William D. MacDonald

The K–T boundary in Oman: identified using magnetic susceptibility field measurements with geochemical confirmation

Abstract Recognizing distal ejecta marker horizons and correlating these among widely separated sections is typically difficult in the absence of visually distinctive marker beds. Here we propose a magnetic susceptibility (MS) field method to locate such horizons, and explore the K–T boundary interval at Abat, Oman, as a test of the method. A distinctive pattern of high MS values was used to approximately locate the K–T boundary interval in a sequence of platform carbonates, which were then sampled in detail. Whole-rock geochemical enrichments in Ir, V, As, Ni, Co, Zn and Zr and a large negative carbon isotope anomaly confirmed the inferred boundary location. Common microspherules whose chemistry reflects the whole-rock geochemistry are associated with the boundary interval. The association suggests that the microspherules formed during or as a result of the impact. The geochemical record at the Abat locality shows two levels where Ir is high (>1 ppb) suggesting input from two impacts, separated in the section by 1.35 m. The upper level is chosen as the boundary interval based on the high Ir (1.19 ppb), δ13C negative excursion (∼7‰), and distinctive MS pattern.

Fault-mediated melt ascent in a Neoproterozoic continental flood basalt province, the Franklin sills, Victoria Island, Canada

The Neoproterozoic Franklin large igneous province on Victoria Island, Canada, is characterized by continental flood basalts and a sill-dominated feeder system. Field relationships indicate that fault-guided transfer zones allowed magma to jump up-section to form higher-level intrusions. Where sills connect to dikes and magmas moved up-section, roof and wall rocks are characterized by wide and intense contact-metamorphic haloes, consistent with throughflow of magma. The geometric constraints suggest that conduits may have opened episodically and then closed when magma pressure waned. The episodic nature of conduit opening events can explain the pulsed ascent of crystal slurries, and may also play a role in the deposition of Ni-sulfides.

Cretaceous-Early Tertiary Metamorphic K-Ar Age Values from the South Caribbean

Fourteen K-Ar age values from the Santa Marta schists (Colombia), of greenschist and amphibolite metamorphic facies, indicate two metamorphic episodes: mid-Cretaceous (or older) and Paleocene. Argon retentivity of hornblende appears to be little affected by a large Eocene diorite batholith, except near the contact. Whole-rock phyllitesamples from the same terrain give mid-Tertiary age values, suggesting considerable argon loss.

Magnetic, structural and geochronologic evidence bearing on volcanic sources and Oligocene Deformation of Ash Flow Tuffs, northeast Nevada

Magnetic properties of mid-Tertiary volcanic rocks west of Jiggs in northeast Nevada were investigated for the purposes of interpreting igneous, structural, and tectonic processes in this part of the Basin and Range province. Anisotropy of magnetic susceptibility (AMS) patterns reflect flow fabrics and suggest previously unknown sources for these ash flow tuffs. Paleomagnetic and structural evidence suggest counterclockwise relative rotation of the southern part of the area with respect to the north. New stratigraphic, chemical and K-Ar isotopic data support these interpretations. Dacite to rhyolite ash flow tuffs of the Indian Well Formation were subdivided into two main units: the lower and predominant tuff of Jiggs (35.8–37.0 Ma) and the unconformably overlying but areally restricted tuff of Hackwood (30.8 Ma). The Jiggs unit has two polarities whereas the Hackwood has only a reversed polarity remanence. Together these units with tilt correction define a pole (92°E, 87°N, dp = 6°, dm = 8°) for approximately 30–37 Ma. This pole is concordant with coeval North American reference poles, indicating that this zone of approximately 30 km NS extent has not undergone significant vertical axis rotation relative to the North American reference. Andesite lavas of normal magnetic polarity and of 32.5-Ma age characterize the Diamond Hills immediately to the south. We interpret this region, from both structural evidence and discordant paleomagnetic direction, to have rotated approximately 25° counterclockwise relative to the Indian Well volcanic units to the north. The apparent rotation of the Diamond Hills is possibly the result of drag on the left-lateral Garcia fault which limits the Diamond Hills on the southwest. Analysis of AMS data suggests, by patterns of the K1 axes, two distinct sources for the Jiggs unit: a northern buried source and a central partially buried source. Lithologic evidence consistent with proximal vent facies is found near the latter source. An unusual pattern in the AMS K3 axes, suggested as resulting from the tilt correction applied to initial dip, is consistent with the interpreted flow directions. Tight age constraints can be placed on the deformation of the main ash flows of the Jiggs unit, in the 35.8- to 30.8-Ma interval. Because the Diamond Hills unit is slightly younger and appears to have been affected by the same deformative episode as the Jiggs, the regional deformation affecting both units can be more narrowly confined to the 32.5–30.8 Ma interval. This deformation is inferred to coincide with the major early Tertiary episode of crustal extension in this region of the Basin and Range.

