African Landmass Research Articles

Predictability of Indian Monsoon Circulation with High Resolution ECMWF Model in the Perspective of Tropical Forecast During the Tropical Convection Year 2008

To address some of the issues of project Year of Tropical Convection (YOTC) and the project ATHENA as ongoing international activities, an endeavor has been made for the first time to study the predictability of Indian summer monsoon in the backdrop of tropical predictability using 850 hPa atmospheric circulations with the high resolution (T1279) ECMWF model during the boreal summer of 2008 as one of the focus years of YOTC. The major findings obtained from the statistical forecast have been substantiated by the dynamical prediction in terms of the systematic error energy, its growth rate and the attribution of the dominant nonlinear dynamical processes to error growth. The systematic error energy of T1279 (16 km resolution) ECMWF model are generated in African landmass, India and its adjoining oceanic region, in near equatorial west Pacific and around the Madagascar region where the root mean square errors are observed and the zonal wind anomaly shows poor forecast skill. As far as the inadequate predictability of Indian summer monsoon by T1279 ECMWF model (revealed from the results of project ATHENA) is concerned, the systematic error energy and the error growth over Arabian Sea, in the eastern and western India due to the nonlinear convergence and divergence of error flux along with the erroneous Mascarene high may possibly be the determining factors for not showing any discernable improvement in Indian monsoon during the medium range forecast up to 240 h. This work suggests that the higher resolution of ECMWF model may not necessarily lead to the better forecast of Indian monsoon circulations during 2008 unless a methodology can be devised to isolate the errors due to the nonlinear processes that are inherent within the system.

Interannual Tropospheric Aerosol Variability in the Late Twentieth Century and Its Impact on Tropical Atlantic and West African Climate by Direct and Semidirect Effects

Abstract A new high-resolution global tropospheric aerosol dataset with monthly resolution is generated using version 4 of the Community Atmosphere Model (CAM4) coupled to a bulk aerosol model and forced with recent estimates of surface emissions for the period 1961–2000 to identify tropospheric aerosol-induced interannual climate variations. The surface emissions dataset is constructed from phase 5 of the Coupled Model Intercomparison Project (CMIP5) decadal-resolution surface emissions dataset to include reanalysis of tropospheric chemical composition [40-yr Reanalysis of Tropospheric Chemical Composition (RETRO)] wildfire monthly emissions data. A four-member ensemble run is conducted using the spectral configuration of CAM4, forced with the new tropospheric aerosol dataset and prescribed with observed sea surface temperature, sea ice, and greenhouse gases. CAM4 only simulates the direct and semidirect effects of aerosols on the climate. The simulations reveal that variations in tropospheric aerosol levels can induce significant regional climate variability on the interannual time scales. Regression analyses over tropical Atlantic and Africa suggest that increasing dust aerosols can cool the North African landmass and shift convection southward from West Africa into the Gulf of Guinea in the spring season. Further, it is found that carbonaceous aerosols emanating from the southwestern African savannas can significantly cool the region and increase the marine stratocumulus cloud cover over the southeast tropical Atlantic Ocean by aerosol-induced diabatic heating of the free troposphere above the low clouds. Experiments conducted with CAM4 coupled to a slab ocean model suggest that present-day aerosols can cool the tropical North Atlantic and shift the intertropical convergence zone southward and can reduce the ocean mixed layer temperature beneath the increased marine stratocumulus clouds in the southeastern tropical Atlantic.

