Fan-delta System Research Articles

‘Singwang strike-slip duplex’ around the Pohang Basin, SE Korea: its structural evolution and role in opening and fill of the Miocene basin

In order to elucidate the structural characteristics of the Yangsan Fault in southeastern Korea the Ganggu-Angang area has been studied in terms of fault branching and fault linkages. According to the results of this study, eight faults (e.g., the Winmal, Docheon, Yugye, Gojusan, Naengsu, Malgol, Ogeum and Moa faults) branch from the Yangsan Fault, and divide it into at least nine segments. The strike separations of these faults suggest that they are much longer than their presently exposed lengths and that the majority of them are linked to the Jangsa Fault as connecting faults. This distribution of faults comprises the ‘Sing-wang strike-slip duplex’ with the geometry of an extensional strikeslip duplex or pull-apart basin. Block faulting took place along the connecting faults and related transfer faults, resulting in the deposition of coarse grained sediments in fan-delta systems. The shape of basalt masses around the northern tip of Yangsan Fault reveals that this fault had a sinistral motion, possibly in relation to the Pliocene (?) rotation of the Pohang-Ulsan Block. At a recent stage, the Yangsan Fault was displaced by WNW-ESE, E-W or ENE-WSW trending faults. Quaternary reactivation occurred along the Yangsan Fault but not along the Jangsa Fault.

Stable oxygen and carbon isotopes of carbonate concretions of the Miocene Yeonil Group in the Pohang Basin, Korea: Types of concretions and formation condition

Carbonate concretions in the Miocene sedimentary rocks of the Yeonil Group in the Pohang Basin (Korea) were investigated in terms of stable oxygen and carbon isotope composition to delineate the origin and associated diagenetic environment for their formation. Carbonate concretions are widely distributed in all the sedimentary rocks in the Pohang Basin, showing that the calcitic concretions are preserved within the mass-flow deposits and the dolomitic ones mostly in the hemipelagic siliceous rocks (diatomites). Concretions can be classified into four different types, on the basis of the stable isotopic signatures, each of which represents its own geochemical range. Type I concretions are calcitic and are composed of micrite to microspar. They occur in the conglomerates and sandstones which were deposited by mass flows (debris flow to turbidity current). It shows relatively lower δ 18O (− 14.0 to − 9.3‰) and δ 13C (− 19.6 to − 8.4‰) values. These concretions grew in a sulfate reducing zone under the influence of residual ambient seawater which had been significantly modified by volcanogenic sediments. Type II concretions are also calcitic, composed mostly of micrite with minor microspar and found in the sandstones. These concretions are characterized by relatively high δ 18O (+ 1.8 to + 2.4‰) and variable δ 13C (− 17.3 to − 0.4‰) values. These isotopic signatures reflect that Type II concretions formed from just beneath the sediment/water interface down to the sulfate reducing zone through the early stage of methanogenesis. Type III concretions are also calcitic, and composed largely of micrite with a minor contribution of microspar. They are observed in hemipelagic mudrocks which were deposited under the influence of mass flows. They are characterized by intermediate to high δ 18O (− 4.6 to + 1.6‰) and high δ 13C (− 1.3 to + 8.8‰) values. These concretions grew in a methanogenic zone by residual ambient seawater and/or seawater slightly modified by reaction with volcanogenic sediments. Type IV concretions are dolomite with calcite inclusion, and occur in hemipelagic siliceous rocks. These concretions are mostly composed of micrite and characterized by variable δ 18O (− 9.1 to + 0.7‰) and high δ 13C (+ 3.1 to + 17.9‰) values, suggesting formation in the methanogenic zone, although the residual ambient seawater is slightly modified by volcanogenic sediments. The same type of the concretions is widely distributed throughout the basin and always shows its own distinctive stable isotopic signature. This means that the formation of the given type depends upon the lithology and composition of host sediments that are closely related to the depositional process of the fan-delta systems regardless of their localities. Further, the different types of concretions are also found at the different, but closely spaced stratigraphic levels in the same locality, displaying the distinctive diagenetic conditions for each type. Such preservation of the unique diagenetic signatures in individual type of concretion suggests that the concretions formed in a completely closed diagenetic system. Therefore, caution should be made to simplify and generalize the diagenetic condition for the formation of any concretions in a large sedimentary basin.

