Reconnaissance Geologic Map Research Articles

Reconnaissance Basement Geology and Tectonics of North Zealandia

AbstractNew rock dredge samples supply key information to establish the tectonic and geological framework of the northern two‐thirds of the 95% submerged Zealandia continent. The R/V Investigator voyage IN2016T01 to the Fairway Ridge, Coral Sea, obtained poorly sorted poly‐lithologic pebbly to cobbly sandstones, well sorted fine grained sandstones, mudstones, bioclastic limestones, and basaltic lavas. Post‐cruise analytical work comprised petrography, whole rock geochemical and Sr and Nd isotopic analyses, and U‐Pb zircon, Rb‐Sr, and Ar‐Ar geochronology. A Fairway Ridge cobbly sandstone has a ∼95 Ma (early Late Cretaceous) depositional age; two biotite granite cobbles are 111 ± 1 and 128 ± 1 Ma in age, and some volcanic pebbles are also likely Early Cretaceous. Fairway Ridge basalts have intraplate alkaline chemistry and are of Late Eocene age (∼40–36 Ma). By analogy with South Zealandia, we interpret strong positive continental magnetic anomalies of North Zealandia to mainly result from Late Cretaceous to Cenozoic intraplate basalts, many of them rift‐related lavas. A new basement geological map of North Zealandia shows the position of the Mesozoic Gondwana magmatic arc axis (Median Batholith) and other major geological units. This study completes onland and offshore reconnaissance geological mapping of the entire 5 Mkm2 Zealandia continent.

SE Asia as part of Gondwana: a 1960s regional mapping project over Southern Thailand that lay behind the hypothesis

Abstract In the 1960s, anticipating the award of concessions in the Gulf of Thailand, BP carried out reconnaissance geological mapping over all of Southern Thailand including its Gulf islands. The resulting eight geological maps at 1:250 000 scale provided a database that underpinned discussions of Thailand's place in global plate-tectonic reconstructions that were emerging at that time. In particular, new light was thrown on the Paleozoic succession, and important differences were found between the Upper Thai Peninsula and the Lower Peninsula (respectively, NW and SE of the prominent bend in the peninsula's outline). The Phuket Group of the Upper Peninsula is a very thick diamictite-bearing succession of possible Devonian to Early Permian age, apparently sourced in the west, a region now occupied by the Indian Ocean. That in turn suggested a possible Gondwana origin for SE Asia, a contentious concept at the time, but one that is now widely accepted. The emphasis of field mapping these days is on obtaining detailed data that necessarily means over areas of limited extent. But a very broad regional mapping project such as BP's in Thailand revealed stratigraphic and tectonic insights that could be revealed only from a wide-ranging regional study of this kind.

Stratigraphic and volcanic signatures of Miaolingian-Late Ordovician rift pulses in the Alborz Mountains, northern Iran

• The Cambrian-Ordovician succession of the eastern Alborz Mountain Range displays uplift episodes associated with erosion. • The SPICE chemostratigraphic shift marks the Miaolingian–Furongian boundary. • Alkaline to subalkaline magmatic activities are separated by quiescent intervals of up to 35 m.y. • Intraplate (continental rift) volcanism reflects OIB-type mantle sources . • Plume-related rift pulses are linked to a large igneous province about 600 km across. Recent reconnaissance geological mapping, identification of unconformities and chronostratigraphic dating of Miaolingian to Ordovician strata in the eastern Alborz Mountains have led to recognition of synsedimentary uplift episodes associated with block tilting and partial erosion of the Cambrian Mila and Ordovician Simeh Kuh, Qumes and Lashkarak formations. A chronostratigraphic revision of the Miaolingian-Late Ordovician fossil content is presented, and the Steptoean Positive Carbon Isotope Excursion (SPICE) chemostratigraphic shift, marking the Miaolingian–Furongian boundary, is constrained close to the Members 2/3 contact of the Mila Formation. Upper Ordovician basaltic lava flows embedded in the Abarsaj Formation and Devonian sills and dykes intruded in the Mila and Simeh-Kuh formations were geochemically analysed and compared with the voluminous lava eruptions recognized in the Katian-Llandovery Soltan-Maidan and Gorgan Schists formations, and in the ?Emsian-Eifelian Mighan Formation. They represent successive magmatic pulses of alkaline to subalkaline activities, separated by background (quiescent) intervals of up to 35 m.y., characterizing an intraplate field (continental rift) affinity compatible with OIB-like (ΔNb greater than 0) mantle sources. Successive Miaolingian-Mid Ordovician (volcanic-free) uplift episodes were related to lithospheric doming preceding the onset of stepwise plume-related rift pulses, which shaped a large igneous province with transects of about 600 km, linking the Damghan and Alestan domains of the eastern Alborz Mountains with South Kopet-Dagh and some blocks of Central Iran.