The oldest African bat from the early Eocene of El Kohol (Algeria)

The Afro-Arabian Paleogene fossil record of Chiroptera is very poor. In North Africa and Arabia, this record is limited, thus far, to a few localities mainly in Tunisia (Chambi, late early Eocene), Egypt (Fayum, late Eocene to early Oligocene), and Sultanate of Oman (Taqah, early Oligocene). It consists primarily of isolated teeth or mandible fragments. Interestingly, these African fossil bats document two modern groups (Vespertilionoidea and Rhinolophoidea) from the early Eocene, while the bat fossil record of the same epoch of North America, Eurasia, and Australia principally includes members of the "Eochiroptera." This paraphyletic group contains all primitive microbats excluding modern families. In Algeria, the region of Brezina, southeast of the Atlas Mountains, is famous for the early Eocene El Kohol Formation, which has yielded one of the earliest mammalian faunas of the African landmass. Recent fieldwork in the same area has led to the discovery of a new vertebrate locality, including isolated teeth of Chiroptera. These fossils represent the oldest occurrence of Chiroptera in Africa, thus extending back the record of the group to the middle early Eocene (Ypresian) on that continent. The material consists of an upper molar and two fragments of lower molars. The dental character association matches that of "Eochiroptera." As such, although very fragmentary, the material testifies to the first occurrence of "Eochiroptera" in Algeria, and by extension in Africa. This discovery demonstrates that this basal group of Chiroptera had a worldwide distribution during the early Paleogene.

Horned dinosaurs venture abroad

The discovery in Europe of fossils of a small horned dinosaur, a member of a group previously known only from Asia and North America, will prompt a rethink of biogeography at that time in the past. The ceratopsians, or horned dinosaurs, were distinctive features of the fauna in the Late Cretaceous of East Asia and western North America, and are usually thought of as endemic to those areas. There have been hints that ceratopsians might have occurred elsewhere, but nothing definitive until now. The discovery of a new species of ceratopsian in what is now Hungary shows that Late Cretaceous biogeography still has surprises in store. Much of Europe at that time was an archipelago lying between the African and Eurasian land masses. Although poorly known, it could still yield insights into dinosaur biogeography.

Why is the stability of the Agulhas Current geographically bi‐modal?

The Agulhas Current, the largest western boundary current of the southern hemisphere, becomes fully constituted along the eastern coast of South Africa at about 25°S, from where is flows poleward along this coast. South of the continent it retroflects with most of its water being returned to the Indian Ocean in the Agulhas Return Current whereas the rest is injected into the South Atlantic by the shedding of Agulhas Rings at the retroflection point of the current. The flow of the Agulhas Current between its initiation and retroflection points can be divided into two distinct segments by trajectorial characteristics: a northern and a southern Agulhas Current. The path of the northern Current is exceptionally stable, whereas the southern Current meanders in a way more characteristic of western boundary currents. By applying a two‐layer, linear, stability theory that specifically considers the effect of a lateral boundary (i.e. the shelf‐break) on the stability of an adjacent current flowing parallel to it, we show that the stability of the northern Current is attributable to its proximity to the African land mass, which is located only one tenth of the internal radius of deformation west of the current. By comparison, the southern Current on average flows at a distance of over nine‐tenths of the internal radius of deformation from the shelf break, where according to the theory the instability is over twice that of the northern Current.

Goldschmidt Abstracts 2009 – V

We present a detailed reconstruction of hydrological changes near the equator in East Africa from before the LGM to the present, using proxies extracted from the sediment record of Lake Challa on the lower east slope of Mt. Kilimanjaro (3° S, 36° E). Our results show that monsoon rainfall in this region varied at half-precessional (~11,500-year) intervals, because the southeasterly and northeasterly Indian Ocean monsoons, which together create the bimodal seasonal distribution of equatorial rainfall, were strengthened in alternation when the inter-hemispheric insolation gradient was maximized. Dry conditions prevailed when neither monsoon was particularly strong, and minima in local March or September insolation weakened the rain season that followed. The distinct timing of late-glacial drought on the equator (20.5-16.5 ka BP) does not fit well with NH glaciation being its primary driver. It is best attributed to partial failure of the short rain season due to modest local September insolation, perhaps exacerbated by the southeasterly monsoon being weakened by northern cooling still affecting the North African land mass. Our data further highlight that orbital-scale ITCZ ‘migration’, or shifts in its mean annual latitudinal position, mainly relates to variation in how far the ITCZ is displaced into the NH or SH during summer and winter; the cross-equatorial position of the East African region with twice-annual ITCZ passage is not much affected. On (sub)millennial time scales the temporal pattern of hydrological change on the East African equator bears clear signatures of northern high-latitude climate variability, but on the orbital time scale it mainly responded to low-latitude insolation forcing. Important keys to this history are the low-latitude position of its continental regions of convergence and its relative isolation from the Atlantic Ocean domain, where strong meridional overturning circulation more tightly coupled tropical climate regimes to high-latitude climate dynamics.

