosional hiatuses. Based on this chronostratigraphic scheme, the Cenozoic subsurface succession of the Venetian –Friulian Basin is subdivided into five depositional sequences ( S1-S5 ) bounded by four major unconformities ( U1-U4 ) and by their correlative conformity surfaces, regionally recognised throughout the basin.

NEW DATA ON THE AGE OF THE SIMONI MELANGE, NORTHERN MIRDITA OPHIOLITE NAPPE, ALBANIA

In the Albanian sector of the Dinaric-Hellenic Chain, the ophiolites have been subdivided into two parallel belts: the Western Belt (WB) and the Eastern Belt, due to their different stratigraphical, petrological and geochemical characteristics: WB represents oceanic lithosphere generated at a mid-oceanic ridge (MOR ophiolites), EB represents an oceanic basin developed over a subduction zone (SSZ ophiolites) (ISPGJ-IGJN, 1990; Beccaluva et al., 1994; Bortolotti et al., 1996, and bibl. therein). EB overthrust westwards WB. The radiolarian cherts (Kalur Cherts) covering the volcanites of both the successions have the same ages, comprised between latest Bajocian-early Bathonian (UAZones 5 of Baumgartner et al., 1995) and middle Callovian-early Oxfordian (UAZones 8) (Chiari et al., 2004, and bibl. therein). Thin levels of radiolarian cherts intercalated in the basalts of EB have late Bajocian to latest Bajocian-early Bathonian ages (UAZones 4-5) (Chiari et al., 1994 and Chiari et al., 2004). A “blocks in matrix-type” sedimentary melange, the Simoni Melange, unconformably covers both the successions, indistinctly lying on the radiolarian cherts or on the upper portions of the underlying volcanites (Bortolotti et al., 1996; Carosi et al., 1996). It grades upwards to the Firza Flysch (latest Tithonian-Valanginian age after Shallo 1991; Gardin et al., 1996), a pelagic sediment with frequent ophiolitebearing polimict pebbly sandstones and mudstone intercalations. The melange includes blocks of different sizes (up to several hundred meters) of continental derived (predominant) and ocean derived rocks, in a shaly matrix. It can be interpreted as “syn-orogenic....deposited after the inception of ophiolite deformation” (Bortolotti et al., 1996). The age of the melange was indirectly determined, being comprised between middle Callovian-early Oxfordian, the more recent age found at the top of the Kalur Cherts (Marcucci and Prela, 1996), and the latest Tithonian-early Berriasian?, found at the base of the Firza Flysch (Gardin et al., 1996). The aim of this note is to present new biostratigraphical data that define a more precise age of the Simoni Melange, dating a cherty-silty level found at the top of the Kalur Cherts near the Lumi i Zi (northern Albania).

The Ninyerola Gypsum unit: an example of cyclic, lacustrine sedimentation (Middle Miocene, E Spain)

The Ninyerola Gypsum unit is a Middle Miocene lacustrine deposit, close to 200 m in thickness, which crops out mainly along an anticline structure trending NNE-SSW near the city of Valencia. This anticline structure is comprised of Keuper (Upper Triassic) diapiric materials in the core and a Miocene succession on its fl anks. The Ninyerola Gypsum unit appears at the base of this succession and is overlain by alluvial deposits. The latter deposits grade upwards to terrigenous, coastal marine sediments presumably of Tortonian age. The anticline fl anks of the Miocene succession differ markedly not only in terms of their depositional and diagenetic facies but also in thickness and cyclicity. These differences suggest that synsedimentary deformation of the Triassic basement exerted control on Miocene sedimentation. On the eastern fl ank, where the succession is thicker (>230 m) and well exposed, we distinguished the following stratigraphic intervals from base to top: nodular gypsum (a), bioturbated gypsum (b), alternation of laminated gypsum and carbonate (c), a calcareous interval (d), red lutites and bioturbated gypsum (e), and a clastic alluvial alternation (f); this assemblage unconformably underlies a clastic, coastal marine interval (g). The Ninyerola Gypsum unit constitutes the lacustrine sediments −intervals (a) to (e)− of the succession. In this unit, numerous carbonate-gypsum cycles are recorded in intervals (a) to (c), whose average thickness approaches 6 m. The lacustrine stages of this succession were interpreted as the result of the gradual development of a saline lake of the sulphatecarbonate type overlying the Triassic materials, which extruded during the Lower Miocene. Interal (c) represents the maximum extension, depth and subsidence of this saline lake. This deep lake stage (c) was preceded by a shallow lake stage (b), which initially experienced anhydritic sabkha conditions (a). Stage (c) was followed by a diluted lake stage (d) and a fi nal evaporitic lake stage (e). Subsequently, an input of alluvial clastic materials (f) and marine transgression (g) brought an end to the evaporative conditions in the area.

