scholarly journals Badenian (middle Miocene) continental paleoenvironment in the Novohrad–Nógrád Basin (Central Paratethys): a volcano-sedimentary record from the Páris-patak Valley in Hungary

2021 ◽  
Vol 151 (2) ◽  
pp. 159
Author(s):  
Emese M. Bordy ◽  
Orsolya Sztanó
Keyword(s):  
Grain Size ◽  
Middle Miocene ◽  
River System ◽  
High Energy ◽  
Volcanic Field ◽  
Bedload Transport ◽  
Coarse Grained ◽  
Facies Association ◽  
Cross Bedding ◽  
Facies Associations

Two levels of volcaniclastics, comprising conglomerates, sandstones and mudstones, are interbedded with upper middle Miocene (upper Badenian) andesite pyroclastics near the Hungarian-Slovakian border in the distal region of the Central Slovakian Neogene Volcanic Field. Based on the field sedimentological investigations, the facies of the volcaniclastics (e.g., lateral and vertical grain size changes, sedimentary structures, textures, clast composition), their geometry and field relationships are documented herein with the aim of reconstructing the depositional environment. The silica-cemented volcaniclastics are mostly andesite clasts with only ~ 5% being granitoid, quarzitic, and tuff clasts as well as charred fossil wood fragments. The coarse-grained facies association includes crudely stratified, tabular or lenticular, clast-supported pebble-cobble conglomerates with erosive basal surfaces, b-axis imbrication, alternating with sets of cross-bedding. The fine-grained facies association comprises cross-bedded pebbly to medium-grained sandstone and lenses of tuffaceous clayey siltstone with rare horizontal lamination and water-escape structures. Rip-up mudstone clasts, with diametre up to 1 m, are present in both facies associations, revealing the co-existence of abandoned silty palaeo-channel plugs. Facies associations are arranged in several 0.5-4-m-thick, fining-upwards successions that likely formed in shallow channels as downstream- to laterally accreting longitudinal bars, extensive gravel sheets and bars that migrated in peak flow during floods. Palaeocurrent indicators (i.e., clast imbrication, direction of planar cross-bedding, orientation of petrified wood logs) show bedload transport by traction currents, initially towards ~S, and later towards ~W. Intermittently debris flows also occurred. Cross-bedded sandstones formed as in-channel transverse bars during medium/low discharge. Variation of grain size shows frequent discharge fluctuations during permanently wet conditions in the late Badenian. The 4-5-m-deep, low-sinuosity channels were part of a high-energy, gravel-bed braided-river system on the south-eastern foothills of the Lysec palaeo-volcano. Here, pyroclastics were reworked and redeposited as volcaniclastics during inter-eruption, high-discharge episodes.

scholarly journals Famennian glaciation in the eastern side of Parnaíba Basin, Brazil: evidence of advance and retreat of glacier in Cabeças Formation

2015 ◽  
Vol 45 (suppl 1) ◽  
pp. 13-27 ◽  
Author(s):  
Roberto Cesar de Mendonça Barbosa ◽  
Afonso César Rodrigues Nogueira ◽  
Fábio Henrique Garcia Domingos
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
High Energy ◽  
Normal Faults ◽  
Facies Association ◽  
Delta Front ◽  
Cross Bedding ◽  
Facies Associations ◽  
Detachment Surface ◽  
Parnaíba Basin

ABSTRACTGlaciotectonic features studied in the siliciclastic deposits of Cabeças Formation, Upper Devonian, represent the first evidence of Famennian glaciation in Southeastern Parnaíba Basin, Brazil. Outcrop-based stratigraphic and facies analyses combined with geometric-structural studies of these deposits allowed defining three facies association (FA). They represent the advance-retreat cycle of a glacier. There are: delta front facies association (FA1) composed of massive mudstone, sigmoidal, medium-grained sandstone with cross-bedding and massive conglomerate organized in coarsening- and thickening-upward cycles; subglacial facies association (FA2) with massive, pebbly diamictite (sandstone, mudstone and volcanic pebbles) and deformational features, such as intraformational breccia, clastic dikes and sills of diamictite, folds, thrust and normal faults, sandstone pods and detachment surface; and melt-out delta front facies associations (FA3), which include massive or bedded (sigmoidal cross-bedding or parallel bedding) sandstones. Three depositional phases can be indicated to Cabeças Formation: installation of a delta system (FA1) supplied by uplifted areas in the Southeastern border of the basin; coastal glacier advance causing tangential substrate shearing and erosion (FA1) in the subglacial zone (FA2), thus developing detachment surface, disruption and rotation of sand beds or pods immersed in a diamicton; and retreat of glaciers accompanied by relative sea level-rise, installation of a high-energy melt-out delta (FA3) and unloading due to ice retreat that generates normal faults, mass landslide, folding and injection dykes and sills. The continuous sea-level rise led to the deposition of fine-grained strata of Longá Formation in the offshore/shoreface transition in the Early Carboniferous.