Macroinvertebrate richness patterns in North African streams

AbstractAim To test the hypothesis that macroscale environmental variables explain local taxonomic richness of stream macroinvertebrates, and then to identify the relationships between these variables and benthic fauna richness in North Africa.Location North Africa, from West Morocco to East Tunisia.Methods Using a large‐scale data base made of 211 sampling sites gathered from an area of 500,000 km2, an artificial neural network model has been built to identify and predict the influence of macroscale environmental variables on local macroinvertebrate richness.Results The correlation coefficient (r) between observed and estimated taxon richness values was 0.75 (P < 0.001), and the model explained more than 55% (r2 = 0.563) of the macroinvertebrate richness variation. Macroinvertebrate richness was, therefore, accurately predicted using only three environmental variables accounting for hydrology (number of rainy days), geographical factors, i.e. connections between European and North African land masses (longitude) and climatic gradient (altitude).Main conclusions Stream macroinvertebrate richness in North Africa results from a combination of climatic, geographical and hydrological parameters. Although consistent with current biogeographic and ecological concepts mainly derived from European and North American streams, this study underlines the specificity of dry Mediterranean ecosystems. The shape of diversity patterns results from climatic conditions and their associated environmental seasonal dynamics, which screens geographical processes.

Aerosol optical depth over a remote semi-arid region of South Africa from spectral measurements of the daytime solar extinction and the nighttime stellar extinction

Spectral daytime aerosol optical depths have been measured at Sutherland, South Africa (32°22′S, 20°48′E), from January 1998 to November 1999. Sutherland is located in the semi-arid Karoo desert, approximately 400-km northeast from Cape Town. The site, remote from major sources of aerosols, hosts the South African Astronomical Observatory (SAAO), where nighttime stellar extinction is being measured. The comparison of daytime and nighttime measurements for the years 1998–1999 makes it possible to validate the astronomical dataset of aerosol optical depth ( τ a) dating back to 1991. The 1998 and 1999 annually averaged daytime τ a at 500 nm are 0.04±0.04 and 0.06±0.06, respectively. Half-day averages vary between 0.03 and 0.44, with peak values in August–September. This pronounced seasonality is linked to the biomass-burning season in the Southern Hemisphere. Smoke haze layers transported to Sutherland originated primarily on the African landmass at latitudes between 10° and 20°S and passed over Namibia and Angola. On one occasion, aerosols from fires in Brazil transported across the Atlantic Ocean were likely detected. The haze layers reaching Sutherland are therefore at least 2–3 days old. The spectral dependence of the aerosol optical depth for the smoke layers supports the bimodality of the volume size distribution for biomass burning aerosols. The accumulation mode has a volume modal diameter of 0.32 μm, consistent with the hypothesis of aged haze. The stellar measurements (1991–2001) show that, due to the eruption of Mt. Pinatubo, the atmospheric extinction depth at 550 nm in the years 1991–1993 increased by 33% with respect to the average value (0.14±0.03) for the period 1994–2001. Outside the Pinatubo event, extinction is largest in the period 1997–1999.

