Discontinuous Rim Research Articles

Charakteristika měděného slitku ze zlatého dolu na Zlatém vrchu v Mníšku pod Brdy (střední Čechy, Česká republika)

A copper pig with dimensions 3.3 × 2.3 × 1.7 cm and weighing 25 g was found together with few iron artefacts (several mining chisels, several fragments of mining hoes, a hook) in dump material of ancient gold mine at the Zlatý vrch Hill near Mníšek pod Brdy town, central Bohemia, Czech Republic. The gold mines were in operation in the surrounding area in the 15-16th centuries with some later attempts in 18th century. A material characterization of the copper artefact, based on reflected light microscopy and electron microprobe study, is presented in this paper. The composition can be roughly approximated as an As-poor Cu-As alloy, with predominating matrix composed of low-As copper and most impurities being concentrated in intergranular spaces of low-As copper grains, where domeykite and inclusions of native lead were identified. The copper matrix contains 1.54 - 2.23 wt. % As, and low Sb and Ni. Strongly substituted domeykite (Cu3As) contains high Sb (9.3 - 15.6 wt. %) and lesser amounts of Pb, Ni, S, Ag, Sn, and Fe. Metallic lead is characterized by elevated contents of Bi, Ag, Sb, Cl, and S. In addition, a discontinuous rim with composition close to cuprite (Cu2O), locally containing inclusions of a Sb-As oxide, was identified on the surface of the copper pig. The element assemblage observed in the studied metallic artefact may indicate the source of the copper ore in the neighbouring base-metal (Ag-Pb-Zn) Příbram ore area, however, there are not known indications of historical smelting of sporadically occurring copper ores in the Příbram area. Therefore, we suggest that the studied artefact likely has more distal provenance.

Genesis of greenockite (CdS) and associated sulphide‐gold mineralization from Mahakoshal belt, Central Indian Tectonic Zone

The Zn‐Cd‐S critical minerals system is a significant mineralogical indicator of hydrothermal ore‐forming processes. The understanding of greenockite (CdS) formation in natural mineral systems is very restricted due to low crustal abundances (Cd <0.2 ppm). We report the first occurrence of greenockite from the Bagada orogenic gold prospect, Paleoproterozoic Mahakoshal belt, Central Indian Tectonic Zone (CITZ). This provides an ideal opportunity to access the occurrence and its genesis. Greenockite occurs in discrete sheared quartz veins hosted in phyllites. Sulphide mineralization is closely associated with hydrothermal chlorite alterations. Pyrrhotite, arsenopyrite, pyrite, and chalcopyrite assemblages are formed at high temperatures, while sphalerite and galena precipitated when the temperature decreased. Greenockite, native silver and bismuth with gold precipitated during the breakdown of arsenopyrite to scorodite in progressively oxidizing conditions. Greenockite precipitation occurred in a transforming hydrothermal environment from a reducing to an oxidizing one during late‐stage hydrothermal alteration. Sphalerite and chlorite thermometry yielded temperatures in the range from 266 to 366°C for this late‐stage hydrothermal alteration. Textural relations show two modes of occurrence of greenockite, one (Gck‐1) as exsolution within sphalerite (Sp‐2) and the other (Gck‐2) occurring as a discontinuous rim replacing the Cd‐rich sphalerite (Sp‐3). The Gck‐1 contains high Zn and low Cd, while as compared to Gck‐2. Zinc and Cd in greenockite show diadochy relation (Zn2+ ↔ Cd2+). Laser ablation ICPMS analyses of sphalerite (Sp‐2) reveal remarkably high Cd contents (1.9–12 wt%). Cd >0.02 ppm suggests possible Cd derivation from sphalerite (Sp‐2) under variable fS2, T, pH, fO2 conditions, fluid–rock interaction, and change in the Zn/Cd ratio. Sphalerite compositions display a low Zn/Cd (av.22) ratio and high temperature of formation, with characteristics intermediate between volcanogenic massive sulphides (VMS) and magmatic‐hydrothermal Cu‐Pb‐Zn system.