scholarly journals Incised Valley Depositional System of the Cretaceous Yolde Formation of the Gongola Sub-basin Northern Benue Trough N.E. Nigeria

2020 ◽  
Vol 5 (8) ◽  
pp. 870-876
Author(s):  
Shettima Bukar ◽  
Mohammed Bukar ◽  
Asabe Kuku ◽  
Bintu Shettima ◽  
Ishaku H. Kamale
Keyword(s):  
Current System ◽  
River System ◽  
High Energy ◽  
Facies Association ◽  
Benue Trough ◽  
Incised Valley ◽  
Stratigraphic Architecture ◽  
Facies Associations ◽  
Valley Fills ◽  
Sandstone Facies

This research was carried out in the Gongola Sub-basin of the Northern Benue Trough aimed at deciphering of the paleo-depositional environment of the Yolde Formation based on facies on facies analysis. Six lithofacies were identified to include trough crossbedded sandstone facies (St), massive bedded sandstone facies (Sm), planar crossbedded sandstone facies (Sp), ripple laminated sandstone facies (Sr), parallel sandstone facies (Sl) and mudstone facies (Fm). These build into two facies association of fluvial channel and tidally influenced fluvial channel facies associations. The fluvial successions typical characterizes the lower stratigraphic horizons and their contained dominances of trough crossbedded sandstone facies with high channel to overbank facies and contained mud-clast reflecting deep, high energy braided river system. The submergences of these channels by surging sea level rise generated the tidally influenced fluvial facies association and this package characteristically defines the upper interval stratigraphic architecture of this formation, displaying occasional bi-directional current system and abundant marine ichnogenera. This architectural symmetry is reflective of an incised valley fills, developing as a consequence of Cenomanian transgressive phase induced by the mid-Cretaceous global marine transgression.       

scholarly journals Eocene-Oligocene sedimentation in the external areas of the Moldavide Basin (Marginal Folds Nappe, Eastern Carpathians, Romania): sedimentological, paleontological and petrographic approaches

2009 ◽  
Vol 60 (5) ◽  
pp. 397-417 ◽  
Author(s):  
Crina Miclăuş ◽  
Francesco Loiacono ◽  
Diego Puglisi ◽  
Dorin Baciu
Keyword(s):  
Source Area ◽  
Late Eocene ◽  
Low Rank ◽  
Coarse Grained ◽  
Facies Association ◽  
Compositional Uniformity ◽  
Eastern Carpathians ◽  
Extensional Tectonic ◽  
Facies Associations ◽  
Depositional Systems

Eocene-Oligocene sedimentation in the external areas of the Moldavide Basin (Marginal Folds Nappe, Eastern Carpathians, Romania): sedimentological, paleontological and petrographic approachesThe Marginal Folds Nappe is one of the most external tectonic units of the Moldavide Nappe System (Eastern Carpathians), formed by Cretaceous to Tertiary flysch and molasse deposits, piled up during the Miocene closure of the East Carpathian Flysch basin, cropping out in several tectonic half-windows, the Bistriţa half-window being one of them. The deposits of this tectonic unit were accumulated in anoxic-oxic-anoxic conditions, in a forebulge depozone (sensuDeCelles & Giles 1996), and consist of a pelitic background sporadically interrupted by coarse-grained events. During the Late Eocene the sedimentation registered a transition from calcareous (Doamna Limestones) to pelitic (Bisericani Beds) grading to Globigerina Marls at the Eocene-Oligocene boundary, and upward during the Oligocene in deposits rich in organic matter (Lower Menilites, Bituminous Marls, Lower and Upper Dysodilic Shales) with coarsegrained interlayers. Seven facies associations were recognized, and interpreted as depositional systems of shallow to deeper water on a ramp-type margin. Two mixed depositional systems of turbidite-like facies association separated by a thick pelitic interval (Bituminous Marls) have been recognized. They were supplied by a "green schists" source area of Central Dobrogea type. The petrography of the sandstone beds shows an excellent compositional uniformity (quartzarenite-like rocks), probably representing a first cycle detritus derived from low rank metamorphic sources, connected with the forebulge relief developed on such a basement. The sedimentation was controlled mainly by different subsidence of blocks created by extensional tectonic affecting the ramp-type margin of the forebulge depozone.

