Stratigraphic Groups Research Articles

Physical volcanology, stratigraphy, and lithogeochemistry of an Archean volcanic arc: evolution from plume-related volcanism to arc rifting of SE Abitibi Greenstone Belt, Val d'Or, Canada

The eastern segment of the Southern Volcanic Zone (SVZ) of the Abitibi greenstone belt is a complex sequence of volcano-sedimentary rocks interleaved with syn- to post-volcanic plutonic suites. The eastern segment can be broken into two stratigraphic groups based on regional tectonics and volcano-sedimentary stratigraphy, the basal Malartic Group (MG), composed of the La Motte-Vassan, Dubuisson and Jacola Formations (JFs), and the upper Louvicourt Group (LG), containing the Val d'Or (VDF) and Héva formations (HF). The MG represents a Archean oceanic floor controlled by extensional mantle plume tectonics and characterized by effusive komatiites and basalts, and intrusive dykes and sills. Although plume processes were still operating, the LG signaled the change to subduction-related processes featuring a deep-marine volcanic arc. This complex volcano-sedimentary sequence evolved between 2714±2 and 2702±2 Ma. U–Pb age determinations of 2704±2 Ma for the Val d'Or Formation (VDF) and 2702±1 Ma for the HF constrain the duration of the 5–7 km-thick Val d'Or Arc to ca. 5 My. The JF is characterized by the interstratification of komatiitic, plume-related volcanism and basaltic, subduction-related volcanism based on stratigraphy, geochemistry, and geochronology. The contact between the mafic–ultramafic JF and intermediate-felsic VDF is gradational and indicated by andesitic volcaniclastic debris deposited via mass flow processes within the lower VDF. The 3–5 km-thick VDF is a complex subaqueous volcano-sedimentary sequence with rapid lateral and vertical changes in the depositional environment of volcaniclastic deposits and associated andesitic to rhyolitic lavas. The 1–50 m-thick volcaniclastic deposits are normal- to reverse-graded beds composed of angular to subrounded tuff- to breccia-sized clasts, that locally have a vesicularity index between 15 and 35%. The 1–100 m-thick massive, pillowed/lobate and brecciated lava flows are variably vesicular (1–30%). In addition, numerous small felsic-dominated volcanic centers of limited areal extent, containing massive sulfide deposits, define the Val d'Or Arc. Effusive eruptions and their autoclastic to hydroclastic counterparts form these centers. The 2–3 km-thick tholeiitic HF is defined by effusive mafic and felsic volcanism with local volcaniclastic deposits. A basal spherulitic dacite unit within the HF, traceable for 40 km along strike, serves as the marker horizon for the LG. Up section, massive to pillowed mafic flows with gabbroic dykes and sills are dominant, suggesting more fissural-type volcanism. Local tuff turbidites and reworked scoriaous pyroclastic deposits attest to explosive eruption(s). The JF represents a deep marine ridge-like oceanic setting controlled by mantle plume volcanism with intercalated volcanics originating from a proto-arc. Andesitic volcaniclastic rocks and lavas at the base of the VDF indicate subduction-related volcanism. Fragment-dominated calc-alkaline volcanism of the VDF marks arc construction forming numerous small and overlapping subaqueous volcanic centers. The tholeiitic HF indicates a return to an extensional regime with fissure-type volcanism during arc dissection. The Val d'Or Arc forms a homoclinal, south-facing volcano-sedimentary succession evolving from plume- to subduction-related volcanism.

Geological setting of the Argentine Frontal Cordillera in the flat-slab segment (30°00′–31°30′S latitude)