Field guide to Laramide basin evolution and drilling activity in North Park and Middle Park, Colorado

This two-day field trip provides an overview of the geologic history of the North Park–Middle Park area and its past and recent drilling activity. Stops highlight basin formation and the consequences of geologic configuration on oil and gas plays and development. The trip focuses on work from ongoing U.S. Geological Survey research in this area (currently part of the Cenozoic Landscape Evolution of the Southern Rocky Mountains Project funded by the National Cooperative Geologic Mapping Program). Surface mapping is integrated with perspective from petroleum exploration within the basin. The starting point is the west flank of the Denver Basin to compare and contrast the latest Cretaceous through Eocene basin fill on both flanks of the Front Range. The next stop continues on the south end of the North Park–Middle Park area, about 60 miles [95km] west from the first stop. A general clockwise loop is described by following U.S. Highway 40 from Frasier via Granby and Kremmling to Muddy Pass after which CO Highway 14 is followed to Walden for an overnight stay. On the second day after a loop north of Walden, the Continental Divide is crossed at Willow Creek Pass for a return to Granby via Highway 125. The single structural basin that underlies both physiographic depressions of North Park and Middle Park originated during the latest Cretaceous to Eocene Laramide orogeny (Tweto, 1957, 1975; Dickinson et al., 1988). It largely filled with Paleocene to Eocene sediments and is bordered on the east by the Front Range, on the west by the Park Range and Gore Range, on the north by Independence Mountain and to the south by the Williams Fork and Vasquez Mountains (Figure 1). This larger Paleocene-Eocene structural basin is continuous underneath the Continental Divide, which dissects the basin in two approximately equal physiographic depressions, the ‘Parks.’ Therefore Cole et al. (2010) proposed the name ‘Colorado Headwaters Basin’ or ‘CHB,’ rather than North Park–Middle Park basin (Tweto 1957), to eliminate any confusion between the underlying larger Paleocene-Eocene basin and the two younger depressions that developed after the middle Oligocene. The name was derived from the headwaters of the Colorado, North Platte, Laramie, Cache La Poudre, and Big Thompson Rivers which are all within or near the study area. In this field guide, we will use the name Colorado Headwaters Basin (CHB) over North Park–Middle Park basin. Several workers have described the geology in the basin starting with reports from Marvine who was part of the Hayden Survey and wrote about Middle Park in 1874, Hague and Emmons reported on North Park as part of the King Survey in 1877, Cross on Middle Park (1892), and Beekly surveyed the coal resources of North Park in 1915. Further reconnaissance geologic mapping was performed by Hail (1965 and 1968) and Kinney (1970) in the North Park area and by Izett (1968, 1975), and Izett and Barclay (1973) in Middle Park. Most research has focused on coal resources (Madden, 1977; Stands, 1992; Roberts and Rossi, 1999), and oil and gas potential (1957, all papers in the RMAG guidebook to North Park; subsurface structural geologic analysis of both Middle Park and North Park (the CHB) by oil and gas geologist Wellborn (1977a)). A more comprehensive overview of all previous geologic research in the basin can be found in Cole et al. (2010). Oil and gas exploration started in 1925 when Continental Oil's Sherman A-1 was drilled in the McCallum field in the northeast part of the CHB. It produced mostly CO2 from the Dakota Sandstone and was dubbed the ‘Snow cone’ well. Later wells were more successful finding oil and/or gas, and exploration and production in the area is ongoing, most notably in the unconventional Niobrara play in the Coalmont-Hebron area.