Regional Gravity Anomalies and Crustal Structure in Northern Colombia

The central range of the Colombian Andes gives way northward to a series of Cenozoic fault-bordered basins and uplifts near the Caribbean Sea. Pre-Cenozoic structures exposed in the uplifts curve increasingly toward the east to become parallel to the continental margins along the south side of the Caribbean. Major Cenozoic faults, with large vertical and horizontal displacements, cut across older structures which include Permian-Triassic(?) and Late Cretaceous to early Tertiary metamorphic zones, Precambrian gneiss, and Jurassic batholiths. Gravity anomalies have large amplitudes in the Santa Marta area. Bouguer anomalies rise to +130 mgals over the crystalline rocks of the high Santa Marta massif. Over adjacent Cenozoic basins, they range down to −80 mgals over the Lower Magdalena basin and to −65 mgals over the Baja Guajira basin. Steep gravity gradients characterize the Santa Marta and Oca faults on the west and north sides of the massif, respectively. In the Guajira Peninsula region, Bouguer anomalies increase to +105 mgals over a serpentinite zone at Cabo de la Vela and to +55 mgals over Cretaceous volcanic rocks in the southern peninsula. Two smaller basins on the peninsula are characterized by negative Bouguer anomalies. Steep gradients characterize many of the major Cenozoic faults, and two concealed faults are postulated on this basis. Though useful for evaluating the relative vertical displacements, which may exceed 10 km along faults bounding the Santa Marta massif, the gravity data yield no definitive information on the large horizontal displacements postulated along some of the major faults of the area. The Bouguer anomalies do indicate, however, the extension of some of the Cenozoic basins into offshore areas. Strong positive Bouguer anomalies of the Santa Marta massif and its great relief, which exceeds 9 km relative to the floor of the adjacent Caribbean, indicate thin continental crust, lack of isostatic balance, and relatively recent uplift for the massif. After corrections are made for the gravitational effects of Tertiary sedimentary basins in the Guajira Peninsula, most of the peninsular region also has positive anomalies, suggesting a relatively thin continental crust and a lack of isostatic balance. A mechanism of overthrusting, in relatively recent time, of the continental margin over the adjacent Caribbean upper mantle and crust to the northwest can account for the observations.

Composite magnetic fabrics and S-C structure in granitic gneiss of Cerro de los Viejos, La Pampa province, Argentina

The Cerro los Viejos gneiss, in La Pampa Province, Argentina, is a mylonitized granite which lies along the late Paleozoic deformed boundary zone between the Patagonia block and ‘mainland’ Gondwana to the north. Petrofabric and anisotropy of magnetic susceptibility (AMS) analyses were compared for this body. Unexpectedly, it was found that the Kmin poles deviate from the poles of the main foliation s1 by about 25°. Similarly, but not so noticeably, the Kmax poles trend NE/SW, with a near-horizontal plunge, whereas the principal lineation l1 plunges gently SW. This unusual situation is attributed to the existence of superimposed fabrics arising from S–C structures. Examination of oriented cores in a modified universal stage and of oriented thin-sections revealed that the Kmin poles are clearly associated with a younger but less obvious metamorphic ‘C’ foliation (s2). Biotite and magnetite, the main contributors to the AMS response, are distributed along both s1 and s2. The dominance of s2 in the AMS anisotropy suggests that magnetite is preferentially distributed on s2.