The Argentine Frontal Cordillera, between latitudes 30°20′ and 31°20′S, presents an eastward-verging polycyclic structure. Two large groups of rocks with markedly different structural nature can be distinguished: a Paleozoic basement with characteristic thin-skinned structure and the Andean cover showing remarkable extensional Mesozoic structures inverted by Tertiary compressional tectonic events. These groups are divided by a large tectonic unconformity separating not only rocks with different style and degree of deformation but also different paleogeographic settings. The Cenozoic tectonic inversion is linked with the subhorizontal subduction of the Nazca Plate in the Pampean flat-slab segment and subsequent uplift of the Andean Cordillera. The Paleozoic basement is composed of Devonian and Permo-Carboniferous marine sedimentary rocks, separated by an angular unconformity. These rocks were deposited in siliciclastic and mixed sedimentary platforms, respectively, and intruded by Permo-Triassic granitoid rocks. The Devonian rocks were deformed by the Famatinian orogeny in Late Devonian times. The most important Paleozoic structures are related to the Gondwanic orogeny, of Permian age. These include east-vergent thrusts and related folds that record a large shortening component. The Andean cover, lying unconformably over the Paleozoic basement, is primarily volcanic and volcanoclastic, with some interbedded continental sedimentary rocks. Two main stratigraphic groups can also be defined here: an older sequence linked to an extensional tectonic event and a synorogenic sequence connected to a compressional tectonic event that produced an inversion of previous extensional features. The lower unit (Choiyoi Group: Permian and Triassic) is affected by normal faults with downthrown western blocks and intruded by Jurassic granodioritic rocks. The faults in this unit involve the Gondwanic basement in a typical thick-skinned tectonic style and are grouped in bands with N–S strike. The uppermost units (Vizcachas, Melchor, and Olivares groups: Tertiary and Plio-Quaternary) also lie unconformably over the latter. The normal faults were inverted in the upper Miocene by the uplift of the western block, deforming the lower and upper units in a compressional context during the Andean orogenic cycle. Displacement on reverse faults, measured in the Tertiary synorogenic rocks, is usually less than 1 km. The Tertiary crustal shortening calculated from cross-sections is about 10%, in contrast with the estimated 50% shortening of the Precordillera unit. All this shows that most of the crustal shortening of the Andean Cordillera during the compressional stage has been transferred to the Precordillera unit through the lower detachment fault. These facts also show that the Cordillera Frontal unit is an uplifted block in which the extensional structures have been preserved, and that the Rodeo–Upsallata basin is a piggy-back type.

Sediment records of phosphorus-driven shifts to phytoplanktondominance in shallow Florida lakes

Primary producer community structure (PPCS) in shallow lakes is influenced by phosphorus (P) load and water column P concentration. Theoretically PPCS may shift between phytoplankton and macrophyte states with intermediate P loading, but phytoplankton dominate when P loading exceeds a critical threshold. We analyzed 2 sediment cores from five shallow, eutrophic lakes (size range: 0.6 to 125 km ) that are phytoplankton dominated to determine whether the development of the current state was associated stratigraphically with an increase in sediment total P (TP) and a shift in PPCS. We used sponge biogenic silica (BSi ) concentrations and total Sponges carbon to total nitrogen ratios (TC:TN) as proxies for macrophyte abundance and sediment organic matter source, respectively. Three stratigraphic groups of sediments were identified with k-means cluster analysis. These samples were grouped by increasing TP concentrations and decreasing age and identified as macrophyte, transitional and phytoplankton sediments. Results show that as P loading increased in the late 19th and early 20th centuries, the lakes produced sediments with an increasing contribution from phytoplankton. Four of our lakes may represent a 2 subset of shallow lakes because of their large size (30 to 125 km ) and relatively rapid historic P enrichment. In these Florida lakes, PPCS shifted to phytoplankton dominance with no paleolimnological record of lake-wide alternating stable states or of lake-wide phytoplankton dominance before anthropogenic P enrichment.