Rainfall variability in Northern Tanzania in the March-May season (long rains) and its links to large-scale climate forcing

Mean rainfall in Northern Tanzania is characterized by 2 annual maxima: in March–May, the long rains, and in October–December, the short rains. The interannual variability of rainfall in the March–May season and its relationships to large-scale fields from the NCEP/NCAR Reanalysis in the African-Indian Ocean sector were analysed. It was found that the March–May precipitation has characteristics different to that in October–December, the former being much less persistent within the season and being associated with intraseasonally changing large-scale patterns of surface temperature, sea-level pressure and winds. March and April rainfall anomalies are linked to zonal thermal contrast between the Indian Ocean and the Eastern African land mass, to zonal surface winds anomalies and to vertical velocity anomalies. On the other hand, May rainfall anomalies are associated with a meridional surface temperature contrast between the Indian Ocean and the Asian continent and meridional surface winds anomalies, indicating a relationship to the Indian Monsoon. Local convergence seems to play a smaller role than for March anomalies. Indeed, Northern Tanzanian rainfall anomalies in May are correlated with the area-averaged Indian rainfall in the following months of June, July and August. The consequences for possible statistical seasonal prediction schemes are briefly discussed.

A conceptual model for understanding rainfall variability in the West African Sahel on interannual and interdecadal timescales

AbstractThis article describes and validates a new conceptual model for understanding Sahel rainfall variability. This conceptual model provides a framework that can readily incorporate and synthesize the roles played by the oceans, the African landmass and local meteorological factors. The most important ‘local’ factors are the location of the African Easterly Jet (AEJ) and the associated shears. The position of the AEJ helps to distinguish between a ‘wet mode’ and a ‘dry mode’ in the Sahel, while other factors determine which of two spatial patterns prevail during years of the dry regime. We test the paradigm by contrasting selected circulation parameters for the years 1958–1967 (representing the wet mode) and 1968–1997 (representing the dry mode). In doing so, we have identified several changes in the general atmospheric circulation that have accompanied the shift to drier conditions. The AEJ is further southward and more intense, the Inter‐tropical Convergence Zone (ITCZ) is further south, the Tropical Easterly Jet (TEJ) is weaker, the equatorial westerlies are shallower and weaker, the southwesterly monsoon flow is weaker, and the relative humidity is lower (but not consistently so).The results of this study suggest that the key factor controlling the occurrence of the ‘wet Sahel’ mode versus the ‘dry’ mode is the presence of deep, well‐developed equatorial westerlies. These displace the AEJ northward into Sahelian latitudes and increase the shear instabilities. The westerlies appear to be at least partially responsible for the well‐known association between a weaker AEJ and wetter conditions in the Sahel, because the thermal wind induced by the Sahara/Atlantic temperature gradient is imposed upon a westerly basic state. Since one of the strongest contrasts between the ‘wet Sahel’ and ‘dry Sahel’ modes is the strength of the TEJ, the TEJ probably also plays a pivotal role in rainfall variability. In the dry mode, the equatorial westerlies are poorly developed and the core of the AEJ lies well to the south of the Sahel. The dry mode consists of two basic spatial patterns, depending on whether the Guinea Coast Region is anomalously wet or dry (the well‐known dipole and no‐dipole patterns, respectively). Which occurs is determined by other factors acting to reduce the intensity of the rainbelt. One of the relevant factors appears to be sea‐surface temperatures (SSTs) in the Gulf of Guinea. Copyright © 2001 Royal Meteorological Society

SST-induced climate change in tropical North Africa: The intermediary role of lower tropospheric oscillations

The growth rates of amplifying mid-tropospheric perturbations in tropical North Africa is known to reduce with increased vertical shear in the troposphere. This phenomenon leads to a reduction in the frequency of generation of squall lines – the main rain-producing mechanism in tropical North Africa – because squalls are initiated by amplifying modes of African Easterly Waves (AEW). Ultimately, therefore, tropical North Africa experiences a shortfall, with respect to long-term averages, in annual rainfall. Weakening of AEW intensity is shown to be linked with the warming up to the sea-surface temperatures (SST) of the South Atlantic, Pacific and Indian Oceans. These findings are consistent with the observed reduction in the incidence of intense hurricanes along the entire westem Atlantic in Sahelian dry years. It is shown that the frequency of occurrence of Atlantic tropical storms and hurricanes is unaffected by the dryness or otherwise in the Sahel, but the paths of the storms are determined by the zonal exit point, from the African continental land mass to the Atlantic, of West African disturbance lines. These results have applications, and implications, in the level of preparedness for the economic impacts of Atlantic storms and hurricanes.