Elemental and Nd isotopic compositions of zoned titanite in mafic microgranular enclaves of the Early Cretaceous Sanguliu granitic pluton in the North China Craton: Insights into magma mixing process

Widespread Mesozoic granitic plutons in the North China Craton (NCC) are products of remelting of the crust associated with large-scale Mesozoic destruction of the NCC. Mafic microgranular enclaves (MMEs) are common in these plutons and are attributed to magma mixing. However, the involved magmatic processes of the mixing are poorly known. In this paper, we reported complex zoning patterns, in situ U–Pb ages, and elemental and Nd isotopic compositions of titanite grains in the MMEs and host monzogranite of the Sanguliu pluton in the NCC. Titanite grains in the MMEs and monzogranite have similar U–Pb ages of ca.130 Ma, and display different internal textures and compositions. They can thus be divided into four types, i.e., types 1 and 2 from the host monzogranite, and types 3 and 4 from the MMEs. Type 1 is euhedral in shape, whereas type 2 has a euhedral inner domain and a thin deuteric rim with a sharp contact between them. Types 3 and 4 are angular grains. Type 3 shows a typical core-mantle-rim texture with a resorbed core, a mantle and an irregular rim, whereas type 4 contains a homogeneous inner domain and a discontinuous rim. Most titanite grains of the four types display oscillatory, fir-tree and sector zoning and have crystallization temperatures ranging from 680 to 750 °C based on the Zr-in-titanite thermometry, indicating a magmatic origin. The core of type 3 titanite has the highest εNd(t) of −11.3 to −12.5 among others, close to that of the mafic dykes (εNd(t) = −7.8 to −11.6) that intrude the Sanguliu pluton. The core can be interpreted as crystallized from the mixed magmas with more mafic components that were derived from metasomatized subcontinental lithospheric mantle (SCLM) beneath the NCC. The mantle of type 3 and the inner domain of type 4 titanite have εNd(t) (−13.0 to −15.3) nearly identical to that of the MMEs (−14.3 to −15.4), but distinctly higher than that of types 1 and 2 titanite (−15.9 to −18.3). The rims of types 3 and 4 titanite have εNd(t) (−16.1 to −18.3), identical to that of type 1 and the inner domain of type 2 titanite, however, they have REE concentrations and Th/U and Nb/Ta similar to the rim of type 2 titanite, clearly indicative of crystallization from evolved, hydrated, granitic magmas. A three-stage growth model is thus proposed to explain the core-mantle-rim texture of the titanite in the MMEs by a magma mixing process. The core likely crystallized from the mixed magma with more mafic components. It was then partially or totally resorbed by mixed, dioritic magma due to chemical disequilibrium, which was followed by the re-precipitation from the dioritic magma, forming the mantle. The rim is an overgrowth from the evolved, hydrated, granitic magma. This study demonstrates that the complex zoning patterns and compositions of titanite in the MMEs can be used to investigate magmatic processes including magma mixing.

The Djilouet granite suite (Djanet terrane, eastern Hoggar, Algeria): petrography, mineralogy, geochemistry, and relations with quartz-cassiterite-wolframite vein systems

The Djilouet complex forms a cupola made up of leucocratic granites associated with Sn-W mineralization. It could represent the Hoggar’s easternmost rare metal granite (RMG) comparatively to those of the Taourirt province in the central Hoggar. It is located in the Djanet terrane, 12 km NE of the town of the same name in the far east of the Tuareg shield in Algeria. The Djanet terrane is made of a thick low-grade (greenschist facies) sedimentary sequence which was intruded by several generations of granitic rocks. The subcircular Djilouet body is made of leucocratic granites with progressive mutual transitions. Most of the outcrop is occupied by a porphyritic coarse-grained biotite granite. A muscovite granite is found in the center of the cupola, whereas a garnet (almandine—spessartite) granite forms a discontinuous rim all around it. Black micas from the biotite granite are lithian annite (“protolithionite”). The white micas from the muscovite granite and the garnet granite are classified as Fe-Li muscovite. The muscovite granite and the garnet granite contain accessory minerals as rutile, xenotime, monazite, zircon, and minerals that may be members of the pyrochlore supergroup. All the facies are leucogranite with high SiO2 content and high K2O-Na2O ratio. They are poor in calcium and in mafic components and have very low phosphorus content (P2O5 ~ 0.1%). The peraluminous composition is low to mild (1.08 1.46). The evolved granites of the Djilouet suite are enriched in Th and U, but the tantalum and the niobium are not highly concentrated (Ta + Nb = 10.4–17.1). The total REE content is low (57–84 ppm), and the patterns are typically wing shaped with a strong negative Eu anomaly and a small tetrad effect. The Sn-W mineralization consists two systems of veins: large quartz veins with ferberite (H/F ~ 0.8) and quartz veinlets or stockworks with homogeneous cassiterite and minor wolframite (H/F ~ 48). The iso-content contours of tin and tungsten, as produced from a sampling covering the whole cupola, overlap very little. The differences which were noted throughout the study between the Taourirt granites and those of the Djilouet suite are to be related to the lithological nature of the crust rather than to a difference in the geodynamic environment.