GEOLOGICAL CONTROLS ON OVERPRESSURE IN THE NORTHERN CARNARVON BASIN

A small, but significant fraction of wells drilled in the Northern Carnarvon Basin have encountered problems with overpressure: better pore pressure prediction would improve safety and economy for drilling operations. In the Northern Carnarvon Basin the occurrence of overpressure and likely mechanisms are under investigation as part of the Australian Petroleum Cooperative Research Centre (APCRC) Research Program on Pore Pressure Prediction. Previous workers have proposed a number of mechanisms as the main cause of overpressure including undercompaction, hydrocarbon generation, horizontal stress and clay reactions.A preliminary regional study was undertaken incorporating over 400 well completion reports which identified approximately 60 wells with mud weights greater than 1.25 S.G. A subset of these wells was investigated and more reliable but much scarcer pressure indicators such as kicks or direct pressure measurements were examined. Depth-pressure profiles of wells across the region are variable and commonly show pressure compartmentalisation. Using a range of indicators, it was observed that overpressured strata in the Barrow Subbasin:occur over a wide depth range (2,500 to 4,000+ mbsl);occur over a wide stratigraphic range (Late Triassic to Late Cretaceous);are not regionally limited by major structural boundaries;are associated with sequences dominated by finegrained sediments with variable clay mineralogy; and in depositionally, or structurally, isolated sandstones; andmainly to the west of the Barrow and Dampier Subbasins around the Alpha Arch and Rankin Trend, coinciding with thickest Tertiary deposition.Previous published work in the study area has tended to support hydrocarbon generation as the primary cause of overpressure, though more recent publications have emphasised compaction disequilibrium. The log response (DT, RHOB and NPHI) of overpressured clay-rich strata has been investigated to constrain the type of overpressure mechanism. A normal compaction trend has been derived for four stratigraphic groupings; Muderong Shale, Barrow Group, Jurassic and Triassic. All overpressure occurrences were accompanied by an increase in sonic transit time. Not all wells have suitable log data for evaluation, but all stratigraphic groups show some evidence of elevated porosity associated with overpressure consistent with disequillibrium compaction as a dominant mechanism. Overpressures in the Barrow Group in Minden-1 and the Jurassic section within Zeepaard–1 do not have accompanying porosity anomalies suggesting a different overpressure mechanism model is needed.

Zircon ages constrain the timing of deformation events in the the Granites‐Tanami region, northwest Australia*

Dating of extensive granitoids in the The Granites‐Tanami Block of northern Australia has enabled time limitations on deformational events, including five Palaeoproterozoic events (D1‐D5) and one older, almost Neoarchaean event. D1 is present in the basal Tanami Group, D2 to D5 help define other stratigraphic groups in the region. The intrusive rocks have been dated by traditional zircon and stepwise lead evaporation techniques. A post‐D2/pre‐D3 foliated and boudinaged leucocratic tonalite gave 1870 ± 3 Ma. A syn‐D3 foliated and boudinaged tonalite 1858 ± 3 Ma and a post‐D3/pre‐D4 foliated tonalite 1840 ± 11 Ma. Two unmetamorphosed and post‐D5 granitoids gave 1797 ± 1 Ma and 1787 ± 3 Ma, respectively. Some deformation events are separated by as little as 10 to 20 million years. These times of intrusive igneous activity exactly replicate other igneous events, many of which are volcanic, in the Halls Creek Province 400 km to the northwest, which strengthens proposed geological relationships between the two regions. A granitic gneiss in a basement inlier gives 2487 ± 3 Ma, a Neoarchaean age. Tectonic uplift in the Carboniferous is deduced from concordia lower intercept data that is similar to the eastern Arunta Block. Further age definition of D1 D2 and D5 is under current investigation.

The Oman Exotics: a key to the understanding of the Neotethyan geodynamic evolution

The study of the exotic blocks of the Hawasina Nappes (Sultanate of Oman) leads to give apposit data that allow us to propose a new paleogeographic evolution of the Oman margin in time and space. A revised classification of exotic blocks into different paleogeographical units is presented. Two newly introduced stratigraphic groups, the Ramaq Group (Ordovician to Triassic) and the Al Buda’ah Group (upper Permian to Jurassic) are interpreted as tilted blocks related to the Oman continental margin. The Kawr Group (middle Triassic to Cretaceous) is redefined and interpreted as an atoll-type seamount. The paleogeography and paleoenvironments of these units are integrated into a new scheme of the Neotethyan rifting history. Brecciae and olistoliths of the Hawasina series are interpreted to have originated from tectonic movements affecting the Oman margin and the Neotethyan ocean floor. The breccias of late Permian age were generated by the extension processes affecting the margin, and by the creation of the Neotethyan oceanic floor. The breccias of mid-late Triassic age coincide in time with the collision of the Cimmerian continents with Eurasia. In constrast, the breccias of late Jurassic and Cretaceous age are interpreted as resulting to the creation of a new oceanic crust (Semail) off the Oman margin.