Facies architecture and depositional evolution of alluvial fan to fan-delta complexes in the tectonically active Miocene Köprüçay Basin, Isparta Angle, Turkey

This paper provides the first general account of the facies associations that fill a Miocene fault bounded basin located in the central part of the Isparta Angle (Turkey), in order to define process-controlled sedimentary units reflecting environmental changes in a general tectonic and stratigraphic framework. The northern and western parts of the Köprüçay Basin are occupied by extensive conglomeratic successions belonging to three distinct alluvial fan–fan delta systems (AFD), which pass laterally into pelagic mudstones in the deeper part of the basin. In spite of syn- and post-sedimentary tectonics, most of the sedimentary units still display their original relationships. This allows definition and interpretation of the conspicuous facies changes in the clastic successions according to their position along complete sections across the basin. The bulk of the paper is thus devoted to the description of 11 sub-facies types that have been identified in the three AFDs, and their environmental interpretation. As a result, the origin and sedimentary infill of the Köprüçay Basin appears primarily controlled from Langhian to Tortonian by the N–S trending Kirkkavak Fault. After uplift and erosion of the western part of the basin, the final closure of the Isparta Angle resulted in a westward displacement of the Anatolian block during Upper Tortonian.

PARADICTYODORA ANTARCTICA: A NEW COMPLEX VERTICAL SPREITE TRACE FOSSIL FROM THE UPPER CRETACEOUS-PALEOGENE OF ANTARCTICA AND TIERRA DEL FUEGO, ARGENTINA

Abstract A new ichnotaxon, Paradictyodora antarctica, is proposed for complex spreite trace fossils from the Upper Cretaceous of Antarctica and Eocene of Tierra del Fuego, Argentina. This complex, three-dimensional, vertical spreite structure enlarges upwards, displaying a prismatic-to-conical shape that consists of subvertical folded laminae, produced by the lateral migration of a subvertical J-shaped tube anchored at its base. Overall morphology and mode of formation distinguishes this new ichnotaxa from other ichnogenera such as Daedalus, Dictyodora, and Heimdallia. Paradictyodora occurs in fodinichnia-dominated ichnocoenosis developed in distal fine-grained deposits of deepwater fan delta systems.

Quantitative Taphonomy in Sandstones from an Ancient Fan Delta System (Lower Pleistocene,Western Emilia, Italy)

Other| June 01, 2004 Quantitative Taphonomy in Sandstones from an Ancient Fan Delta System (Lower Pleistocene,Western Emilia, Italy) STEFANO DOMINICI STEFANO DOMINICI 1Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, 50121 Florence, Italy,stefano.dominici@geo.unifi.it Search for other works by this author on: GSW Google Scholar Author and Article Information STEFANO DOMINICI 1Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, 50121 Florence, Italy,stefano.dominici@geo.unifi.it Publisher: SEPM Society for Sedimentary Geology Accepted: 22 Jan 2004 First Online: 03 Mar 2017 Online ISSN: 1938-5323 Print ISSN: 0883-1351 Society for Sedimentary Geology PALAIOS (2004) 19 (3): 193–205. https://doi.org/10.1669/0883-1351(2004)019<0193:QTISFA>2.0.CO;2 Article history Accepted: 22 Jan 2004 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation STEFANO DOMINICI; Quantitative Taphonomy in Sandstones from an Ancient Fan Delta System (Lower Pleistocene,Western Emilia, Italy). PALAIOS 2004;; 19 (3): 193–205. doi: https://doi.org/10.1669/0883-1351(2004)019<0193:QTISFA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract Many ancient settings differ from Holocene shallow seas, and studies on quantitative taphonomy carried out in modern shell beds do not necessarily answer all questions that the fossil record may raise. In order to help characterize past environments, sedimentological shell beds from a Lower Pleistocene fan-delta system are analyzed. The studied sandstone intervals, cyclically alternated with a mudstone facies, were deposited in a tectonically active setting during phases of advance of fan deltas. High-density flows triggered by river floods mixed skeletal remains from different shallow-water communities. Standardized quantitative taphonomic procedures tested in modern settings are applied for the first time to fossil bivalves. The study shows that intrinsic factors did not overprint the pattern dictated by extrinsic agents. Damage levels are inversely related to sedimentation rate and directly related to the volume of high-density flows, although the influence of other agents cannot be eliminated. The most revealing taphonomic variables are fine-scale alteration, edge modification, fragmentation, and discoloration, while bioerosion and encrustation were insignificant, due to generally high rates of sedimentation. The study suggests different sources of skeletal remains. Fine-scale alteration, edge modification, and fragmentation are higher in shells from aerobic muddy sediments, and it is suggested that these remains came from erosion of older, tectonically exposed sediments of the mudstone biofacies. The majority of shells are fresh and came from nearshore sandy and muddy biofacies. These are interpreted as contemporary, or nearly so, to the depositional events. The overall results underline the importance of active tectonics in causing repeated burial/exhumation cycles and episodic burial of articulated, or otherwise pristine, skeletal remains, causing the frequent mixing of taphonomic signatures through lateral transport. This situation may be characteristic of other fan-delta systems from ancient structurally confined basins. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Santa Bárbara Formation (Caçapava do Sul, southern Brazil): depositional sequences and evolution of an Early Paleozoic postcollisional basin