FACIES ANALYSIS, STRATIGRAPHY AND PALAEONTOLOGY (MOLLUSCS AND VERTEBRATES) IN THE UPPER PLIOCENE SANDY FLOOD-BASIN DEPOSITS OF THE UPPER VALDARNO BASIN (NORTHERN APENNINES)

The Upper Valdarno Basin, one of the most investigated Neogene–Quaternary basins of the Northern Apennines, developed during three main phases, as testified by the occurrence of three unconformity-bounded stratigraphic units (UBSUs) in the basin infill. Despite numerous studies having been carried out, biochronological, paleoecological and stratigraphical issues in the lower portion of the Montevarchi Synthem (second phase) have yet to be understood. Sandy deposits (Montecarlo Sand and Silt Unit), stratigraphically located in this portion of the Montevarchi Synthem, are the focus of this multidisciplinary study. These deposits conformably overlie sandy fluvio-eolian sediments and are, in turn, capped by fluvio-palustrine deposits through a progressive unconformity. Facies analysis suggest a sandy flood-basin environment for these deposits, characterised by variations in water discharge and flood event energy. Mollusc and fish remains, pointing to quiet or slow-moving shallow waters, have been affected by transport processes before final burial in overbank areas. Fish remains of the primary marine family Mugilids highlight a connection between the basin and the sea that was previously only supposed. Small mammal remains, referred to the rodent Mimomys polonicus , are coherent with a water-rich environment. Cyclic variations in shell content and sedimentological characteristics testify to the occurrence of short-term climatic oscillations during this warming phase. This study fits with paleomagnetic and radiometric datings and mammal biochronology, in indicating that the Montecarlo Sand and Silt Unit belongs to a time interval preceding the Reunion paleomagnetic event. The depositional evolution of the Montecarlo Unit was driven by climatic change from arid to humid conditions, related to a global increase in temperature that occurred between 2.4 and 2.2 Ma.

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).

EVOLUÇÃO GEOLÓGICA DA PORÇÃO MERIDIONAL DA PROVÍNCIA BAUXITÍFERA DE PARAGOMINAS DURANTE O NEÓGENO/PLEISTOCENO (NOROESTE DA BACIA DO GRAJAÚ, NORDESTE DO PARÁ E EXTREMO OESTE DO MARANHÃO)