Synthesis and revision of groups within the Newark Supergroup, eastern North America

The Newark Supergroup currently includes nine stratigraphic groups, each of which applies to part or all of the rock column of only one or a few basins. Because the group nomenclature within the Newark Supergroup is neither inclusive nor parallel in its concepts, nearly half of the strata within the Newark Supergroup lacks any group placement. A new system is proposed herein that (1) establishes unambiguous group boundaries, (2) places all Newark Supergroup strata into groups, (3) reduces the number of group names from nine to three, (4) creates parallelism between groups and three major successive tectonic events that created the rift basins containing the Newark Supergroup, and (5) coincidentally provides isochronous or nearly isochronous group boundaries. These proposed groups are (1) the Chatham Group (Middle Triassic to basal Lower Jurassic sedimentary rocks), (2) the Meriden Group (Lower Jurassic extrusive volcanic and sedimentary rocks), and (3) the Agawam Group (new name) (Lower Jurassic sedimentary rocks above all early Mesozoic igneous intrusive and extrusive rocks). This new rock classification system makes use of the fact that a discrete interval of synchronous or nearly synchronous volcanism and plutonism occurred throughout the early Mesozoic rift system of eastern North America. The presence or absence of volcanic rocks provides a powerful stratigraphic tool for establishing regional groups and group boundaries. The presence of sedimentary rocks injected by diabase dikes and sills, in the absence of extrusive volcanic rocks, places Newark Supergroup rocks in the Chatham Group. The presence of extrusive volcanic rocks, interbedded with sedimentary rocks injected by diabase dikes and sills, places Newark Supergroup rocks in the Meriden Group. The presence of sedimentary rocks lacking both extrusive volcanic rocks and diabase dikes and sills, places Newark Supergroup rocks in the Agawam Group. Application of this new regional group stratigraphy to the early Mesozoic rift basins requires revision of the stratigraphy of several basins to make formation boundaries match group boundaries.

Provenance of mudstones in the Karoo Supergroup of the Ellisras basin, South Africa: Geochemical evidence

The Karoo Supergroup in the Ellisras basin consists of sediments that were deposited predominantly during the Permian Period. Mudstone samples of the Swartrant and Grootegeluk Formations of the Ecca Group and the succeeding Eendragtpan Formation of the Beaufort Group were collected from borehole core. Based on the concentration and ratios of relatively immobile elements that have been shown to be directly related to their source rocks, the Grootegeluk Formation and Eendragtpan Formation mudstones have provenances which are approximately granodioritic in composition and have similar Late Proterozoic SmNd model t Nd-DM ages (∼1.2 Ga). The provenance of the Swartrant Formation was relatively more mafic and has Early Proterozoic SmNd model t Nd-DM ages (∼2.1 Ga). The model t Nd-DM ages for all the sediments are too low to support a cratonic origin. It is not possible to identify a unique source for the Swartrant sediments from only the SmNd isotopic data, but a source from approximately the northwest of the coalfield in the Damaran mobile belt or from Antarctica to the east, in its Late Proterozoic to Late Palaeozoic palaeo-position, cannot be discounted. Palaeo-current data for the Swartrant Formation indicates a source from the east. This is consistent with palaeo-drainage patterns from Antarctica and therefore a fluvio-glacial source from Antarctica, in its Late Paleozoic geographic position, is favoured. The Grootegeluk and Eendragtpan mudstone Formations are most compatible with a source from retreating continental ice-sheets to the north of the Ellisras basin at the time of Early Permian deglaciation. Although the Swartrant and Grootegeluk Formations have been assigned to the same stratigraphic group (Ecca), the Grootegeluk Formation is geochemically distinct from the Swartrant Formation and indistinguishable from the overlying Eendragtpan Formation of the Beaufort Group.

Rincón de la Vieja volcano, Guanacaste province, Costa Rica: geology of the southwestern flank and hazards implications