The Santa Bárbara Formation, located in southernmost Brazil, comprises red-colored conglomerates, sandstones, and siltstones. It was deposited in the Early Paleozoic under continental conditions in a restricted, strike-slip basin (Santa Bárbara basin) in response to the postcollisional stresses of the Brasiliano/Pan-African cycle (900–500 Ma). Five facies associations are recognized: alluvial fans, fan-delta front, lacustrine, sandy braided, and gravel bed braided river deposits. The application of sequence stratigraphy enables the recognition of three depositional sequences. The two basal sequences (I and II) represent a coherent depositional pattern, with axial fluvial and fan-delta systems that deposit northeastward and lateral contribution from alluvial fans. Such coarse deposits are composed mainly of metamorphic clasts derived from the erosion of ‘Caçapava high’, the eastern steep margin of the Santa Bárbara basin. Sequence III lies unconformably over the basal subunits and reflects the inversion of the axial systems, in that the paleocurrents of the gravel bed deposits systematically point south/southwestward. The alluvial fan deposits of Sequence III also suggest a tectonic rearrangement of the basin, with partial erosion of the basal sequences and the presence of granitoid fragments, which reflects a deeper denudation stage for the Caçapava high and possibly a significant hiatus at the base of Sequence III.

Facies analysis and tectonic significance of lacustrine fan-deltaic successions in the Pliocene–Pleistocene Mugello Basin, Central Italy

The Mugello Basin is an intermontane asymmetric basin, trending WNW–ESE and filled with Pliocene–Pleistocene alluvial and lacustrine deposits. The study focuses on the sedimentary succession deposited at the basin's northern margin and uses facies analysis to reconstruct the margin's depositional and deformation history. The controversial concepts of “fan delta” and “hyperconcentrated flow,” adopted in this study, are firstly discussed. The early Pleistocene succession consists of two higher order, coarsening-upward successions (time span ∼10 5 years), separated by an angular unconformity and referred to as the lower fan delta (LFD) and upper fan delta (UFD) systems. They show an alternation of alluvial fan, fan delta front, lacustrine, and floodplain facies associations. The alluvial facies indicate sediment dispersal by flashy, hyperconcentrated flows in the form of sheetfloods and moderately channelized flows. Deposits of the former predominate in the LFD system and deposits of the latter in the UFD system, and this change is attributed to the greater volumes of water yielded by enlarged fan catchments. The fan deltas are of Gilbert and shoal water types, indicating variable water depths at the lake margin. Each of the successions consists of fining- and coarsening-upward cyclothems of medium (tens of meters) and small scale (meters). These different-scale cycles are interpreted as the sedimentary response to pulses of a compressional deformation of the basin margin at variable frequencies, related to the contemporary Northern Apennine active thrusts (ca. 50 km away from the basin). The high-frequency flood events are attributed to episodes of heavy rainfall. The episodes of compression downwarped the basin margin alluvium, made the central lake shift periodically toward the margin, and created progressive unconformities in the sedimentary succession. The immediate effects of a compressional pulse included lake transgression and accentuation of the structural hinge of the basin margin, causing a decline of sediment supply from the catchments. As the hinge relief was subsequently reduced by denudation, the alluvial fans prograded and fan deltas were formed in normal conditions of graben subsidence. The time lag determined the amount of fine-grained sediment accumulated at the lake margin, and thus the water depth and fan delta type.