Utilizing the image processing techniques in mapping the geology of Al Taif area, central western Arabian Shield, Saudi Arabia

Al Taif area is located in the central western portion of the Arabian Shield, Saudi Arabia. The area was a part of a geosyncline, in which sediments were deposited, followed by granitic and dioritic intrusions. During the Hijaz orogeny (650–800 Ma), the area was subjected to a progressive regional metamorphism to lower amphibolite facies which produced chlorite schists and micaceous quartzo­feldspathic schists. This metamorphism also affected the igneous rocks to produce gneissic granite and metadiorite. The metamorphism was followed by the intrusion of other granitic plutons (490–600 Ma). This work aimed to discriminate the lithologic rock units and structural framework of the study area in order to map the geology of Al Taif area. The study area was not subjected previously to detailed geological mapping using the modern image processing techniques. However, it was subjected only to reconnaissance geological mapping. The study area was geologically remapped in this work through image processing techniques of Landsat 7 Enhanced Thematic Mapper (ETM)+ digital data and field verifications. The fusion between multispectral ETM+ data and panchromatic ETM+ band 8 produced a color composite fused image for the study area of scale 1:50,000. The fused image was delineated the main regional folds, faults, and fractures of the study area. It was also defined and recognized the chlorite schist and the intruded monzogranitic rocks. The processed Landsat ratio image and supervised classification map discriminated the most lithologic units of the study area. The results of this work produced detailed and accurate geological map of scale 1:50,000 for the investigated area.

Field reconnaissance geologic mapping of the Columbia Hills, Mars, based on Mars Exploration Rover Spirit and MRO HiRISE observations

Chemical, mineralogic, and lithologic ground truth was acquired for the first time on Mars in terrain units mapped using orbital Mars Reconnaissance Orbiter's High Resolution Imaging Science Experiment (MRO HiRISE) image data. Examination of several dozen outcrops shows that Mars is geologically complex at meter length scales, the record of its geologic history is well exposed, stratigraphic units may be identified and correlated across significant areas on the ground, and outcrops and geologic relationships between materials may be analyzed with techniques commonly employed in terrestrial field geology. Despite their burial during the course of Martian geologic time by widespread epiclastic materials, mobile fines, and fall deposits, the selective exhumation of deep and well-preserved geologic units has exposed undisturbed outcrops, stratigraphic sections, and structural information much as they are preserved and exposed on Earth. A rich geologic record awaits skilled future field investigators on Mars. The correlation of ground observations and orbital images enables construction of a corresponding geologic reconnaissance map. Most of the outcrops visited are interpreted to be pyroclastic, impactite, and epiclastic deposits overlying an unexposed substrate, probably related to a modified Gusev crater central peak. Fluids have altered chemistry and mineralogy of these protoliths in degrees that vary substantially within the same map unit. Examination of the rocks exposed above and below the major unconformity between the plains lavas and the Columbia Hills directly confirms the general conclusion from remote sensing in previous studies over past years that the early history of Mars was a time of more intense deposition and modification of the surface. Although the availability of fluids and the chemical and mineral activity declined from this early period, significant later volcanism and fluid convection enabled additional, if localized, chemical activity.

Submarine geology of Hana Ridge and Haleakala Volcano's northeast flank, Maui

We present a morphostructural analysis of the submarine portions of Haleakala Volcano and environs, based upon a 4-year program of geophysical surveys and submersible explorations of the underwater flanks of Hawaiian volcanoes that was conducted by numerous academic and governmental research organizations in Japan and the U.S. and funded primarily by the Japan Agency for Marine–Earth Science and Technology. A resulting reconnaissance geologic map features the 135-km-long Hana Ridge, the 3000 km 2 Hana slump on the volcano's northeast flank, and island-surrounding terraces that are the submerged parts of volcanic shields. Hana Ridge below 2000 m water depth exhibits the lobate morphology typical of the subaqueously erupted parts of Hawaiian rift zones, with some important distinctions: namely, subparallel crestlines, which we propose result from the down-rift migration of offsets in the dike intrusion zone, and an amphitheater at its distal toe, where a submarine landslide has embayed the ridge tip. Deformation of Haleakala's northeast flank is limited to that part identified as the Hana slump, which lies downslope from the volcano's submerged shield, indicating that flank mobility is also limited in plan, inconsistent with hypothesized volcanic spreading driven by rift-zone dilation. The leading edge of the slump has transverse basins and ridges that resemble the thrust ramps of accretionary prisms, and we present a model to describe the slump's development that emphasizes the role of coastally generated fragmental basalt on gravitational instability of Haleakala's northeast flank and that may be broadly applicable to other ocean-island slumps.