A giant spleen with multiple cysts: a rare case of isolated splenic hemangiomatosis.

A 63 year old female presented with the complaint of lump in left upper quadrant of abdomen since two years which had gradually increased in size reaching up to umbilicus. It was associated with dull aching abdominal pain. Her vitals and initial laboratory investigations were in normal range. Her family history was insignificant. Contrast enhanced computed tomography (CECT) of abdomen revealed a grossly enlarged spleen with parenchyma greatly replaced by cysts of variable sizes, largest one measuring 72mm x 69mm x 69mm. Few cysts had peripheral discontinuous rim like calcifications (A,B). Splenectomy done via subcostal incision revealed a spleen weighing 1800 gram and measuring 28cm x 16cm x 9cm with irregular surface showing numerous cysts filled with serous and serosanguinous fluid (C,D). Normal architecture of spleen was replaced almost completely by cysts filled with proteinaceous fluid containing red blood cells. There was dystrophic calcification in the periphery of the cysts. The endothelial cells were diffusely positive for CD34 and factor VIII, focally positive for CD31 but not for CD8. The diagnosis of splenic hemangiomatosis was made. Splenic hemangiomatosis is extremely rare with unknown incidence and is most frequently seen in cases of diffuse angiomatosis. The imaging characteristics of splenic hemangiomas are non-specific. The differential for a cystic lesion of the spleen includes lymphoma, metastases, abscess, cyst, lymphangioma and hematoma. Due to poor specificity of imaging and the risks associated with percutaneous biopsy of the spleen, splenectomy is performed when definitive characterization of splenic lesions is needed.

The Geomorphology of Submarine Channel Systems of the Northern Line Islands Ridge, Central Equatorial Pacific Ocean

More than 844,000 km² of the northern Line Islands Ridge mapped with multibeam bathymetry and backscatter provide unprecedented views of the geomorphology of this isolated area in the central equatorial Pacific Ocean. A compilation of all available multibeam data in the area reveals six extensive submarine dendritic channel systems that encompass a combined drainage area that exceeds 60,000 km². The channel systems occur in a predominately carbonate environment and are the longest calciclastic submarine channel systems mapped in the oceans to date. The channel systems occur in a carbonate-dominated region well above the carbonate compensation depth and have developed into the surface of basins that are surrounded by small guyots and seamounts that make up a discontinuous rim around the summit of the northern Line Island Ridge. The channels have mostly straight or gently curved well-developed tributaries and main reaches. Although the Line Island Ridge has been dated at 86 to 68 Ma old, the channels occur on the surface and are not buried by any significant sediment accumulations. Levees are very rare along the channel banks and no bathymetric expression of submarine fans was found where the channels exit onto the adjacent abyssal basins. There is sparse evidence of landslide deposits throughout the ridge although the flanks of the guyots exhibit numerous headwall scarps. The presence of plunge pools below the northwest escarpment, together with well-defined channels meters to hundreds of meters deep relative to the surrounding seafloor, suggests the channels might be relatively recent (perhaps late Neogene or even younger) features developed long after the ridge subsided more than a kilometer below sea level.