Paleomagnetism of Late Cretaceous Poços de Caldas alkaline complex, Southern Brazil

Abstract The paleomagnetism of the Late Cretaceous Pocos de Caldas alkaline complex (46.6°W, 21.9°S) was investigated through 42 oriented cores from seven sites. Six sites, reversed relative to the present magnetic field of the Earth, yield a pole at 127°W, 82°S ( dp = 8°,dm = 13° ). This pole is located close to other Late Cretaceous poles for South America obtained by Creer [1] from untreated paleomagnetic samples. The results are significantly different from those for the nearby Early Cretaceous Serra Geral basalt but close to the Triassic pole for South America. The polar wandering path for South America for the Mesozoic seems to be more complicated than anticipated. The available paleomagnetic information may not yet be precise enough to determine the time of opening of the Atlantic.

Triassic Paleomagnetism of Northern South America

Reconnaissance sampling of three radiometrically dated Late Triassic igneous units in northern South America provides new data on Triassic paleomagnetism and a new estimate of the position of the Triassic pole for South America. The new poles compare favorably with other Triassic poles from South America. Poles in Venezuela for Guacamayas Group (195 m.y.) and Bolivar dikes (199 m.y.), combined with the mean pole for the Las Cabras lavas (204 m.y.) of Argentina, give a mean pole for the South America Triassic at 72°S, 91°W, ^agr95 = 15°. Bullard et al. showed that isobaths of the continental margins of Africa and South America have a minimum misfit for a rotation of 57.6° about a pole at 42.5°N, 29.9°W (500 fm isobath). Triassic paleomagnetic poles for Africa, when rotated about that same best-fit pole, merge with the Triassic poles for South America. A mean-composite pole for African and South American poles together is at 72°S, 106°W, ^agr95 = 10°, N = 5. A dipole geomagnetic field is indicated over the South American and African landmasses during Triassic time.

Precambrian Gneisses from Northern Colombia, South America

This note summarizes the geologic occurrence and age measurements of high-grade metamorphic rocks from northern Colombia. These rocks crop out on the northern flank of the Sierra Nevada de Santa Marta near the southern shore of the Caribbean Sea. The Rb-Sr ages are in the range 1400 ± m.y., confirming the geological deductions of Gansser (1955) on their Precambrian age. As part of an uplifted remnant of the Guayana Shield, these gneisses suggest a great northwestern extension of the Precambrian shield beneath the younger sedimentary rocks of the low-lying llanos (plains) into the folded and uplifted Andes of Venezuela and Colombia. These Precambrian rocks lie close to the probable northwestern limit of the South American Precambrian cratonic basement.

Magnetic fabric of peridotite with intersecting petrofabric surfaces, Tinaquillo, Venezuela

Abstract Two intersecting structural surfaces of different ages and origins are characteristic of the Tinaquillo (Venezuela) peridotite. An older penetrative mylonitic foliation is cut by younger serpentine veinlets. The anisotropy of magnetic susceptibility (AMS) is influenced by both petrofabric components. The dominant fabric surface from the AMS perspective is defined by serpentine veinlets, probably because of the abundant secondary magnetite associated with them. The minimum AMS axis ( K 3 ) is typically orthogonal to the plane of the dominant veinlet set, and the maximum ( K 1 ) and intermediate ( K 3 ) axes lie in the veinlet plane. The mylonitic foliation surface also influences the magnetic fabric, so that the K 1 axis lies parallel to the line of intersection between the mylonitic foliation and the veinlet plane. A macroscopically prominent mineral lineation, defined by stretched enstatite laths in the foliation surface, shows no AMS correlation. Remanent magnetization directions are multicomponent and show much scatter, probably due to the deformation and complex magnetization history of the rock.