Regional tectonic effects on the composition and mode of distribution of barite mineralization in Akpet Area, SE Nigeria

Reconnaissance geologic mapping for barite mineralization within Akpet Central and environs had been successfully done. The results of the study show that the mineralization of barite is largely structurally controlled although minor sedimentary processes have also contributed. Barite mineralization in the area therefore probably displays both metamorphic/igneous and/or sedimentary origins. Those that are derived from igneous processes occur mainly as veins and exhibit thicknesses which depend on the width of the fractures found in the rocks. On the other hand, barite mineralisation in the northeastern flank of the study area occurs as alluvial deposits and in few cases as domes. Chemical compositions for samples obtained from the northwestern and southwestern parts of the area show greater concentration of BaSO4 than in those samples from northeastern axis which exist in alluvial forms. Hence, both regional tectonic activities and the mode of occurrence of barites constitute important factors in the evaluation of barite mineralization. All the barite samples of the study areas analyzed are of high quality to stand the test of time in the world market. KEY WORDS: Mineralization; Barite; Fractures; quality; alluvial. Global Journal of Geological Sciences Vol.2(2) 2004: 177-181

Crustal growth through successive arc magmatism: reconnaissance U–Pb SHRIMP data from the northeastern Superior Province, Canada

North of the La Grande subprovince in northern Quebec, the Superior Province is dominated by plutonic rocks of the Bienville subprovince and Minto block, which have been further subdivided into domains on the basis of reconnaissance geological mapping and geochronology, and projected along strike using aeromagnetic anomalies. To test the validity of the extrapolations, U–Pb ages were obtained through SHRIMP analyses of zircon from 18 small archival samples selected from a suite of ∼2000 specimens collected during a helicopter reconnaissance survey conducted in the 1960s. The results reveal a history of successive arc magmatism between 3.07 and 2.70 Ga, followed by metamorphism, emplacement of crustally-derived granites (2.702–2.685 Ga) and late hydrothermal events (ca. 2.65 Ga). In the northeastern Minto block, the Douglas Harbour domain, previously extrapolated from dated localities to the northeast, is substantiated through an age of 2.87 Ga. The older tonalitic crust is punctured by 2.734–2.725 Ga granodioritic plutons of the Leaf River suite that locally carry 3.01 Ga inherited zircon. The northern Utsalik domain is made up of pyroxene and hornblende-bearing Leaf River plutons with consistent 2.725–2.723 Ga ages and local inheritance of 2.82 and 2.77 Ga. However, the southern extension of Utsalik domain contains pyroxene-bearing granodiorites both older (2.762 Ga) and younger (2.692 Ga) than the Leaf River suite. A tonalitic rock from the southern Goudalie domain with an age of 2.833 Ga supports a linkage to 2.84–2.83 Ga units of the Qalluviartuuq domain along strike to the north. Tikkerutuk domain represents a north-trending, 50-km wide calc-alkaline magmatic arc with ages between 2.71 and 2.70 Ga, that extends southward into the Bienville subprovince on the basis of aeromagnetic anomalies and geochronology. Zircon inheritance ages indicate that the Tikkerutuk arc was built upon antecedent arcs of 2.84, 2.77 and 2.725 Ga. The Inukjuak domain in the west consists mainly of granite, dated in one locality at 2.72 Ga.

Exploration Applications of RADARSAT Imagery In Western Canada And Tropical Regions