Magma Mixing Genesis of the Mafic Enclaves in the Qingshanbao Complex of Longshou Mountain, China: Evidence from Petrology, Geochemistry, and Zircon Chronology

The Qingshanbao complex, part of the uranium metallogenic belt of the Longshou-Qilian mountains, is located in the center of the Longshou Mountain next to the Jiling complex that hosts a number of U deposits. However, little research has been conducted in this area. In order to investigate the origin and formation of mafic enclaves observed in the Qingshanbao body and the implications for magmatic-tectonic dynamics, we systematically studied the mineralogy, petrography, and geochemistry of these enclaves. Our results showed that the enclaves contain plagioclase enwrapped by early dark minerals. These enclaves also showed round quartz crystals and acicular apatite in association with the plagioclase. Electron probe analyses showed that the plagioclase in the host rocks (such as K-feldspar granite, adamellite, granodiorite, etc.) show normal zoning, while the plagioclase in the mafic enclaves has a discontinuous rim composition and shows instances of reverse zoning. Major elemental geochemistry revealed that the mafic enclaves belong to the calc-alkaline rocks that are rich in titanium, iron, aluminum, and depleted in silica, while the host rocks are calc-alkaline to alkaline rocks with enrichment in silica. On Harker diagrams, SiO2 contents are negatively correlated with all major oxides but K2O. Both the mafic enclaves and host rock are rich in large ion lithophile elements such as Rb and K, as well as elements such as La, Nd, and Sm, and relatively poor in high field strength elements such as Nb, Ta, P, Ti, and U. Element ratios of Nb/La, Rb/Sr, and Nb/Ta indicate that the mafic enclaves were formed by the mixing of mafic and felsic magma. In terms of rare earth elements, both the mafic enclaves and the host rock show right-inclined trends with similar weak to medium degrees of negative Eu anomaly and with no obvious Ce anomaly. Zircon LA-ICP-MS (Laser ablation inductively coupled plasma mass spectrometry) U-Pb concordant ages of the mafic enclaves and host rock were determined to be 431.8 5.2 Ma (MSWD (mean standard weighted deviation)= 1.5, n = 14) and 432.8 4.2 Ma (MSWD = 1.7, n = 16), respectively, consistent with that for the zircon U-Pb ages of the granite and medium-coarse grained K-feldspar granites of the Qingshanbao complex. The estimated ages coincide with the timing of the late Caledonian collision of the Alashan Block. This comprehensive analysis allowed us to conclude that the mafic enclaves in the Qingshanbao complex were formed by the mixing of crust-mantle magma with mantle-derived magma due to underplating, which caused partial melting of the ancient basement crust during the collisional orogenesis between the Alashan Block and Qilian rock mass in the early Silurian Period.

Radiology Estimates of Viable Tumor Percentage in Hepatocellular Carcinoma Ablation Cavities Correlate Poorly With Pathology Assessment

No study has evaluated radiology/pathology correlation of percentage viable tumor (PVT) estimates in ablated hepatocellular carcinoma (HCC) to examine the reliability of radiologic estimates. To determine how well interdisciplinary PVT estimates correlate and identify pathologic factors that influence this correlation. Pathologists and radiologists established blinded PVT estimates in 22 HCC ablation cavities. Paired sample t tests examined the differences between the interdisciplinary estimates. Fifteen cavities had pathologic viable tumor (VT) (68%) and 6 had radiographic VT (22%). Radiology's sensitivity for detecting VT was 40% and the specificity was 100%. Pathology detected significantly more VT than radiology (pathology mean = 22.3% versus radiology mean = 2.6%; P = .005). Five cavities had tumor growth in a discontinuous rim pattern, 7 in a nodular pattern, and 3 in a solid pattern. Radiology did not detect VT in cavities with a discontinuous rim pattern (sensitivity = 0%) but did detect VT in 3 cavities with a nodular pattern (sensitivity = 43%), and in all cavities with a solid pattern (sensitivity = 100%). There was no significant difference in PVT estimates in cavities 3.5 cm or larger (P = .07), but there was a significant difference in cavities smaller than 3.5 cm (P = .01). This study clarifies that the risk of underestimation by imaging is greatest in small lesions (<3.5 cm), though the sensitivity of detection depends primarily on the tumor growth pattern within the cavity. This underestimation raises the question of whether basing treatment decisions on a radiologic impression of complete ablation is valid.