Evolution of Qinling Mianlue Belt: Evidence from Sedimentology and Tectonics of the Northern Yangtze, China

Evolution of Qinling Mianlue Belt: Evidence from Sedimentology and Tectonics of the Northern Yangtze, China

MIDDLE PENNSYLVANIAN GASTROPODS FROM THE FLECHADO FORMATION, NORTH-CENTRAL NEW MEXICO

The lower Desmoinesian part of the Flechado Formation near Taos, New Mexico, contains a diverse marine fauna within predominantly dark gray shale and siltstone facies representing deposition in shallow marine environments adjacent to prograding lobes of a fan delta system. Gastropods are particularly abundant and diverse in a 60-m-thick section south of the town of Talpa; 157 species are described in this paper, including 19 new species and two new genera. New species are Euphemites hermosus, Luciellina ocultabanda, Spiroscala georgiannae, Glabrocingulum (Glabrocingulum) globosum, G. (Ananias) talpaensis, Worthenia legrandi, Cyclites diminutus, Platyzona hespera, Anomphalus? blancus, Stegocoelia (Donaldospira) taosensis, S. (Hypergonia) hoffmani, S. (H.) agraciada, Bicuerda procolumnare, new genus and species; Hermosanema varium, new genus and species, Pseudozygopleura (Stephanozyga) granda, P. (S.) lisaspira, Hemizyga (Hemizyga) larga, Strobeus immanis, and Meekospira delgada. Species of Pleurotomarioidea, Neritoidea, Trochoidea, and Murchisonioidea comprise 64 percent of the total gastropod specimens, with most specimens of each of the last three groups being contributed by large numbers of one or two very abundant species. The 20 most abundant species are represented by about 65 percent of all specimens, whereas 62 species (39 percent of total species richness, but only 1.1 percent of all specimens) are represented by six or fewer specimens, illustrating the significant contribution of rare species to total species richness. The Flechado gastropod fauna contains many species known from Desmoinesian strata in the Midcontinent and Appalachian Basin regions of the U.S.; many elements of Desmoinesian gastropod assemblages were distributed widely in shallow marine environments across North America. Potential paleogeographic barriers in the Colorado–New Mexico region, however, may have facilitated evolution of some endemic species in this area.

Bored pebbles and ravinement surface clusters in a transgressive systems tract, Sant Llorenç del Munt fan-delta complex, SE Ebro Basin, Spain

A 25-m thick transgressive systems tract in the Sant Llorenç del Munt, wave-influenced, fan-delta system (Eocene, SE Ebro Basin) has an internal framework consisting of a cluster of transgressive erosion surfaces, each of which has minor relief and which are collectively stacked vertically no more than two metres apart. Each erosion surface bounds a cycle containing a conglomeratic lag (up to 0.5-m thick) followed by a coarsening-upward sandstone to conglomeratic unit (the uppermost levels of which can be nonmarine). Individual cycles become entirely nonmarine landwards of the termination of the basal-bounding erosion surface, whereas they thin and eventually become entirely marine basinwards. The individual erosion surfaces within the transgressive tract, some 16 of them within a 20-m thick lithosome, are interpreted as wave-ravinement surfaces that repeatedly eroded into the conglomeratic shoreface during transgression. This interpretation, rather than one invoking nonmarine flooding or other marine erosion surface types, is consistent with the arrangement of bivalve and sponge borings on the top surfaces of clasts and with the associated lag pavements. Multiphase boring around the entire surface of clasts, as well as erosion of the clasts at some horizons, particularly in reaches of the tract where the ravinement trajectory is sub-horizontal, suggest repeated reworking of previously generated lag pavements in zones of minimal aggradation during transgression. Where the transgressive shoreline trajectory rises more steeply and there has been more rapid aggradation during transgression, the lag pavements show only single-phase borings, with the borings on the upper surface of the pebble-pavement only. The close spacing of erosion surfaces within the transgressive systems tract, together with estimates of time span in the tract, suggest that transgressive erosion occurred with a frequency of less than 500 years.