The surface geology of the southern Paragominas Bauxite Province, which lies in the northwestern Grajau basin, essentially consists of two sedimentary-lateritic sequences. The oldest one is composed of siliciclastic deposits of the Cretaceous Itapecuru Group overlain by a lateritic/bauxitic mantle, which sustains the Paleogene Sul-Americana surface, and a clayey overburden – the Belterra Clay, whose top is marked by a fairly plane regional surface. The youngest sequence unconformably overlies the Itapecuru deposits in the low areas, which separate the plateaus of the Sul-Americana surface. The lower unit of this sequence consists of a reddish, poorly sorted and friable clayey sandstone up to 30 m thick, which contains dispersed laterite fragments and quartz pebbles, besides lateritic stone-lines. Although it is normally massive, the sandstone occasionally shows an incipient stratification. The reddish sandstone is overlain, in sharp contact, by a ferruginous duricrust up to 3 m thick, that sustains the Late Velhas surface of Neomiocene age. According to its degradation degree, the duricrust displays a columnar, blocky or nodular structure or a pisolitc facies. When completely degraded, the duricrust is transformed into a ferruginous stone-layer. Conglomeratic accumulations, composed of laterite blocks, pelitic fragments and strongly subordinate quartz pebbles locally occur, corresponding to colluvial deposits resulting from the dismantling of the oldest residual cover. The ferruginous duricrust is overlain in sharp contact by a homogeneous, yellowish sandy clay, up to 5 m thick, that contains minute laterite fragments. The surface which marks the top of this overburden is situated 60 to 80 m below the surface sustained by the Belterra Clay. The reddish sandstone represents a continental deposit originated by successive debris-flows and is correlated with the Pirabas and Barreiras Formations of Neo- Oligocene to Mesomiocene age. The ferruginous horizon, interpreted as a water-table laterite, was probably formed during the Neomiocene, while the clayey sandy overburden, comparable with the continental Pos-Barreiras sediments described to the north, in the Bragantina Platform, should be of Pliocene age.

GEOLOGY OF THE HOMO -BEARING PLEISTOCENE DANDIERO BASIN (BUIA REGION, ERITREAN DANAKIL DEPRESSION)

This paper deals with the geological context of the northernmost site in the East Africa Rift system which has yielded Homo erectus -like remains. They are dated ca. 1 Ma and have been found in the deltaic deposits of the Alat Formation belonging to the Dandiero group. This newly defined group crops out extensively in an elongated belt from the Gulf of Zula to the North to the Garsat area to the south. In the Buia-Dandiero area it ranges in age from the Early to the Middle Pleistocene, and incorporates six formations, from bottom up: the fluvial Bukra Sand and Gravel, the deltaic and lacustrine Alat Formation, fluvial Wara Sand and Gravel, the lacustrine Goreya Formation, the fluvio-deltaic Aro Sand and alluvial Addai Fanglomerate. This succession is bounded by two major unconformities, which separate it from the Neoproterozoic basement and from the overlaying Boulder Beds fanglomerate, and has been designated the Maebele Synthem. The latter is the result of two lacustrine transgression and regressions evidenced by two depositional sequences. The unconformities bounding the Maebele Synthem are related to the tectonic history of the basin fill and its substrate. The development of the two sequences was, instead, mainly controlled by lake level fluctuations and, hence, by climatic variations connected with the weakening and strengthening of the monsoons in the northwestern Indian ocean. The environment where the Buia Homo lived was a savannah with some scattered water pools. This environment probably extended farther north along the western coastal plain of the Red Sea, and was a preferential pathway for the dispersal of the hominids from East Africa toward Eurasia.

An electron diffraction and bond valence sum study of the space group symmetries and structures of the photocatalytic 1:2 B site ordered A3CoNb 2O 9 perovskites ( A=Ca 2+, Sr 2+, Ba 2+)

A careful investigation has been carried out into the space group symmetries, structures and crystal chemistries of the 1:2 B-site ordered triple perovskites A 3Co 2+Nb 5+ 2O 9 ( A=Ca 2+, Sr 2+, Ba 2+) using a combination of bond valence sum calculations, powder XRD and electron diffraction. A recent investigation of these compounds by Yin et al. reported a random distribution of Co 2+ and Nb 5+ ions onto the perovskite B-site positions and hence Pm 3 ̄ m ( a= a p, subscript p for parent perovskite sub-structure) space group symmetry for the A=Ba and Sr compounds and a monoclinically distorted variant thereof (although no space group was given) in the case of the A=Ca compound. An electron diffraction study, however, shows that the A=Ba compound occurs in a 1:2 B site ordered trigonal P 3 ̄ m1 , a h = b p − c p , b h = c p − a p , c h = a p + b p + c p structure type while both the A=Ca and Sr compounds occur at room temperature in monoclinic P12 1/ c1, a m =[ 1 ̄ 1 ̄ 2] p , b m =[1 1 ̄ 0] p , c m =3[110] p , perovskite-related structure types. The latter monclinic structure type can be described as a modulated variant of the 1:2 B site ordered trigonal P 3 ̄ m1 structure type with the additional modulations associated with a b – b − c + octahedral tilt sequence. Bond valence sum calculations are used to explain why this should be so as well as to provide a useful zeroth-order approximation to the structures of each of the compounds.