Distribution of Basal/Myoepithelial Markers in Benign and Malignant Bronchioloalveolar Proliferations of the Lung

We investigated the staining pattern of commonly used basal cell/myoepithelial markers, such as p63 (a p53-homologous nuclear protein), basal cell-specific cytokeratin antibody (34betaE12, K903), and smooth muscle myosin heavy chain (SMMHC) in benign and malignant bronchioloalveolar proliferations of the lung. We studied 85 lung lesions consisting of 35 bronchioloalveolar carcinoma, 30 well-differentiated adenocarcinoma, and 20 cases of benign lung lesions. In normal lung, p63, K903, and SMMHC decorated the basal cells of large and small airways and occasional cells of terminal bronchioles. In reactive processes, a distinctive staining pattern was present in 19/20 (95%) of the cases characterized by staining of basal cells of the airways and bronchiolar epithelium and squamous metaplastic epithelium for p63 and K903, whereas 12/20 (60%) stained with SMMHC. Respiratory ciliated cells, alveolar epithelial cells, and nonepithelial cells were negative. In bronchioloalveolar carcinoma, a discontinuous peripheral rim of p63-immunoreactive cells was retained surrounding and intermingled with the malignant bronchioloalveolar proliferation in 31/35 (88.5%) cases, SMMHC in 28/35 (80%) cases, and K903 in 20/35 (57%) cases. For adenocarcinoma, a majority of the cases (28/30, 93%) were negative for p63 and K903; however, SMMHC showed artifactual staining in the desmoplastic stroma in 6/30 (20%) cases. Our results highlighted the differential expression of basal cell markers across various bronchioloalveolar lesions. The staining pattern of basal cells in bronchioloalveolar carcinoma supports that these neoplasms may actually be carcinoma in-situ.

Geochronologic, geochemical and isotopic constraints on the origin of monzonitic and related rocks in the Laramie anorthosite complex, Wyoming, USA

We present new U-Pb zircon ages for monzonitic rocks of the 1.43 Ga Laramie anorthosite complex, a major Proterozoic anorthosite plutonic suite in southeastern Wyoming, USA, associated with contemporaneous reduced rapakivi-type anorogenic granites of the Sherman batholith. Our results constrain (1) the geochemical evolution of Fe-enriched residual liquids to anorthosite, (2) the duration and cooling history of mid-Proterozoic magmatism in the region, and (3) the composition and structure of the crust below the exposed plutonic rocks. Monzonitic intrusions and dikes, ranging in composition from monzodiorite to quartz monzonite, account for nearly one-quarter of the Laramie anorthosite complex, forming a discontinuous rim around the anorthositic intrusions. All of the monzonitic rocks crystallized at high temperatures (>950°C) and under relatively reducing conditions (fO2 1000°C was relatively rapid (20-25°C per million years) during the initial 30 million years following emplacement and slowed considerably once ambient temperatures of about 350°C were reached. Each of the monzonitic intrusions and all of the monzodioritic dikes within the Laramie anorthosite complex are characterized by a wide range of initial 87Sr/86Sr and limited variation of initial eNd, consistent with simple binary mixing between anorthosite residual liquids and Archean crust. The ages and isotopic compositions of most of the monzonitic and related rocks indicate that their ferrodioritic parents were extracted from anorthosite at depth and that they were contaminated during ascent through Archean crust prior to emplacement within and along the margins of the currently exposed anorthosites in the Laramie Mountains. The combined geochronological and geochemical results of this study demonstrate that Proterozoic anorthosite plutonic suites are characterized by extensive, relatively long-lived (∼10 million years) conduit systems, with intrusions distributed at different depths throughout the crust, which are linked and cut by dikes derived from deeper crustal levels. © 2003 Elsevier Science B.V. All rights reserved.