Acid rain and ongoing eruptive activity at Rincón de la Vieja volcano in northwestern Costa Rica have created a triangular, deeply eroded “dead zone” west-southwest of the Active Crater. The barren, steep-walled canyons in this area expose one of the best internal stratigraphic profiles of any active or dormant volcano in Costa Rica. Geologic mapping along the southwestern flank of the volcano reveals over 300 m of prehistoric volcanic stratigraphy, dominated by tephra deposits and two-pyroxene andesite lavas. Dense tropical forests and poor access preclude mapping elsewhere on the volcano. In the “dead zone” four stratigraphic groups are distinguished by their relative proportions of lava and tephra. In general, early volcanism was dominated by voluminous lava emissions, with explosive plinian eruptions becoming increasingly more dominant with time. Numerous phreatic eruptions have occurred in historic times, all emanating from the Active Crater. The stratigraphic sequence is capped by the Río Blanco tephra deposit, erupted at approximately 3500 yr B.P. Approximately 0.25 km 3 (0.1 km 3 DRE) of tephra was deposited in a highly asymmetrical dispersal pattern west-southwest of the source vent, indicating strong prevailing winds from the east and east-northeast at the time of the eruption. Grain-size studies of the deposit reveal that the eruption was subplinian, attaining an estimated column height of 16 km. A qualitative hazards assessment at Rincón de la Vieja indicates that future eruptions are likely to be explosive in style, with the zone of greatest hazard extending several kilometers north from the Active Crater.

Tectonic evolution of the emergent part of a volcanic ocean island: Lanzarote, Canary Islands

A reconstruction of the structural evolution of Lanzarote confirms the existence of two principal stages of construction of the island, separated by a main erosional event. The first (pre-erosional) is the shield stage, Miocene-Pliocene in age, which is divided in three stratigraphic groups, where different formations can be distinguished using unconformities mappable over the whole area of outcrop. In the second (post-erosional) stage, developed during the Quaternary, nine stratigraphic levels are tentatively identified and are grouped in two phases of volcanic activity (Shields phase and Pyroclastic cones phase). Data from 580 photogeologic lineaments, as well as field data from 204 fault planes (195 with sense of movement), 273 dykes, and 45 volcanic alignments, are presented. The tectonic style of the deformations is brittle and polyphasic. Strike-slip faults are dominant, but normal and reverse faults are also present. Both the so-called “Atlantic” and “African” directions are well developed in trends of strike-slip faults and volcanic indicators. At least two deformation phases can be identified from the geometrical analysis of faults and from the stress tensor calculated using lineations on fault planes. The reconstructed stress field responsible for the measured deformations changed orientation during the evolution of the island of Lanzarote. It is suggested that a permutation between principal stress axes σ 1, and σ 3 occurred subparallel to the length of the island between the pre-erosional and post-erosional stages.

Submarine geology and post-opening tectonic movements in the southern region of the Sea of Japan

The submarine geologic structure of the southern region of the Sea of Japan was analyzed on the basis of seismic reflection profile data and bottom samples. The seismic reflection patterns depict sedimentary layers with well-developed onlap and toplap terminations and truncations by submarine erosion. The planes marked by series of onlap, toplap, and erosional truncations enabled us to define three stratigraphic groups composed of six formations in this region. They are the Kasumioki Group, the Hamasakaoki Group and the Tottorioki Group in ascending order. Detailed analysis of the seismic reflection profiles reveals subtle structures of these sedimentary units. The oldest Kasumioki Group is folded and faulted, forming submarine ridges. The Hamasakaoki Group mainly occurs in the depressions between these ridges. The youngest Tottorioki Group covers widely both of the older formations with a pronounced marine onlap. Fossil evidence indicates that the depositional ages of the Kasumioki Group, the Hamasakaoki Group and the Tottorioki Group are Early Miocene to Earliest Pliocene, late Early to Late Pliocene, and Late Pliocene to Holocene, respectively. These observations provide the basis to understand two main structural features in this region; (1) E-W trending folds, and (2) northwestward tilting blocks. The former construct a fold zone composed of two to four ranges of anticlines that have steeper southern flanks truncated by reverse faults. The latter are shown by slightly-inclined floors formed by submarine erosion, step-like asymmetric basins bounded by NE-SW trending reverse faults, and onlap-toplap sedimentation patterns. The lateral change in bed-thickness, sedimentation patterns, and age of submarine sediments indicate that the E-W trending folds were formed in the early Middle or Late Miocene to the Early Pliocene by N-S compression, whereas the northwestward tilt blocks were formed in the late Early to Late Pliocene by NW-SE compression. The sedimentary records in this region dictate an abrupt change in tectonic setting in the late Early Pliocene. This change is interpreted to show that the Sea of Japan has been closing since then due to either the reactivation of the Amurian Plate, or the directional change of the Philippine Sea Plate motion.