For the past two decades or so, remote sensing satellite technology has been widely used by the petroleum industry to support exploration in both frontier and mature areas. The application of this technology covers a wide range of geological and logistical aspects of the petroleum sector including:reconnaissance geological mapping of large areas,detailed mapping of prospective structures,delineation of fractured related reservoir trends,direct detection of surface alterations and vegetation changes related to oil seeps,logistical application associated with the collection and planning of seismic data, andenvironmental monitoring of petroleum related activities.To date, most remote sensing studies have been conducted with images that are obtained from passive, optical satellite imaging systems, such as the American LANDSAT and French SPOT satellites. However, these systems rely on short wavelength energyemitted from the sun, limiting their imaging capabilities. Radar imaging systems have proven to be excellent surface mapping tools in temperate and tropical areas because they are sensitive to variations in topography and surface roughness, and thus are capable of imaging the topographic expression of structures which are completely obscured by vegetation and other surficial material. Moreover, images can be obtained through heavy cloud cover due to radar's longer wavelength. The shadow effect which results from radar's active nature greatly enhances the surface expression of structures on the image. This effect can be maximized by carefully selecting the beam alignment to coincide with preferred structural orientations. In addition, images over the same area with different incidence angles and same or opposite look directions can be used to obtain stereo images which improve the structural mapping capabilities of radar. The advantages of radar imaging systems as compared to passive imaging technology (such as the sensors on the LANDSAT and SPOT satellites) can be seen in Chevron's exploration attempts in the Sumapaz Area of the Magdalena Basin in Colombia (Figures 1a and 1b). LANDSAT images of the area without significant cloud cover were difficult to find. The best of these images is shown in Figure 1a and the corresponding RADARS AT image is shown in Figure 1b. Not only does radar defeat the cloud cover problem, it captures, in much greater detail, the topographic expression of the structures. Figures 1c and 1d illustrate the benefit of using radar systems in areas with more subtle structures, showing a comparison between imaging technology and radar images over the mildly deformed region of the Canadian fore-foothills. Note that the imaging technology used in this area is very flat, showing no topographic relief, whereas the shadowing effect of radar enhances the topographic expression of subtle elongated anticlines which form significant hydrocarbon traps in the area. Successful application of radar imaging systems for geological exploration has been achieved with airborne synthetic apeture radar (SAR) surveys. However, airborne surveys suffer from two main shortfalls. First, the surveys can become prohibitively expensive when used as regional reconnaissance tools. Second, airplane access to remote or politically unstable areas is problematic.

Erosional processes as a control on the structural evolution of an actively deforming fold and thrust belt: An example from the Pamir‐Tien Shan region, central Asia

The foreland of the Pamir Mountains displays an unusual orogenic topography in which an active north vergent, thin‐skinned fold‐and‐thrust belt shows no topographic foredeep in advance of the thrust sheets. Instead, the thrusts are impinging on a topographic barrier formed by the southern edge of the Tien Shan. The deformation front of this thrust belt is coincident with a deeply incised river valley occupied by a large river, the Surkhob. We suggest that this river played a key role in the evolution of this orogenic system by acting as a lateral conveyor belt for the transport of mass delivered to the deformation front by thrusting. On the basis of reconnaissance geologic mapping it is clear that the present system developed in the Late Miocene or Pliocene, when the Tien Shan were uplifted within an existing foreland basin. As the Tien Shan highland developed, a longitudinal river within the foreland basin was trapped between the north directed thrusts and the Tien Shan highland. With continued deformation the river was squeezed into a narrow canyon and since that time the system has been in an unusual equilibrium where thrusting acts to push the deformation front northward toward the Tien Shan, but fluvial erosion and transport act to keep it in a constant position. Geologic relationships indicate that the Vakhsh thrust, the frontal thrust of the north directed system, is now emerging on an erosion surface beneath a thin Quaternary cover throughout the canyon, suggesting that the river has been unable to keep pace with the tectonic mass flux, and the thrust front, as well as the river, have been pushed northward across the beveled edge of what was originally the southern edge of the Tien Shan. We suggest that the original deformation front of the Tien Shan now lies ∼10 km south of the present‐day topographic front of the Tien Shan, beneath the north vergent Vakhsh thrust, as indicated by: (1) a near‐vertical zone of intense basement seismicity (the “Petrovsky” seismic zone); (2) an en échelon array of backthrusts along the south flank of the Peter the First Range; and (3) footwall cutoff relationships of the Vakhsh thrust system. Order‐of‐magnitude, mass balance considerations based on sediment budgets support this conclusion but suggest that the mass imbalance may be small. Thus large volumes of material may have passed through the thrust fron and been carried out of the system by erosion, a relationship that may account for the narrow (30–50 km wide) band of deformed sedimentary rock between the Pamir salient and the Tien Shan. The present near equilibrium is maintained by long‐term tectonic transport rates and fluvial transport rates, the latter closely linked to climate, particularly precipitation. Thus, although there is apparently a mass “pile up” within this system, other systems of this type might show grossly different structural and stratigraphic relationships depending on the amount of rainfall and corresponding variations in rates of fluvial sediment transport. Similar settings can be recognized in the active thick‐skinned thrust systems of the Tien Shan and ancient systems like the Laramide belt of western North America.