CORNEA CASE STUDY: LESSONS FORTHE FUTURE

The Cornea Field was discovered in January 1997. The Cornea structure represents a relatively simple trap configuration comprising a large, elongated, four-way dip closure, formed by the drape of Albian age sediments over a northeast to southwest trending positive basement feature. It is located on an active hydrocarbon migration fairway with proven charge and is capped by a regionally extensive Albian seal. An associated Direct Hydrocarbon Indicator extends over an area of 50 km2 and this feature coincides with gas-fluid contacts throughout the field.Subsequent to the initial Cornea discovery wells in permit WA–241-P, further exploration drilling took place within WA–241-P, and also within two, newly-acquired, adjacent permits, WA–265-P and WA–266-P, during 1998 and 1999. Detailed field assessment during and subsequent to these wells, involving geological, geophysical and reservoir engineering disciplines, indicated that the field consisted of an extensive gas cap over a thin, laterally discontinuous oil rim. The poor quality of the Lower Cretaceous marine reservoir succession and the extremely low expected oil recovery indicated that recoverable volumes would be low.This paper contains a review of the data and studies that led to a full understanding of the field. Key learning points from the case study are introduced here and may help future exploration campaigns in other areas.

Are Outliers of the Huronian Supergroup Preserved in Structures Associated With the Collapse of the Sudbury Impact Crater?

Several outliers of the Paleoproterozoic Huronian Supergroup are preserved in a 20 km wide arcuate belt northwest of the Sudbury Igneous Complex. Forty years ago it was suggested that these outliers represented the remnants of a discontinuous rim syncline developed around a domal structure. Thirteen years later it was proposed that they were preserved in grabens formed as a result of collapse of the central uplift of a multi‐ring impact crater and the simultaneous emplacement of the Sudbury Igneous Complex. This interpretation has been accepted by most investigators without question, to the extent that the position of the outliers has been used to estimate the size of the supposed crater. Our observations indicate that Huronian strata in the outliers were deposited in a series of paleo‐valleys on a terrane with considerable topographic relief. The asymmetric nature of the fill in some of the outliers suggests deposition may have taken place in half‐grabens. The outliers are preserved in synclines developed by orogenic activity prior to the 1850 Ma impact event. They are cut by later transverse and longitudinal faults. The geometry of the outliers, combined with the character and directional attributes of the sedimentary fill, indicates that the outliers are not related to the Sudbury Event.

Differential response of myofibrillar and cytoskeletal proteins in cells treated with phorbol myristate acetate.

Muscle-specific and nonmuscle contractile protein isoforms responded in opposite ways to 12-o-tetradecanoyl phorbol-13-acetate (TPA). Loss of Z band density was observed in day-4-5 cultured chick myotubes after 2 h in the phorbol ester, TPA. By 5-10 h, most I-Z-I complexes were selectively deleted from the myofibril, although the A bands remained intact and longitudinally aligned. The deletion of I-Z-I complexes was inversely related to the appearance of numerous cortical, alpha-actinin containing bodies (CABs), transitory structures approximately 3.0 microns in diameter. Each CAB consisted of a filamentous core that costained with antibodies to alpha-actin and sarcomeric alpha-actinin. In turn each CAB was encaged by a discontinuous rim that costained with antibodies to vinculin and talin. Vimentin and desmin intermediate filaments and most cell organelles were excluded from the membrane-free CABs. These curious bodies disappeared over the next 10 h so that in 30-h myosacs all alpha-actin and sarcomeric alpha-actinin structures had been eliminated. On the other hand vinculin and talin adhesion plaques remained prominent even in 72-h myosacs. Disruption of the A bands was first initiated after 15-20 h in TPA (e.g., 15-20-h myosacs). Thick filaments of apparently normal length and structure were progressively released from A segments, and by 40 h all A bands had been broken down into enormous numbers of randomly dispersed, but still intact single thick filaments. This breakdown correlated with the formation of amorphous cytoplasmic aggregates which invariably colocalized antibodies to myosin heavy chain, MLC 1-3, myomesin, and C protein. Complete elimination of all immunoreactive thick filament proteins required 60-72 h of TPA exposure. The elimination of the thick filament-associated proteins did not involve the participation of vinculin or talin. In contrast to its effects on myofibrils, TPA did not induce the disassembly of the contractile proteins in stress fibers and microfilaments either in myosacs or in fibroblastic cells. Similarly, TPA, which rapidly induces the translocation of vinculin and talin to ectopic sites in many types of immortalized cells, had no gross effect on the adhesion plaques of myosacs, primary fibroblastic cells, or presumptive myoblasts. Clearly, the response to TPA of contractile protein and some cytoskeletal isoforms not only varies among phenotypes, but even within the domains of a given myotube the myofibrils respond one way, the stress fibers/microfilaments another.