Unusual features of sediment supply-dominated, transgressive–regressive sequences: Paleogene clastic wedges, SE Pyrenean foreland basin, Spain

Abstract The Paleogene marine to nonmarine succession in the southeastern Pyrenean foreland basin consists of a series of 3rd-order (3–5 m.y.) transgressive-to-regressive sequences that can be related directly to the stacking and southward displacement of successive basement-cover thrust sheets. The regressive limbs of the 3rd-order clastic wedges are coeval with thrust movement, with maximum subsidence developing towards peak regression. This relationship between sedimentation and tectonics is sediment supply-driven. One of these 3rd-order wedges, which was produced by the Montserrat and Sant Llorenc del Munt fan-delta systems, demonstrates how such steep-sloped, coarse-grained, high sediment supply systems generate a hierarchy of transgressive–regressive architectures. Striking features of this rapidly subsiding, supply-dominated basin margin, in addition to the tectonics/wedge architecture relationship, include: • the nonperiodic character of the contained high-frequency sequences; • the unusual thickness and landward-thickening geometry of transgressive systems tracts; • a tendency for “misfit” between alluvial fan and shorezone sectors of sequences; • a lack of marked incision associated with the tops of coarse-grained, regressive phases of sequences. Most of the features are symptomatic of high subsidence rates and very high sediment-supply. The last feature, suggestive of few major relative falls of sea level, is also consistent with continuous rapid subsidence. However, it should be remembered that lack of incision may be a poor indicator for lack of base-level fall in a setting where the steepness of alluvial slopes exceeds the slope of the coeval shelf.

Nearshore and alluvial facies in the Sant Llorenç del Munt depositional system: recognition and development

Abstract The Eocene Sant Llorenc del Munt fan-delta complex forms a part of the coarse-grained foreland basin fill of the southeastern margin of the Ebro Basin. It was sourced from the rising Catalan Coastal Range in the SE and shows an overall northwesterly progradation during a time period of approximately six million years. Coarse clastic deposits (coarse sands and gravels) dominated this fan delta, whereas finer-grained sands, silts, and muds were deposited laterally off active sedimentation sites or in the more marine area. Depositional processes were clearly influenced by the high sediment input, giving a dominance of sediment gravity flow deposits. The sediments studied have been grouped into four distinct facies belts including coastal plain, proximal fan-delta front, distal fan-delta front and fan-delta slope sediments, respectively. The facies belts combine to form the nearshore and coastal alluvial reaches of the fan-delta system, developed during a transgressive phase lasting some 50,000 years.

Tectonic and sedimentary evolution of the Korean peninsula: a review and new view

This review focuses on the tectonics and sedimentation of major sedimentary basins and orogenic belts (Late Proterozoic–Neogene) in the Korean peninsula. The Korean peninsula is part of the Amuria Plate and represents an important link between continental blocks of North and South China and the island arcs of Japan. The basement rocks, exposed in the Kyonggi and Yongnam massifs, consist of 2.7 to 1.1 Ga high-grade gneiss and schist. These massifs are separated by the Okchon Fold Belt which comprises metasedimentary rocks and bimodal meta-volcanic rocks. The stratigraphy of the Okchon Group is unclear at present. The Okchon Basin was probably initiated as an intraplate rift prior to the Late Proterozoic. The Hwanggangri Formation (clast-bearing phyllite) most likely represents deposition by subaqueous debris flows in slope environments of an enclosed basin. The stratigraphic relationship between the Okchon Group and the Choson Supergroup (Cambro-Ordovician) of the Taebaeksan Basin is poorly constrained. The Choson Supergroup unconformably overlies the Yongnam Massif and consists mainly of carbonate sequence that formed mostly in shallow marine and tidal environments, reflecting numerous sea-level fluctuations. The sequence is disconformably overlain by siliciclastic sequence of Pyongan Supergroup (Carboniferous–?Triassic) which formed most likely in shallow marine, deltaic, and fluvial environments. The Imjingang Belt is an east-trending fold and thrust zone and consists of metasedimentary rocks and volcaniclastics (Devonian–Carboniferous), underlain unconformably by Proterozoic basement rocks. Late Proterozoic amphibolites of oceanic affinity were metamorphosed at about 8–13 kbar and 630–740°C during the late Permian to the early Triassic. The south-vergent contraction and top-down-to-the-north normal faulting are suggestive of a suture belt between the North China Block (Sino-Korea Craton) and the South China Block (Yangtze Craton), an extension of Sulu Belt across the Yellow Sea. Entire peninsula experienced strong deformation and metamorphism during the suturing event, namely Songrim orogeny. During this orogenic event, the Kyonggi Massif (and the Okchon Basin) accreted to the Yongnam Massif (and Taebaeksan Basin) along the South Korean Tectonic Line running northeast–southwest. A series of northward-trending thrust formed along the boundary zone to the east (Kaktong and Kongsuwon thrusts and others). Piggyback basins locally developed along the thrust faults, forming the Taedong Group. The crustal deformation resumed in the early to late Jurassic (Daebo event) under contractional setting. Dextral ductile shearing associated with thrusting and folding continued in the mid-southern part of the peninsula. It was due to orthogonal (northwestward) subduction of the Izanagi Plate under the Asian continent. The Yongnam Massif experienced continuous dextral offset along the Honam Shear Zone. In the early Cretaceous, the Izanagi Plate began to subduct northward and caused formation of strike–slip basins in retroarc setting, i.e., Kyongsang Basin in the southeastern part and a number of small-scale basins in the mid-southwestern part of the peninsula. Small-scale alluvial fans and fluvial channel networks formed in the basin margin and were transitional to ephemeral lacustrine systems under semiarid to arid conditions. Extensive intrusion of granitoids occurred from Triassic to Early Tertiary with a gap between 160 and 100 Ma, representing continental magmatic arc. In the Tertiary, the southeastern margin of the Korean peninsula experienced back-arc opening. Pull-apart basins formed in the Miocene, bounded by the Yangsan and Hupo faults. The Yonil Group, sedimentary fill of the Pohang Basin, comprises more than 1-km-thick siliciclastic sequence which represents deposition in fan-delta systems on the hanging wall of the Yangsan fault. Thick (more than 10 km thick) sediments in the Ulleung Basin margin were deformed in the late Miocene due to the northward movement of Kyushu Block. Quarternary volcanic events in Cheju Island represent intraplate hot spots.