Delineation of lithostructural domains in northern Orissa (India) from Landsat Thematic Mapper imagery

Image-based reconnaissance geological mapping at 1:100,000 scale using Landsat TM data has delineated a college of Precambrian lithostructural domains within a 50,000-km 2 region which encompasses the northern portion of the Archaean to Proterozoic granulite-grade Eastern Ghats Tectonic Province and the adjacent Archaean-Mesoproterozoic Singhbhum Craton. The domains identified in the present study display distinctive internal structures on satellite imagery. Most are bounded by clearly recognisable major shear zones and faults on imagery; displacement directions may frequently be ascertained through local reorientation of planar structures adjacent to large strike-slip shear zones and through juxtaposition of domains with different structuring. The macroscopic/megascopic structural overview provided by the Landsat interpretation, supported by preliminary field investigation, suggests that the N-S shortening (E-W fold/thrust packages with associated NE and NW strike-slip faulting), is the dominant structural style in the northernmost part of the Eastern Ghats Tectonic Province, followed by regional dextral transpression, expressed in the form of major strike-slip faults. The largest of these structures (Kerajang Fault), which may be traced for over 250 km, has demonstrable Paleozoic-Mesozoic dextral motion associated with coal basin formation. A precursor Kerajang shear zone with dextral movement in excess of 100 km of indeterminate age may also have been instrumental in juxtaposing the Singhbhum Nucleus into its present position to the north of the Eastern Ghats Tectonic Province.

Paleomagnetism, stratigraphy, and petrology of the Upper Cretaceous Roskruge Volcanics at Bell Mountain, southeast Arizona

Reconnaissance geologic mapping and petrologic study of the Upper Cretaceous Roskruge Volcanics at Bell Mountain in the Roskruge Mountains, southeastern Arizona, have identified five compositionally distinct ash flow tuffs, including the basal red Viopuli Tuff, which comprise six or seven separate cooling units. Previous paleomagnetic investigators inferred eleven cooling units at Bell Mountain, but insufficient information was given to determine which tuffs had been sampled. This study was undertaken therefore to establish characteristic remanence directions for each tuff sheet. The available paleomagnetic data corroborate the identification of five map units in the Bell Mountain area, but the number of independent observations is too few for the calculation of a reliable paleopole; paleosecular variation of the geomagnetic field would probably not be averaged out. No correctable tuffs are present at Bell Mountain, so relative vertical axis rotations cannot be determined, although they are probably not large. The absence of regionally correctable tuffs suggests the presence of additional, potentially mineralized, volcanic centers perhaps farther north in the Roskruge Mountains or buried nearby beneath Cenozoic basin fill.

Reconnaissance geologic mapping of a portion of the rain‐forest‐covered Guiana Shield, Northwestern Brazil, using SIR-B and digital aeromagnetic data

This paper documents the result of an integrated analysis of spaceborne radar (SIR-B) and digital aeromagnetic data carried out in the heavily forested Guiana Shield. The objective of the research is to interpret the geophysical data base to its limit to produce a reconnaissance geologic map as an aid to ground work planning in a worst‐case setting. Linear geomorphic features were identified based on the interpretation of the SIR-B image. Digital manipulation of aeromagnetic data allowed the development of a color‐shaded relief map of reduced‐to‐pole magnetic anomalies, a terrace‐magnetization map, and a map showing the location of maximum values of the horizontal component of the pseudogravity gradient (magnetization boundary lines). The resultant end product was a reconnaissance geologic map where broad terrane categories were delineated and geologic faults with both topographic and magnetic expression were defined. The availability of global spaceborne radar coverage in the 1990s and the large number of existing digital aeromagnetic surveys in northwestern Brazil indicate that this approach can be potentially useful for reconnaissance geologic mapping elsewhere in the Guiana Shield.

Regional S-type granites in the Ecuadorian Andes: Possible remnants of the breakup of western Gondwana

Reconnaissance geological mapping of the Ecuadorian Cordillera Real has established the presence of a previously unrecognized regional suite of variably deformed granitoids for which poorly constrained RbSr whole-rock data indicate a minimum Early Jurassic age of 200 ± 12 Ma (initial ratio = 0.7120). This suite, which is associated with low- to medium-grade, semipelitic metamorphic rocks, is dominated by peraluminous monzogranites containing biotite ± garnet ± muscovite. Geochemically, these granites are S-types and can be readily distinguished from juxtaposed I-type granitoids of the Middle-Upper Jurassic Zamora, Abitagua, and Azafrán batholiths located immediately to the east. Intrusion of these S-type granites may be related to the breakup of western Gondwana.