Tracks and Substrate Reworking by Terrestrial Vertebrates in Quaternary Sediments of Kenya

ABSTRACT Reworking of sedimentary substrates by terrestrial vertebrates, especially hoofed herbivores, has stratigraphic significance comparable to the bioturbation of marine sediments by benthic invertebrates. Environmental analysis of the Pliocene-Pleistocene Koobi Fora Formation in northern Kenya reveals many vertebrate footprints and trackways in fluvial and lake-margin strata. Some beds are completely reworked by trampling of many animals, presumably ungulates, with subsequent disarrangement of primary grain fabric and sedimentary structures. Examination of tracks and game trails in similar modern Kenyan environments, and comparison with those in older sediments, indicate characteristics useful for their recognition elsewhere. Preservation is best in mud and sand interbeds of medium thickness where the animal foot punches out a plug of coherent surface sediment (usually mud) and presses it into underlying units of contrasting lithology (usually sand). Thicker and less coherent muds simply mold the foot. In both situations the track is flat to concave upward with a discontinuous rim that surrounds a low spot where later wind- or water-laid sediments and bone fragments may concentrate. Trampling of coherent surface mud disturbs the ground surface allowing wind and water to remove the loose sand below. thereby creating shallow erosional depressions on the landscape. Heavy trampling in wet interbeds of sand and mud homogenizes the previously distinct layers into a thicker, more massive unit, typically without any obvious tracks preserved. Although we have identified individual tracks of hippo, antelope, and bird (and a seven print trackway of an early hominid), exact taxonomic assignments are not easily made.

Tracks and Substrate Reworking by Terrestrial Vertebrates in Quaternary Sediments of Kenya: ABSTRACT

Reworking of sedimentary substrates by terrestrial vertebrates, especially hoofed herbivores, has stratigraphic significance comparable to that of marine sediments by benthic invertebrates. Environmental analysis of the Pliocene-Pleistocene Koobi Fora Formation in northern Kenya reveals many vertebrate footprints and trackways in fluvial and lake-margin strata. Some beds are completely reworked by trampling of many animals, presumably ungulates, with subsequent disarrangement of primary grain fabric and sedimentary structures. Examination of footprints and game trails in similar modern Kenyan environments, and comparison with those in older sediments, indicate characteristics useful for their recognition elsewhere. Preservation is best in mud and sand interbeds of medium hickness where the animal foot punches out a plug of coherent surface sediment (usually mud) and presses it into underlying units of contrasting lithology (usually sand). Thicker and less coherent muds simply mold the foot. In both situations the print is flat to concave upward with a discontinuous rim that surrounds a low spot where later wind- or water-laid sediments and bone fragments may concentrate. Further trampling of coherent surface mud disturbs the ground surface allowing wind and water to remove the End_Page 485------------------------------ loose sand below, thereby creating shallow erosional depressions on the landscape. Heavy trampling in wet interbeds of sand and mud homogenizes the previously distinct layers into a thicker, more massive unit, typically without any obvious tracks preserved. Although we have identified individual prints of hippo and antelope--and a four-print trackway of Homo erectus--exact taxonomic assignments are not yet easily made. End_of_Article - Last_Page 486------------

A discussion concerning the floor of the northwest Indian Ocean - Marine sediments and bottom communities of the Seychelles

The shoal water region of the Seychelles Bank, covering about 31000 km 2 , is floored by relatively thin detrital carbonate sediments. Around the granitic and coral islands at least four submarine platforms can be recognized and are tentatively related to Late-glacial and Postglacial sea levels. A shallow discontinuous rim is developed around the margin of the Bank. The sediments and bottom communities of the open Bank and of the reef flats are described in terms of nine principal environments. Sedimentary facies are distinguished by the proportions of different organic constituents, by variations in grain size and by presence or absence of quartz. Syngenetic pyrite and collophane occur locally as minor constituents. Away from the reef flats the Bank supports a fairly uniform bottom community, the main elements being mollusca and foraminifera. The reefs may be considered as large intertidal pools with many microenvironments and with a complex zonation of communities showing rapid lateral changes in composition. There is a close relation between the sedimentary facies of the reefs and the distribution of the bottom communities.