THE SEDIMENTOLOGY STRATIGRAPHY AND TECTONIC CONTEXT OF THE SÃO FRANCISCO SUPERGROUP AT THE SOUTHWESTERN DOMAIN OF THE SÃO FRANCISCO CRATON, BRAZIL

Neoproterozoic metasedimentary rocks of the São Francisco Supergroup occur in the Southwest part of São Francisco Craton. Three informal lithostratigraphic units composed mainly by psephites, pelites and carbonates represent the São Francisco Supergroup. The last two units formed in storm influenced muddy shelf and platform/ramp systems, respectively. The psephitic unit crops out as isolated bodies nearby the external zone of the southernmost part of Brasilia Fold and Thrust Belt. Its origin is related to fan delta systems that developed eastward, in a foreland basin, as a result of erosion of metamorphic rocks of Brasilia Fold and Thrust Belt.

The sedimentology, stratigraphy and tectonic context of the Sao Francisco Supergroup at the Southwestern Domain of the Sao Francisco craton, Brazil

Neoproterozoic metasedimentary rocks of the Sao Francisco Supergroup occur in the Southwest part of Sao Francisco Craton. Three informal lithostratigraphic units composed mainly by psephites, pelites and carbonates represent the Sao Francisco Supergroup. The last two units formed in storm influenced muddy shelf and platform/ramp systems, respectively. The psephitic unit crops out as isolated bodies nearby the external zone of the southernmost part of Brasilia Fold and Thrust Belt. Its origin is related to fan delta systems that developed eastward, in a foreland basin, as a result of erosion of metamorphic rocks of Brasilia Fold and Thrust Belt.

THE SOURCE OF THE ESPINHAÇO DIAMONDS: EVIDENCES FROM SHRIMP U-PB ZIRCON AGES OF THE SOPA CONGLOMERATE AND PB-PB ZIRCON EVAPORATION AGES OF METAVOLCANIC ROCKS

We present here the results of the SHRIMP U/Pb zircon ages obtained from the diamond-bearing Sopa Conglomerate and also Pb-Pb zircon evaporation age of the associated metavolcanic rocks, outcropping in the Diamantina region of the Espinhaco Range. The Sopa Conglomerate, a clast-supported rock that appear in the lower part of the Espinhaco Supergroup, was deposited mainly in alluvial fan, braided river and fan delta systems. The apparent 238 U- 206 Pb and 207 Pb- 2()6 Pb zircon ages varied from 1442 Ma to 3683 Ma and from 1726 Ma to 3599 Ma peaks, respectively. Different generations of zircon populations showed by the radiometric ages surely demonstrate the age of cratonic rocks, however it is still unknown if one of them could be correlated with a primary source rock of the diamonds. The series of K- and Fe-rich metavolcanics is coeval with the Sopa Conglomerate. The results of a single date of these rocks yielded ages of 1710±12 Ma to the crystallization of the volcanic protolith. Detrital zircons of the Sopa Conglomerate matrix are older than the primary zircons of the metavolcanic rocks outcropping in the lower Espinhaco Supergroup. Therefore, geochronological data do not provide additional evidence for the idea that diamonds are originated from these metavolcanic rocks, as supposed by some authors after geologic regional studies.