GEOLOGIA DO GRÁBEN DE MARTINÓPOLE, ÁREA DE CAMPANÁRIO/PAULA PESSOA (GRANJA, CE): IMPLICAÇÕES NA EVOLUÇÃO LITOESTR ATI GRÁFICA E TECTONOMETAMÓRFICA DO NOROESTE DO CEARÁ

The Martinopole Graben is located in the Northwestern part of Ceara State. A reconnaissance geologic mapping in an area between Campanario and Paula Pessoa cities allowed the stablishement of a lithostratigraphic and tectonometamorphic evolution of this Graben (Table 1). Six major stratigraphic units have been recognized in the study area (from bottom to top): Gnaissico-Migmatitico Complex, Martinopole Group, Sao Jose Group, Casinha Metasediments, and Jaibaras Group. The Gnaissico-Migmatitico Complex underlies all other units and is characterized by an additional deformational phase (D n-1 ) which is not present on the Martinopole Graben. The Sao Joaquim Formation was excluded from the Martinopole Group due to its highly complex pattern of deformation and its different metamorphic grade (upper amphibolite with intermediate pressure facies potassium feldspar/sillimanite isograde and kyanite isograde). On the other hand within the Graben, the rocks have pointed out metamorphism in prehnite-pumpellyite facies(kaolinite + 2quartz X pyrophyllite + H 2 O) to green schist facies (chlorite/biotite isograde or on the upper part of the stability of pyrophyllite). The Martinopole Group has been defined as a volcanoclastic unit, also containing minor amounts of volcanic and siliciclastic rocks. This group remains with the two original formations: Covao (bottom) and Santa Terezinha (top). The Sao Jose Group, which is distinguished by very low-grade metamorphic claystones and sandstones has an uncertain stratigraphic relationship with the Martinopole Group; the informal unit named Casinha Metasediments consists of diamictites with a pre-foliate pebbly framework. It can be correlated with the Olho d'Agua do Caboclo Formation (as defined by Hackspacher et al. 1988). The Jaibaras Group filled small pullapart basins and comprised molassic clastic wedges, that are represented by Massape Formation conglomerates and by the fine clastic rocks of the Pacuja Formation, (both are Brazilian in age). All the units underwent several pulses of shear/faulting with different magnitudes, deformational mechanisms and age.

Grenvillian magmatism in the eastern Grenville Province, Canada

Abstract U-Pb age determinations on six newly identified examples of Grenvillian plutonism from eastern Labrador have yielded the following ages: Gilbert Bay granite 1132−6+7 Ma; Second Choice Lake pegmatite 1003±6 Ma; Southwest Pond granite 963±6 Ma; Chateau Pond granite 964±2 Ma; Riviere Bujeault headwaters quartz syenite 964±5 Ma; Upper St Lewis River (west) granite 956±1 Ma. In addition, eight new K-Ar and Rb-Sr hornblende and biotite ages ranging from 953 Ma to 811 Ma are reported for three of these plutons. In conjunction with three previous U-Pb determinations, it is concluded that the U-Pb dates reflect two periods of Grenvillian plutonism that occurred in separate areas of eastern Labrador, and which were also characterized by distinct emplacement styles. North of the Mealy Mountains terrane boundary (Lake Melville terrane and its border regions with the Mealy Mountains and Hawke River terranes) plutonism occurred between ∼1130 and 1080 Ma and consisted of sporadic, minor granitic intrusions. These intrusions had little structural effect on the host rocks and are inferred to have been emplaced at a high structural level, a model consistent with previous suggestions for tectonic stacking in the region. South of the Mealy Mountains terrane boundary (Mealy Mountains and Pinware terranes) plutonism was brief (966 to 956 Ma), but widespread. Typical magmatic products were circular (in plan) plutons, up to 20 km in diameter, having monzonite, quartz syenite and granite compositions. These plutons exerted marked structural influence on their host rocks and are interpreted to have been emplaced at intermediate structural levels. In a broader regional context, by utilizing a previously demonstrated correlation between positive magnetic anomalies and Grenvillian plutons together with reconnaissance geological mapping, a belt of Grenvillian plutons is inferred to exist across the southern half of the eastern Grenville Province. This zone of plutonism serves to emphasize a distinct difference between an exterior (northern) thrust belt and an interior (southern) magmatic belt. A similar bipartite division is apparent in other parts of the Grenville-Sveconorwegian Orogen. The mineral geochronological data probably do not reflect the cooling histories of individual plutons; alternative explanations include either a slow regional cooling event or the distal effects of younger plutonism/metamorphism to the south, as yet unidentified.