scholarly journals Facies association and depositional environment of fan-delta sequence in southwest Kathmandu Basin, Nepal

1970 ◽  
Vol 12 ◽  
pp. 1-16
Author(s):  
Naresh Kazi Tamrakar ◽  
Pramila Shrestha ◽  
Surendra Maharjan
Keyword(s):  
River System ◽  
Sedimentary Facies ◽  
Facies Association ◽  
Base Level ◽  
Rock Types ◽  
Fan Delta ◽  
Main River ◽  
Facies Associations ◽  
Delta Sequence ◽  
The Difference

Lake marginal sedimentation prevailed around the Paleo-Kathmandu Lake. Owing to the difference in local basin conditions; tectonics, source rock types and river systems therein, the lake marginal environments and sedimentary facies associations differ around the Paleo-Kathmandu Lake. In this study, the basin-fill sediments of southwestern margin of the Kathmandu Basin were studied for the sediments recorded in vertical sequences at various localities and facies analysis was made. Mainly eight facies were recognised. They were matrix-supported massive gravel (Gmm), matrix-supported graded gravel (Gmg), gravelly fine or mud (GF), massive silt (Fsm), massive mud (Fm), ripple-laminated silt or laminated silt/mud/clay (Fl), carbonaceous clay (C), and incipient soil with roots (Fr). Four facies associations that were identified were proximal fan-delta facies association (FA1), mid fan-delta facies associaiton (FA2), distal fan-delta facies association (FA3), and gravelly sinuous river facies association (FA4). Remarkably, these facies associations do not contain any sandy facies and foreset bedding of Gilbert-type. The fan-delta region was characterised by flood-dominated flows and vertical accretion of fines in the flood basins, and vegetated swamps rich in organic sediments. The distribution of facies associations suggests extensive lake transgression followed by rapid lake regression. The recent river system then incised the valley against local upliftment due to faulting or lowering of base level of the main river in the Kathmandu Basin probably related to draining out of the lake water. doi: Bulletin of the Department of Geology, Vol. 12, 2009, pp. 1-16

scholarly journals Facies architecture of the fluvial Missão Velha Formation (Late Jurassic–Early Cretaceous), Araripe Basin, Northeast Brazil: paleogeographic and tectonic implications

2017 ◽  
Vol 67 (4) ◽  
pp. 515-545 ◽  
Author(s):  
Gelson Luís Fambrini ◽  
Virgínio Henrique M.L. Neumann ◽  
José Acioli B. Menezes-Filho ◽  
Wellington F. Da Silva-Filho ◽  
Édison Vicente De Oliveira
Keyword(s):  
Sand Dunes ◽  
High Energy ◽  
Coarse Grained ◽  
Dispersion Pattern ◽  
Northeast Brazil ◽  
River Systems ◽  
Alluvial Deposits ◽  
Architectural Elements ◽  
Overbank Flow ◽  
Facies Associations