THE SOURCE OF THE ESPINHAÇO DIAMONDS: EVIDENCES FROM SHRIMP U-PB ZIRCON AGES OF THE SOPA CONGLOMERATE AND PB-PB ZIRCON EVAPORATION AGES OF METAVOLCANIC ROCKS

We present here the results of the SHRIMP U/Pb zircon ages obtained from the diamond-bearing Sopa Conglomerate and also Pb-Pb zircon evaporation age of the associated metavolcanic rocks, outcropping in the Diamantina region of the Espinhaço Range. The Sopa Conglomerate, a clast-supported rock that appear in the lower part of the Espinhaço Supergroup, was deposited mainly in alluvial fan, braided river and fan delta systems. The apparent 238 U- 206 Pb and 207 Pb- 206 Pb zircon ages varied from 1442 Ma to 3683 Ma and from 1726 Ma to 3599 Ma peaks, respectively. Different generations of zircon populations showed by the radiometric ages surely demonstrate the age of cratonic rocks, however it is still unknown if one of them could be correlated with a primary source rock of the diamonds. The series of K- and Fe-rich metavolcanics is coeval with the Sopa Conglomerate. The results of a single date of these rocks yielded ages of 1710±12 Ma to the crystallization of the volcanic protolith. Detrital zircons of the Sopa Conglomerate matrix are older than the primary zircons of the metavolcanic rocks outcropping in the lower Espinhaço Supergroup. Therefore, geochronological data do not provide additional evidence for the idea that diamonds are originated from these metavolcanic rocks, as supposed by some authors after geologic regional studies.

Upper Triassic – Jurassic sequence stratigraphy and its structural controls in the western Ordos Basin, China

Upper Triassic, Lower–Middle Jurassic and Upper Jurassic strata in the western Ordos Basin of North China are interpreted as three unconformity‐bounded basin phases, BP‐4, BP‐5 and BP‐6, respectively. The three basin phases were deposited in three kinds of predominantly continental basin: (1) a Late Triassic composite basin with a south‐western foreland subbasin and a north‐western rift subbasin, (2) an Early–Middle Jurassic sag basin and (3) a Late Jurassic foreland molasse wedge. Within the Late Triassic composite basin BP‐4 includes three sequences, S4‐1, S4‐2 and S4‐3. In the south‐western foreland subbasin, the three sequences are the depositional response to three episodes of thrust load subsidence, and are mainly composed of alluvial fan, steep‐sloped lacustrine delta and fluvial systems in front of a thrust fault‐bounded basin flank. In the north‐western rift subbasin, the three sequences are the depositional response to three episodes of rift subsidence, and consist of alluvial fan – braid plain and fan delta systems basinward of a normal fault‐bounded basin margin. In the sag basin BP‐5 includes four sequences, S5‐1, S5‐2, S5‐3 and S5‐4, which reflect four episodes of intracratonic sagging events and mainly consist of fluvial, gentle‐gradient lacustrine delta and lacustrine systems sourced from peripheral uplifted flanks. BP‐6, deposited in the foreland‐type basin, includes one sequence, S6‐1, which is the depositional response to thrust load subsidence and is composed of alluvial fan systems. The formation and development of these three kinds of basins was controlled by Late Triassic and Jurassic multi‐episode tectonism of basin‐bounding orogenic belts, which were mainly driven by collision of the North China and South China blocks and subduction of the western Pacific plate.