Tectonic evolution of the Nakasib suture, Red Sea Hills, Sudan: evidence for a late Precambrian Wilson Cycle

The Nakasib suture is a late Proterozoic (Pan-African) ophiolite-decorated structural belt in the central Red Sea Hills of the Sudan. It represents one of the sutures along which the island arc/back-arc terranes and continental microplates of the Arabian-Nubian Shield are welded together. The Nakasib suture separates the 900–850 Ma old Haya terrane in the south from the 830–720 Ma old Gebeit terrane to the north. Five thrust-bounded stratigraphical groups are identified across the suture. These are, from south to north, Arbaat volcanic group (c. 730 Ma rift-related volcanic rocks), Salatib group (early passive margin volcanic and sedimentary rocks), Meritri group (the lower part of which contains alluvial and submarine fans of an incipient ocean basin, while its upper part can be tentatively equated with a platform of clastic sedimentary and carbonate intercalations which mark the stage of mature ocean basin opening), Nakasib ophiolite, and Shalhout group (arc-related volcanic and sedimentary rocks). These are intruded by syn-tectonic and post-tectonic plutonic rocks. The tectonic setting of the above lithological groups suggests that the early stage of the Nakasib suture development was marked by rifting of older crust, probably the Haya terrane, which culminated in development of a passive margin on the south margin of an oceanic basin prior to collapse into an ophiolite-decorated suture. This indicates that the Nakasib suture evolved through a complete Wilson cycle orogeny from rifting, through passive margin formation, to closure of the basin and suturing.

Geology and structural evolution in the Bheri river region, southwest Nepal

The rocks of the Lesser Himalaya in the Bheri River region have been divided into five stratigraphic groups. The meso-grade metamorphic rocks of the Chaurjhari Group is the oldest rock group in the area and is occurring as a klippen in the north. The Kunchha Group and the Nawakot Group are the incipiently metamorphosed sedimentary rocks while the Gondwana Group and Daban Group rocks are generally unmetamorphosed sedimentary sequences.
 In the frontal part of the thrust-fold belt of the Himalaya, leading imbricate fan structure is developed in the rocks of Daban Group with the Main Boundary Thrust (MBT) as the sole thrust, and the Kumak Thrust as the roof thrust. In the region further north, a complex of hinterland dipping duplex structure is envisioned between the Kumak Thrust (main sole thrust) and the Chaurjhari Thrust (main roof thrust) with a number of intermittent thrust-faults which acted as roof thrust to the underlying duplex and as sole thrust to the overlying duplex. The major planes of detachment for the generation of duplex are postulated along the incompetent basal rock members of the Nawakot Group, the Kunchha Group and the Chaurjhari Group respectively. As against the view of some of the earlier workers, a "piggy-back" sequence of thrust from higher to lower (hinterland towards the foreland) is proposed.

Petrology of the Keweenawan Mamainse Point lavas, Ontario: Petrogenesis and continental rift evolution

The Mamainse Point Formation forms a 5250‐m‐thick sequence of flood basalts, ranging from high‐MgO picrites and basalts to basaltic andesites, that formed in the Keweenawan (1100 Ma) Midcontinent Rift of North America. This section is the most continuous and complete sequence of Keweenawan lavas and records almost the entire igneous history of the rift. The section consists of numerous stratigraphic groups of lavas that have distinct chemical characteristics. A variety of petrogenetic processes including partial melting of different sources at decreasing depth, periodically replenished, tapped, and fractionated magma chambers, crustal contamination, and polybaric fractionation were responsible for the evolution of the magmas. The dominant petrogenetic process changes with stratigraphy, which suggests that the physical evolution of the rift controlled which process operated at any time. The initial fracturing of the crust presumably allowed high‐MgO liquids generated at depth to be erupted. The early stage of rifting was apparently slow and allowed the formation of long‐lived magma chambers in which complex processes such replenishment/mixing and assimilation/contamination could occur. The late stage of rifting, however, was apparently vigorous and allowed voluminous magmas from throughout the crust to be erupted. These conclusions from the chemostratigraphy agree with the physical stratigraphy, which indicates a change in tectonic style from an early stage of broad subsidence to a late stage of active graben formation.