TRANSITION FROM CARBONATE PLATFORM TO PELAGIC DEPOSITION (MID JURASSIC- LATE CRETACEOUS), VOURINOS MASSIF, NORTHERN GREECE

A Jurassic- Cretaceous carbonate succession crops out along the Zyghosti Rema, Kozani (Northern Greece). The substratum consists of the ophiolitic succession of the Vourinos Massif (Pelagonian Domain): serpentinites tectonically overlain by basalts, with thin lenses of radiolarian cherts of middle Bathonian age. The contact with the overlying Jurassic limestones is tectonic. Eight informal units have been distinguished within the Mesozoic limestones, from the base upwards. (A) bioclastic, intraclastic and oolitic packstone (Callovian- Oxfordian). (B) bioclastic packstone and coral boundstone (Oxfordian ). (C) bioclastic and oncoidal wackestone with Clypeina jurassica (Oxfordian- Upper Kimmeridgian). (D) (Upper Kimmeridgian- Portlandian): oncoidal packstone and rudstone (facies D1); intraclastic and bioclastic grainstone and packstone (facies D2); neptunian dykes with intraclastic and bioclastic wackestone and packstone filling (facies D3); neptunian dykes with Fe-Mn rich laterite filling and with pink silty filling of early Late Cretaceous age. An unconformity surface, due to emersion and erosion of the platform during the latest Jurassic- Early Cretaceous, is overlain by (E) intraclastic, bioclastic packstone and grainstone (Cenomanian). (F) massive body of debrites with coral, echinoderm, algae and rudist large clasts (facies F1) (Cenomanian); turbiditic beds of bioclastic, intraclastic and lithoclastic rudstone and grainstone (facies F2). (G) thin bedded bioclastic mudstone and wackestone with planktonic foraminifers and radiolarians, alternating with turbiditic beds of bioclastic, intraclastic packstone and rudstone and with conglomeratic levels and slumped beds of the previous turbidites (upper Santonian- lower Campanian). (H): bioclastic packstone with planktonic foraminifers (facies H1) (lower Campanian - ?Maastrichtian); amalgamated turbiditic beds of bioclastic wackestone and packstone with planktonic foraminifers (facies H2); turbiditic beds of bioclastic packstone and rudstone (facies H3). These features allow to recognise the following sequence of events: 1) development of a carbonate platform in the Middle and Late Jurassic; 2) its overthrusting onto the ophiolites and its emersion starting from latest Jurassic time, with erosion and deposition of laterites; 3) marine transgression on the Jurassic platform and on the ophiolites during the early Late Cretaceous, and 4) extensional tectonism and platform demise starting in the Cenomanian, with sedimentation of gravity flows and turbidity currents deposits from the Cenomanian to the Campanian- ?Maastrichtian.

THE RECORD OF FRONTAL TECTONIC EROSION IN A FOSSIL ACCRETIONARY WEDGE: EVIDENCES FROM THE BOCCO SHALE (INTERNAL LIGURIDE OPHIOLITIC SEQUENCE, NORTHERN APENNINE, ITALY)