Abstract Sedimentological analysis of the Missão Velha Formation (Araripe Basin, northeast Brazil) is the aim of this paper through detailed facies analysis, architectural elements, depositional systems and paleocurrent data. The main facies recognized were: (i) coarse-grained conglomeratic sandstones, locally pebbly conglomerates, with abundant silicified fossil trunks and several large-to-medium trough cross-stratifications and predominantly lenticular geometry; (ii) lenticular coarse-to-medium sandstones with some granules, abundant silicified fossil wood, and large-to-medium trough cross-stratifications, cut-and fill features and mud drapes on the foresets of cross-strata, (iii) poorly sorted medium-grained sandstones with sparse pebbles and with horizontal stratification, (iv) fine to very fine silty sandstones, laminated, interlayered with (v) decimetric muddy layers with horizontal lamination and climbing-ripple cross-lamination. Nine architectural elements were recognized: CH: Channels, GB: Gravel bars and bed forms, SB: Sand bars and bedforms, SB (p): sand bedform with planar cross-stratification, OF: Overbank flow, DA: Downstream-accretion macroforms, LS: Laminated sandsheet, LA: Lateral-accretion macroforms and FF: Floodplain fines. The lithofacies types and facies associations were interpreted as having been generated by alluvial systems characterized by (i) high energy perennial braided river systems and (ii) ephemeral river systems. Aeolian sand dunes and sand sheets generated by the reworking of braided alluvial deposits can also occur. The paleocurrent measurements show a main dispersion pattern to S, SE and SW, and another to NE/E. These features imply a paleodrainage flowing into the basins of the Recôncavo-Tucano-Jatobá.

FACIES MODELS IN EXPLORATION — THE CARBONATE PLATFORMS OF NORTH-EAST AUSTRALIA

1988 ◽  
Vol 28 (1) ◽  
pp. 123 ◽  
Author(s):  
Peter J. Davies ◽  
Philip A. Symonds ◽  
David A. Feary ◽  
Christopher J. Pi gram
Keyword(s):  
Great Barrier Reef ◽  
Middle Miocene ◽  
Foreland Basin ◽  
High Energy ◽  
Low Energy ◽  
Barrier Reef ◽  
Carbonate Platforms ◽  
North East ◽  
Facies Associations ◽  
Platform Margin

The carbonate platforms of north-east Australia encapsulate a record of tectonic, eustatic, climatic and oceanographic dynamism that has controlled their formation. Collectively, the Great Barrier Reef and the Queensland and Marion Plateaus, together with the rift basins that separate them, define a new model for carbonate platform evolution with important exploration consequences. Cretaceous rifting, Paleocene breakup, Cainozoic northward drift with concomitant climatic changes, Neogene subsidence pulses, and sea-level perturbations have combined to produce tropical carbonate platforms overlying temperate, mixed carbonate/siliciclastic facies. The Great Barrier Reef tropical shelf platform thins to the south; reefs first developed in the north in the Early to Middle Miocene along the west- to east-trending distal margin of a foreland basin. The reefs of the Queensland and Marion Plateaus developed in the Middle Miocene and are the precursors of the carbonate platforms of the central and southern Great Barrier Reef. The Miocene Marion Plateau barrier and platform reefs backstepped to become the Plio-Pleistocene Great Barrier Reef. Three energy- and climate-related carbonate facies associations define new prospecting scenarios: the tropical, high energy reef model; the tropical, low energy, Halimeda bioherm model; and the subtropical, low energy, deep water, red algal/ foram/bryozoan bioherm model. These facies occur within four distinct structural/sedimentological associations: the progradative platform margin, the backstepped platform margin, the foreland basin, and the fault block association. The models can be readily applied to the Gulf of Papua/Torres Shelf and the Canning Basin and may produce exciting new insights into carbonate plays in these areas.

scholarly journals Lithosphere tearing along STEP faults and synkinematic formation of lherzolite and wehrlite in the shallow subcontinental mantle

Solid Earth ◽  
2019 ◽  
Vol 10 (4) ◽  
pp. 1099-1121 ◽  
Author(s):  
Károly Hidas ◽  
Carlos J. Garrido ◽  
Guillermo Booth-Rea ◽  
Claudio Marchesi ◽  
Jean-Louis Bodinier ◽  
...  
Keyword(s):  
Grain Size ◽  
Lithospheric Mantle ◽  
Volcanic Field ◽  
Mantle Xenoliths ◽  
Coarse Grained ◽  
Fault System ◽  
Mantle Flow ◽  
Second Phase ◽  
Fine Grained ◽  
Wide Range