Volcanological development of the Nakuru area of the Kenya rift valley

The Nakuru area lies within the inner graben of the central Kenya rift valley and has experienced continual outpourings of trachyte magma during the last 6.2 Ma. Basalts are rare, and volcanic rocks of intermediate composition are not exposed. Sedimentary rocks are intercalated with the volcanic rocks. Volcanic rocks older than 6.2 Ma must underlie the area, contributing to the total volcanic and sedimentary rift fill which is constrained by seismic data to be 4 km thick. Five major stratigraphic groups of trachytic volcanic rocks are identified. They consist of voluminous ignimbrites (at least seventeen) with individual volumes probably 20–30 km 3, lava flows, with volumes of 0.01–2.0 km 3, strombolian deposits, widespread (probably mostly sub-plinian) air-fall deposits and hydromagmatic tuff cone deposits. Because the lithology of these volcanic rocks, and apparently their typical volumes, have remained unchanged for 6 Ma, it is considered that the sub-volcanic plumbing systems in this part of the rift also remained similar. The various eruptive styles of trachyte can be explained by eruption from relatively large magma chambers, capped by trachytic magmas and emplaced in the upper crust, like that thought to exist below the Recent volcano Menengai. It is argued that no large volume trachyte lava flows that might have erupted directly maom lower crustal magma chambers exist in the Kenya rift. A caldera complex that may have overlain an earlier magma chamber emplaced in the upper crust was centered on Olongai, NW of Menengai. Menengai, the Olongai complex, and all the Quaternary volcanoes to the S of the Nukura area, lie above a ridge of high gravity that extends along the axis of the inner trough, and which represents a ca 15 km wide linear zone of Late Miocene to Recent mafic dykes and cumulate rocks that formed the roots of trachytic magma chambers.

Popple Hill Gneiss as dacite volcanics: A geochemical study of mesosome and leucosome, northwest Adirondacks, New York

New chemical data from a widespread Precambrian stratigraphic unit, the Popple Hill Gneiss in the northwest Adirondacks, have meaning for depositional setting and correlation of Grenville metasediments. Average chemistry of the predominant rock type is appropriate for dacite volcanics rather than the previously proposed elastic sediments. Particularly unsuitable for shale are the high total alkali, Na 2 O, and SiO 2 content; slightly peraluminous composition; and low index of chemical alteration. This gneissic unit and leucogneisses of probable ash-flow-tuff origin lower in the section (Hyde School Gneiss) are interpreted as massive volcanic outpourings within a platformal-type sedimentary sequence. A recently proposed caldera complex in anorogenic or rift-related environments is an appropriate setting, but vulcanism interrupted as well as preceded carbonate deposition. Popple Hill and Hyde School Gneisses belong to a common depositional sequence with associated metasedimentary units and should be placed in the same stratigraphic group. Popple Hill leucosomes are variable in shape and texture and, most likely, in age and mode of origin. Most are foliated and show grain size reduction; others cut each other and are isoclinally folded. Physical appearance seems unrelated to chemical composition. Confmement of leucosomes to Popple Hill Gneiss and chemical correlations with host mesosome suggest in situ origin and equilibration with mesosome. Many are enriched over mesosomes in K 2 O, Sr, Ba, and Rb; have low total REE and Zr; and have positive Eu anomalies. Restite is not readily observed. Such leucosomes could have formed throughout various phases of folding. Given the dry state of metamorphism as proposed elsewhere for the gneiss, metamorphic differentiation is a likely mode of origin. Other leucosomes with negative Eu anomalies and REE content similar to those of mesosome may be partial melts that formed during early Grenvillian metamorphism.