In the Northern Apennines, the Internal Liguride units are characterized by an ophiolite sequence that represents the stratigraphic base of a Late Jurassic-Early Paleocene sedimentary cover. The Bocco Shale represents the youngest deposit recognized in the sedimentary cover of the ophiolite sequence, sedimented just before the inception of subduction-related deformation history. The Bocco Shale was unconformably deposited over the underlying formations, from the Gottero Sandstone to the Palombini Shale. The deposits belonging to the Bocco Shale can be subdivided into two different groups of deep sea sediments. The first group is represented by slide, debris flow and high density turbidity current-derived deposits, whereas the second group consists thin bedded turbidites. Facies analysis and provenance studies indicate, for the former group, small and scarcely evoluted flows that rework an oceanic lithosphere and its sedimentary cover. We interpret the Bocco Shale as an ancient example of a deposit related to the frontal tectonic erosion of the accretionary wedge slope. The frontal tectonic erosion resulted in a large removal of material from the accretionary wedge front reworked as debris flows and slide deposits sedimented on the lower plate above the trench deposits. These deposits may have been successively deformed and metamorphosed during the following accretion processes. The frontal tectonic erosion was probably connected with subduction of oceanic crust characterized by positive topographic relief. The frontal tectonic erosion can be envisaged as a common process during the convergence-related evolution of the Ligure-Piemontese oceanic basin in the Late Cretaceous- Early Tertiary time span. This interpretation can be also suggested for the analogues deposits from oceanic units of Western Alps and Corsica.

STRATIGRAPHY AND TECTONIC AND METAMORPHIC EVOLUTION OF THE PORTO AZZURRO UNIT IN THE MONTE CALAMITA PROMONTORY (SOUTHEASTERN ELBA ISLAND, TUSCANY)

The Elba Island has a key role in the reconstructions of the stratigraphic, tectonic, metamorphic and magmatic evolution of the Northern Tyrrhenian Sea and of the inner part of the Northern Apennines chain. The Porto Azzurro Unit, cropping out in the SE part of the Island, is the deepest tectonic unit of the central-eastern Elba structural pile of Tuscan, Ligurian and Ligurian-Piedmontese Nappes, which were intruded by Late Tortonian-Lower Pliocene granitoids and mainly acidic dikes. Moreover, in this part of the Island, the relationships between the uplift of the plutonic bodies and the final deformations of the tectonic stack are well exposed. To improve the geological knowledge of SE Elba, the authors carried out a 1:10.000 geological survey of the Calamita Promontory (mostly made up of the Porto Azzurro Unit) and performed petrographic and meso-/micro-structural studies on its rocks. The Porto Azzurro Unit consists of a Paleozoic, likely pre-Carboniferous basement (Mt. Calamita Fm.), which is unconformably overlain by the ?Triassic Verrucano metasiliciclastics (Barabarca Quartzites) and ?Upper Triassic-?Hettangian metacarbonates. In the Mt.Calamita Fm., five main lithofacies were recognized and mapped. In particular, garnet-bearing, albite micaschist (lithofacies a) geometrically underlie a phyllitic-quartzitic unit (lithofacies b); Porphyroids-like rocks (lithofacies e), metabasite bodies (lithofacies d) and graphite-rich siliciclastics (lithofacies c) are also present. The rocks of the lithofacies a are similar to those of the ?pre-Paleozoic-?Paleozoic Micaschist Complex of the Larderello Geothermal Field, whereas the other lithofacies can be probably correletable with the ?Ordovician formations of the Tuscan Metamorphic Units. The complex deformation-metamorphic evolution of the Porto Azzurro Unit consists of the following events: a) a Variscan tectono-metamorphic event (Dx), recognized in the Mt.Calamita Fm., which is defined by pre- Alpine schistosity and mineralogic relics (garnet); b) two Alpine tectono-metamorphic folding events (D1 and D2) in the Greenschists facies, which deformed also the Mesozoic covers; c) a following folding event (D3) which probably occurred during or immediately after the strong thermometamorphic imprint (including the magnetite-rich skarn bodies), due to the Neogene magmatic intrusions; d) Subsequently, the uplift of the magmatic bodies caused low-angle detachments within the Porto Azzurro Unit (between the Mt.Calamita Fm. and the Mesozoic cover) and between the latter and the overlying tectonic Units (e.g. Zuccale Fault between the Porto Azzurro Unit and the Cretaceous Flysch). A final weak antiformal folding (D4) of the whole promontory took place before the development of NW-SE and N-S trending high –angle normal fault systems, locally sealed by hydrothermal, sometimes Fe-rich mineralizations. The lithostratigraphic, tectonic, metamorphic and magmatic evolution of the Porto Azzurro Unit is similar to that defined for the Larderello geothermal region. Thus, the Mt.Calamita area can be considered as a little older, but similar geological model for all the future interpretations of the deep structure of southern Tuscany crossed by the Crop 18 profile.