Abstract. Subduction-transform edge propagator (STEP) faults are the locus of continual lithospheric tearing at slab edges, resulting in sharp changes in the lithospheric and crustal thickness and triggering lateral and/or near-vertical mantle flow. However, the mechanisms at the lithospheric mantle scale are still poorly understood. Here, we present the microstructural study of olivine-rich lherzolite, harzburgite and wehrlite mantle xenoliths from the Oran volcanic field (Tell Atlas, northwest Algeria). This alkali volcanic field occurs along a major STEP fault responsible for the Miocene westward slab retreat in the westernmost Mediterranean. Mantle xenoliths provide a unique opportunity to investigate the microstructures in the mantle section of a STEP fault system. The microstructures of mantle xenoliths show a variable grain size ranging from coarse granular to fine-grained equigranular textures uncorrelated with lithology. The major element composition of the mantle peridotites provides temperature estimates in a wide range (790–1165 ∘C) but in general, the coarse-grained and fine-grained peridotites suggest deeper and shallower provenance depth, respectively. Olivine grain size in the fine-grained peridotites depends on the size and volume fraction of the pyroxene grains, which is consistent with pinning of olivine grain growth by pyroxenes as second-phase particles. In the coarse-grained peridotites, well-developed olivine crystal-preferred orientation (CPO) is characterized by orthorhombic and [100]-fiber symmetries, and orthopyroxene has a coherent CPO with that of olivine, suggesting their coeval deformation by dislocation creep at high temperature. In the fine-grained microstructures, along with the weakening of the fabric strength, olivine CPO symmetry exhibits a shift towards [010] fiber and the [010] and [001] axes of orthopyroxene are generally distributed subparallel to those of olivine. These data are consistent with deformation of olivine in the presence of low amounts of melts and the precipitation of orthopyroxenes from a melt phase. The bulk CPO of clinopyroxene mimics that of orthopyroxene via a topotaxial relationship of the two pyroxenes. This observation points to a melt-related origin of most clinopyroxenes in the Oran mantle xenoliths. The textural and geochemical record of the peridotites are consistent with interaction of a refractory harzburgite protolith with a high-Mg no. melt at depth (resulting in the formation of coarse-grained clinopyroxene-rich lherzolite and wehrlite) and with a low-Mg no. evolved melt in the shallow subcontinental lithospheric mantle (forming fine-grained harzburgite). We propose that pervasive melt–peridotite reaction – promoted by lateral and/or near-vertical mantle flow associated with lithospheric tearing – resulted in the synkinematic crystallization of secondary lherzolite and wehrlite and had a key effect on grain size reduction during the operation of the Tell–Rif STEP fault. Melt–rock reaction and secondary formation of lherzolite and wehrlite may be widespread in other STEP fault systems worldwide.

scholarly journals Grain Size Analysis, Textural Characterisation and Depositional Environment of Turonian Amasiri Sandstone, Southern Benue Trough, Nigeria

2021 ◽  
Vol 4 (2) ◽  
Keyword(s):  
Grain Size ◽  
Depositional Environment ◽  
Function Analysis ◽  
High Energy ◽  
Coarse Grained ◽  
Grain Size Analysis ◽  
Size Analysis ◽  
Benue Trough ◽  
Shallow Marine ◽  
Wide Range

Grain size analysis of Turonian Amasiri Sandstone in southern Benue Trough has been undertaken to determine the controversial depositional environment of the formation. The formation was first studied on outcrops and 26 representative samples were collected and subjected to particle size analysis in line with standard procedures for dry sieving. Various methods of environmental interpretation of grain size distribution data were applied to constrain the depositional of the sandstones. The result indicates that the sands are medium and coarse-grained with mean size ranging from 0.15 to 1.87φ and averaging 0.96 φ. The sandstones are moderately to poorly sorted with standard deviation values ranging from 0.72 to 1.38 φ and averaging 1.07 φ. They exhibit a wide range of distribution from strongly coarse skewed to strongly fine skewed with skewness values ranging from -2.31 to 1.52 φ and averaging -0.04 φ but indicate a narrow range of kurtosis from mesokurtic to leptokurtic distribution with values ranging from 0.99 to 3.49 φ and an average of 2.06 φ. The sediments have bimodal with minor polymodal and unimodal distribution with primary modal size of 1.2 φ. The bivariate plots of size statistical parameters indicate fluvial environment of deposition. However, linear discriminant function analysis and the interpretations of log-probability plots indicate deposition in a fluvial, beach, and shallow marine settings, and thus suggesting a possible deposition in high-energy transitional environment. The C-M pattern of the samples indicates that sediments were transported mainly by rolling and suspension with subordinate fractions moved by rolling as well as suspension. Thus, it is deduced that Amasiri Sandstone was deposited in fluvial, beach, and agitated shallow marine environments.