Stratigraphy of the Aroostook–Percé Anticlinorium in the Gaspé Peninsula, Quebec

In Quebec, the Aroostook–Percé Anticlinorium exposes two stratigraphic groups: the Honorat and the Matapédia. The terrigenous units of the Honorat Group form the core of the anticlinorium and are overlain by the carbonate sequence of the Matapédia Group. Each group comprises two formations. The Honorat is herein divided into two new formations, the Arsenault and the Garin, which are defined for the first time. The Matapédia Group includes the Pabos and White Head formations. The Pabos Formation, a transitional unit between the noncalcareous strata of the Honorat Group and the carbonate facies of the White Head Formation, is here redefined. The White Head Formation comprises four members. These two groups represent continuous sedimentation from the Late Ordovician (Caradocian) to the Early Silurian (Llandoverian).The lithostratigraphy of the type areas of the Honorat Group (Honorat Township region) and Pabos Formation (Chandler region) are fully described, whereas the lithostratigraphy of the type area of the White Head Formation (Percé area) is summarized only. The Arsenault, Garin, and Pabos formations and the Burmingham, Côte de la Surprise, L'Irlande, and Des Jean members of the White Head Formation constitute the stratigraphic sequence in the eastern part of the Aroostook–Percé Anticlinorium. These units are extended to the western part of the anticlinorium in Quebec and in New Brunswick, and also to other Late Ordovician – Early Silurian rocks in the Chaleur Bay and Gaspé – Connecticut Valley synclinoria.Rocks of the Aroostook–Percé Anticlinorium are interpreted as having been deposited by turbidity currents in a relatively deep water basin. Deposition evolved through time from terrigenous to limestone, reflecting a change in source areas.The Grand Pabos fault divides the Aroostook–Percé Anticlinorium between Percé and Carleton into two areas with stratigraphically different sequences. The northern area is dominated by carbonate rocks of the Matapédia Group, whereas the southern area is composed mainly of terrigenous facies of the Honorat Group. Carbonate sedimentation of the White Head Formation started during Late Ordovician time in the northern area but only during Early Silurian times in the southern area. The thickness of the carbonate strata, which reaches 2000 m in the northern area, diminishes to 150 m in the southern area.

Punctuated Succession of Paleogene Mollusks in the Northern Gulf Coastal Plain

Occurrence data for 2,876 molluscan species (including species, subspecies, variations, and unnamedforms) in the northern Gulf states of Texas, Louisiana, Mississippi, andAlabama show a punctuated succession through the Paleogene stratigraphic sequence. These occurrence data are readily divisible into five successional cycles that correspond both to stratigraphic groups and to cycles of marine transgression and regression. The base of each cycle contains a fauna with a high percentage of species making their first appearance and coincides with a marine transgression. This percentage decreases through the cycle as the percentage of species making their last appearance increases. Each cycle is concluded by a marine regression. Individual cycles represent a background normal rate of speciation and extinction events. Punctuations between cycles represent major extinction events produced by marine regression and accompanying increased shelf sedimentation from advancing delta complexes.

Digital image processing of integrated geochemical and geological information

Image analysis computer systems, developed for the study of remotely-sensed data, provide a powerful method for handling spatial data in digital-image formats. Recent developments in image creation techniques, using point and line data, and the digital image processing of the derived images, enable interactive colour mapping and statistical analysis of integrated geological and geochemical data to be performed. The system, developed jointly by the British Geological Survey (BGS) and the Experimental Cartography Unit (ECU), is based on an International Imaging Systems (I 2 S) Model 70 computer and allows the geochemical signatures, or associations of trace elements for particular geological units, to be studied in terms of lithological or stratigraphical groups or subgroups, on a regional or local scale. This paper describes the techniques used to create the geochemical digital images and geological region-of-interest sub-images for the Moray-Buchan region of Scotland. The application of image processing and image analysis methods available for geochemical mapping and the identification of geochemical signatures of specific geological units are discussed with reference to the Moine-Dalradian geochemistry, mineral exploration, heat production, medical geography (epidemiology) and agricultural studies.

Geochemistry of Archaean metavolcanic rocks from the Holenarsipur and Shigegudda volcano—sedimentary belts of Karnataka, South India

Abstract Major and trace element data for ultramafic and mafic metavolcanic rocks from the volcano-sedimentary belts at Holenarsipur and Shigegudda are presented. Although the Holenarsipur belt has been regarded as representing two stratigraphic groups separated widely in time — the Sargur (pre-3,4 Ga) and Dharwar (post-3,4 Ga) Groups — and Shigegudda is clearly younger than the 3.4 Ga gneisses, representative samples from all three suites are part of the same geochemical population. This comprises komatiitic to tholeiitic lavas which are genetically related by progressive fractional crystallization of olivine + pyroxene ± Cr-spinel. Some of the variability in the high-MgO rocks may reflect differential, partial melting of the mantle. There are two sub-populations, separable into light rare-earth-enriched and light rare-earth-depleted, which may reflect different depths of melting of compositionally homogeneous mantle. Many of the geochemical characteristics of the population bear a strong resemblance to modern basic volcanics formed in destructive plate margin environments.