Two Stages of Deformation and Fluid Migration in the West-Central Brooks Range Fold and Thrust Belt, Northern Alaska

The Brooks Range is a north-directed fold and thrust belt that forms the southern boundary of the North Slope petroleum province in northern Alaska. Field-based studies have long recognized that large-magnitude, thin-skinned folding and thrusting in the Brooks Range occurred during arc-continent collision in the Middle Jurassic to Early Cretaceous (Neocomian). Folds and thrusts, however, also deform middle and Upper Cretaceous strata of the Colville foreland basin and thus record a younger phase of deformation that apatite fission-track data have shown to occur primarily during the early Tertiary (60 and 45 Ma). A structural and kinematic model that reconciles these observations is critical to understanding the petroleum system of the Brooks Range fold and thrust belt. New interpretations of outcrop and regional seismic reflection data indicate that from the modern mountain front northward to near the deformation front under the coastal plain, the basal thrust detachment for the orogen is located in the Jurassic and Lower Cretaceous Kingak Shale in the upper part of the regionally extensive, gently south-dipping, north-derived Mississippian to Early Cretaceous Ellesmerian sequence. The frontal part of the orogen lies in middle Cretaceous foreland basin strata and consists of a thin-skinned fold belt at the deformation front and a fully developed passive-roof duplex to the south. Near the mountain front, the orogen is composed of a stacked series of allochthons and thrust duplexes and associated Neocomian syntectonic deposits that are unconformably overlain by proximal foreland basin strata. The foreland basin strata and underlying deformed rocks are truncated by a younger generation of folds and thrusts. Vitrinite reflectance and stable isotope compositions of veins provide evidence of two fluid events in these rocks, including an earlier higher temperature (250–300C) event that was buffered by limestone and a younger, lower temperature (150C) event that had distinctly lower 13C values as a result of oxidation of organic matter and/or methane. Zircon fission-track data from the host rocks of the veins show that the higher temperature fluid event occurred at 160–120 Ma, whereas the lower temperature event probably occurred at about 60–45 Ma. It is proposed that the Brooks Range consists of two superposed contractional orogens that used many of the same mechanically incompetent stratigraphic units (e.g., Kayak Shale, Kingak Shale) as sites of thrust detachment. The older orogen formed in a north-directed arc-continent collisional zone that was active from 160 to 120 Ma. This deformation produced a thin-skinned deformational wedge that is characterized by far-traveled allochthons with relatively low structural relief, because it involved a thin (1–4-km [0.6–2.5-mi]-thick) stratigraphic section. Deeper parts of the deformational wedge are envisioned to have contained relatively high-temperature fluids that presumably migrated from or through limestone-rich source areas in the underlying autochthon or from deeper parts of the orogen. The younger orogen, which formed initially at about 60 Ma and reactivated at 45 Ma, produced a thrust belt and frontal triangle zone with low amounts of shortening and relatively high structural relief, because it involved a structural section 5–10 km (3–6 mi) thick. Fluids associated with this deformation were relatively of lower temperature and suggest that hydrocarbon migration occurred at this time. We conclude that hydrocarbon generation from Triassic and Jurassic source strata and migration into stratigraphic traps occurred primarily by sedimentary burial principally at 100–90 Ma, between the times of the two major episodes of deformation. Subsequent sedimentary burial caused deep stratigraphic traps to become overmature, cracking oil to gas, and initiated some new hydrocarbon generation progressively higher in the section. Structural disruption of the traps in the early Tertiary released sequestered hydrocarbons. The hydrocarbons remigrated into newly formed structural traps, which formed at higher structural levels or were lost to the surface. Because of the generally high maturation of the Colville basin at the time of the deformation and remigration, most of the hydrocarbons available to fill traps were gas.