The Effects of Nanostructure on the Strengthening of NiAl

1998 ◽  
Vol 552 ◽  
Author(s):  
T. Chen ◽  
N. N. Thadhani ◽  
J. M. Hampikian
Keyword(s):  
Grain Size ◽  
Vickers Hardness ◽  
High Energy ◽  
Coarse Grained ◽  
Shock Compaction ◽  
Transmission Electron ◽  
Nial Alloy ◽  
Microhardness Testing ◽  
The Relationship ◽  
Hardness Values

ABSTRACTThe relationship between microhardness and grain size was investigated for nanocrystalline and coarse-grained stoichiometric intermetallic NiAl. The nanocrystalline NiAl specimens were synthesized through mechanical alloying with a high-energy Spex 8000 shaker mill and consolidated by shock compaction at a peak pressure of 4–6 GPa, to 83% dense compacts. The nanocrystalline NiAl compacts were also sintered at 1073, 1173 and 1473 K for 2 h. The Vickers hardness of consolidated and sintered NiAl was determined by microhardness testing, and the grain size and microstructure were investigated with transmission electron microscopy. It was found that the hardness values increased with decreasing grain size of the NiAl alloy. The Vickers hardness values were approximately 650±16, 690±6, 800±43 HV, respectively for NiAl with grain-sizes corresponding to approximately 27±18, 11±6 and 9±6 nm. The possible strengthening mechanisms operating in NiAl are discussed.

Explosive volcanism on Santorini, Greece

1989 ◽  
Vol 126 (2) ◽  
pp. 95-126 ◽  
Author(s):  
T. H. Druitt ◽  
R. A. Mellors ◽  
D. M. Pyle ◽  
R. S. J. Sparks
Keyword(s):  
Explosive Volcanism ◽  
Volcanic Field ◽  
Pyroclastic Flows ◽  
Coarse Grained ◽  
Magma Ascent ◽  
Explosive Eruptions ◽  
Caldera Collapse ◽  
Caldera Wall ◽  
Magma Chambers ◽  
Facies Associations

AbstrctSantorini volcanic field has had 12 major (1–10 km3 or more of magma), and numerous minor, explosive eruptions over the last ~ 200 ka. Deposits from these eruptions (Thera Pyroclastic Formation) are well exposed in caldera-wall successions up to 200 m thick. Each of the major eruptions began with a pumice-fall phase, and most culminated with emplacement of pyroclastic flows. Pyroclastic flows of at least six eruptions deposited proximal lag deposits exposed widely in the caldera wall. The lag deposits include coarse-grained lithic breccias (andesitic to rhyodacitic eruptions) and spatter agglomerates (andesitic eruptions only). Facies associations between lithic breccia, spatter agglomerate, and ignimbrite from the same eruption can be very complex. For some eruptions, lag deposits provide the only evidence for pyroclastic flows, because most of the ignimbrite is buried on the lower flanks of Santorini or under the sea. At least eight eruptions tapped compositionally heterogeneous magma chambers, producing deposits with a range of zoning patterns and compositional gaps. Three eruptions display a silicic–silicic + mafic–silicic zoning not previously reported. Four eruptions vented large volumes of dacitic or rhyodacitic pumice, and may account for 90% or more of all silicic magma discharged from Santorini. The Thera Pyroclastic Formation and coeval lavas record two major mafic-to-silicic cycles of Santorini volcanism. Each cycle commenced with explosive eruptions of andesite or dacite, accompanied by construction of composite shields and stratocones, and culminated in a pair of major dacitic or rhyodacitic eruptions. Sequences of scoria and ash deposits occur between most of the twelve major members and record repeated stratocone or shield construction following a large explosive eruption.Volcanism at Santorini has focussed on a deep NE–SW basement fracture, which has acted as a pathway for magma ascent. At least four major explosive eruptions began at a vent complex on this fracture. Composite volcanoes constructed north of the fracture were dissected by at least three caldera-collapse events associated with the pyroclastic eruptions. Southern Santorini consists of pryoclastic ejecta draped over a pre-volcanic island and a ridge of early- to mid-Pleistocene volcanics. The southern half of the present-day caldera basin is a long-lived, essentially non-volcanic, depression, defined by topographic highs to the south and east, but deepened by subsidence associated with the main northern caldera complex, and is probably not a separate caldera.

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