Reconnaissance geological mapping and mineral exploration in northern sudan using satellite remote sensing

In suitable terrain. Landsat and other satellite imagery can be used for the production of reconnaissance geological maps which contain a comprechensive range of lithological and structural information. The method described here is based on aerial photogeological intrpretation. This approach does not require extremely sophisticated image processing and is most effective wen supported by a liited, controlled field check. Maps produced in this way can be called ‘image-geological’ maps to distinguish them from (aerial) photogeological and conventional geological maps. Using this procedure image-geological maps have been completed at 1:250 000 scale of an area of 92 000 km2 of the Sudance Nubian Desert, based on interpreation of Landsat MSS and RBV imagery and one swath of SIR-A radar imagery as well as on a rapid field check. The work was undertaken to provide a basis for detailed planning and execution of a regional mineral exploration programme that is being conducted by the Egypt-Sudan Integration Project and was initially assisted by the UNDP. This paper describes the main geological and mineral exploration results and the interpretability of the satellite imagery. The complementary nature of methods of geological mapping based on satellite imagery, aerial photography and fieldwork is discussed briefly. It is shown that the absolute reliability and accuracy of each method is a less important issue than an understanding of thir different potential uses in a regional mineral explortion programme.

LUKANGA SWAM: PROBABLY ASTROBLEME

This article summarizes information relevant to the interpretation of Lukanga Swamp — a depression some 52 km in diameter in central Zambia — as a considerably eroded astrobleme. The results of reconnaissance geological mapping, an airborne magnetic survey and petrological study of a small set of samples of impact rocks are presented. Circular topographic features coincide with a remarkably subdued magnetic field. A circular set of tangential faults indicates a diameter of the outer rim approximately 110 km. Polymict breccia occur, carrying marble megaclasts (klippes) up to 50 m long. Strongly shocked quartz clasts with several sets of decorated planar features are present as well as devitrified, flow‐banded silica glass exhibiting decarbonation textures around marble clasts. Provisional correlation of the brecciation event with the local geological record suggests a Paleozoic age of the astrobleme.

Ophiolitic Rocks of Iditarod Quadrangle, West-Central Alaska: ABSTRACT

Reconnaissance geologic mapping during 1984 revealed previously unreported ultramafic rocks of probable ophiolitic origin in the northern End_Page 669------------------------------ part of the Iditarod quadrangle. The ultramafic rocks are poorly exposed on rounded tops of the low rolling hills in the area, and they form a northeast-southwest belt at least 8 mi long and as much as 3 mi wide. Although these rocks are associated with Mississippian to Jurassic(?) chert, tuff, argillite, and basalt of the Innoko terrane, the contact relations are uncertain. The ultramafic rocks consist of harzburgite, lherzolite, pyroxenite, and pods of altered dunite; disseminated chrome spinel is a common but minor constituent. Serpentinization is ubiquitous, all samples showing at least some effects and many being entirely serpentinized. Discontinuous pods of former serpentinized dunite(?) within lherzolite are entirely altered to a magnesite + talc + chrome spinel assemblage, indicative of low-grade metamorphism in the presence of water and carbon dioxide. The ultramafic rocks of Iditarod quadrangle are similar to and on trend with those of the Mount Hurst area, 25 mi to the northeast in the Ophir quadrangle. The Mount Hurst rocks represent part of a dismembered ophiolite, probably related to the Yukon-Koyukuk ophiolite belt. Although several of the classic ophiolite components are missing from the ultramafic sequence in Iditarod quadrangle, an ophiolite origin is strongly suggested by correlation with the Mount Hurst rocks and is supported by the presence of a structural slice of hypersthene gabbro in close proximity to the ultramafic sequence. End_of_Article - Last_Page 670------------