A gravel lobe deposit in the prodelta of the Doumsan fan delta (Miocene), SE Korea

Abstract The Doumsan fan-delta system in the Pohang Basin (Miocene), SE Korea comprises tripartite components of Gilbert-type topset, foreset, and bottomset environments with an extended prodelta in the deeper part (a few hundred metres deep). The present study documents sedimentologic features and origin of a gravel body (here interpreted as a gravel lobe) formed in the prodelta region of the Doumsan fan delta. The gravel lobe lies on sandy mud deposits and is capped by a thick massive sand bed. It extends for more than 1.5 km with a height of up to 30 m and shows a narrow tongue-like geometry. Eight sedimentary facies have been distinguished to describe characteristic features of the gravel lobe and the associated deposits. Of these, three types of gravelly sedimentary facies are important with regard to volumetric contribution and depositional processes: (1) crudely stratified pebble-grade conglomerate; (2) disorganized, clast-rich pebble(-to-cobble)-grade conglomerate; and (3) matrix-rich, bimodal cobble-grade conglomerate. The former two types dominate the central part of the lobe where they are not accompanied by sand beds, whereas the latter, as subordinate units, is prevalent in the fringe which otherwise is dominated by thick sandy mud deposits. The stacked successions of crudely stratified pebble-grade conglomerate are representative of the active aggradational phases of the gravel lobe, whereas the occurrence of channels within the lobe reflects that the gravel lobe prograded under the influence of subaqueous channel systems. The gravel lobe resulted from catastrophic disturbance (slumping) on the foreset region that further caused the development of channel systems, promoting efficient transport of gravelly sediments. This type of deposit may represent an important additional category of low-efficiency subaqueous fans.

The sequence stratigraphic concept and the Precambrian rock record: an example from the 2.7–2.1 Ga Transvaal Supergroup, Kaapvaal craton

Sequence stratigraphic concepts are applied to the 2.7–2.1 Ga Transvaal Supergroup, which constitutes the sedimentary floor to the Bushveld igneous complex. The Transvaal succeeds the foreland-fill of the Witwatersrand Supergroup and, in its lowermost portions, is partly synchronous with the Ventersdorp Supergroup lavas that followed upon the Witwatersrand sediments in the stratigraphic record. The unconformable contacts at the base and the top of the Transvaal Supergroup mark significant changes in the overall tectonic setting, which qualifies them as first-order sequence boundaries. Sedimentation within the Transvaal basin was controlled by cycles of extensional and/or thermal subsidence resulting in the accumulation of sequences, separated by stages of base-level fall generating sequence-bounding subaerial unconformities. The Transvaal first-order sequence is subdivided into five second-order unconformity-bounded depositional sequences, i.e. the Protobasinal, Black Reef, Chuniespoort, Rooihoogte–Timeball Hill and Boshoek–Houtenbek sequences. The span of time of these second-order sequences, averaging 130 Ma, is much longer than the duration of the second-order Phanerozoic cycles, which is explained by the less evolved and thus slower plate tectonic processes that dominated the Late Archaean–Early Proterozoic times. Each Transvaal second-order sequence preserves a variable number of systems tracts, mainly as a function of the strength of the erosional processes associated with the ravinement surfaces and subaerial unconformities. In a complete succession, a sequence would include a basal lowstand systems tract (LST), followed by transgressive (TST), highstand (HST) and falling stage (FSST) systems tracts. The Protobasinal and Rooihoogte–Timeball Hill sequences only preserve the LST and TST, due to the strong post-depositional erosion. The Black Reef sequence is composed of LST, TST and HST. The Chuniespoort sequence preserves the TST, HST and FSST, but lacks the LST due to ravinement erosion. The Boshoek–Houtenbek sequence develops the complete succession of systems tracts. Common features may be emphasized between the same types of systems tract developed within different sequences. The LST includes high-energy coarse prograding facies, such as alluvial fans and fan–delta systems, that fill the irregularities of the pre-existing topography and lead to the peneplanation of the depositional areas. The TST may be recognized from retrogradational stacking patterns that are associated with the transgression of marine environments. The HST marks the normal regression of the previous marine environments, which assumes coeval aggradation of fluvial and basinal facies within an overall progradational framework. The FSST would normally include only basinal facies age-equivalent with the correlative subaerial unconformities. In our case study, the stages of base-level fall resulted invariably in fully nonmarine environments within the Transvaal area, generally leading to the development of subaerial unconformities. In particular circumstances, the nonmarine environment may still accumulate and preserve fluvial, alluvial fan or lacustrine sediments as a function of the relative position between topography and local equilibrium profiles and base-levels. The interpretations of base-level rise and fall resulting from the application of the sequence stratigraphic concept to the 2.7–2.1 Ga Transvaal Supergroup enable estimation of sea level changes during deposition of this succession. With an understanding of local tectonic conditions on the Kaapvaal craton and of coeval sedimentation elsewhere in Africa, eustatic and relative sea level changes and variation in continental freeboard conditions may be suggested for the Transvaal Supergroup.