Warm and cold events in the southeast Atlantic/southwest Indian Ocean region and potential impacts on circulation and rainfall over southern Africa

An important pattern of interannual variability in the southern African region is one where sea surface temperature (SST) in neighbouring waters, particularly in the Agulhas Current, its retroflection region and outflow across the southern midlatitudes of the Indian Ocean, is anomalously warm or cool. Evidence exists of significant rainfall anomalies over large parts of southern Africa during these warm or cool SST events. Here, a general circulation model is used to study the response of the atmosphere in the region to an idealised representation of these SST anomalies. The induced atmospheric circulation and precipitation anomalies over the adjacent southern African landmass on intraseasonal through to interannual time scales are investigated.

SST variability in the South Indian Ocean and associated circulation and rainfall patterns over Southern Africa

A general circulation model is used to study the response of the atmosphere to an idealised sea surface temperature (SST) anomaly pattern (warm throughout the southern midlatitudes, cool in the tropics) in the South Indian Ocean region. The anomaly imposed on monthly SST climatology captures the essence of patterns observed in the South Indian Ocean during both ENSO events and multidecadal epochs, and facilitates diagnosis of the model response. A previous study with this anomaly imposed in the model examined differences in the response between that on the seasonal scale (favours enhancement of the original SST anomaly) and that on the decadal scale (favours damping of the anomaly). The current study extends that work firstly by comparing the response on the intraseasonal, seasonal and interannual scales, and secondly, by assessing the changes in the circulation and rainfall over the adjoining African landmass. It is found that the atmospheric response is favourable for enhancement of the original SST anomaly on scales up to, and including, annual. However, as the scale becomes interannual (i.e., 15–21 months after imposition of the anomaly), the model response suggests that damping of the original SST anomaly becomes likely. Compared to the shorter scale response, the perturbation pressure and wind distribution on the interannual scale is shifted poleward, and is more reminiscent of the decadal response. Winds are now stronger over the warm anomaly in the southern midlatitudes suggesting enhanced surface fluxes, upper ocean mixing, and consequently, a damping of the anomaly. Examination of the circulation and rainfall patterns indicates that there are significant anomalies over large parts of southern Africa during the spring, summer and autumn seasons for both short (intraseasonal to interannual) and decadal scales. It appears that rainfall anomalies are associated with changes in the advection of moist tropical air from the Indian Ocean and its related convergence over southern Africa. Over eastern equatorial Africa, the austral autumn season (the main wet season) showed rainfall increases on all time scales, while parts of central to eastern subtropical southern Africa were dry. The signals during summer were more varied. Spring showed generally dry conditions over the eastern half of southern Africa on both short and decadal time scales, with wet areas confined to the west. In all cases, the magnitude of the rainfall anomalies accumulated over a 90 day season were of the order of 90–180 mm, and therefore represent a significant fraction of the annual total of many areas. It appears that relatively modest SST anomalies in the South Indian Ocean can lead to sizeable rainfall anomalies in the model. Although precipitation in general circulation models tends to be less accurately simulated than many other variables, the model results, together with previous observational work, emphasize the need for ongoing monitoring of SST in this region.

Surface Temperature Related Variations in Tropical Cirrus Cloud as Measured by SAGE II

Data from the Stratospheric Aerosol and Gas Experiment II (SAGE II) solar occultation satellite instrument have been used to study the properties of tropical cloud over the altitude range 10.5–18.5 km. By virtue of its limb viewing measurement geometry, SAGE II has good vertical resolution and sensitivity to subvisual cloud not detectable by most other satellite instruments. The geographical distribution and temporal variation of the cloud occurrence have been examined over all longitudes on timescales from less than 1 day to that of the El Niño-Southern Oscillation (ENSO) cycle. Significant variations in cloud occurrence are found on each of these scales and have been compared with the underlying surface temperature changes. Maximum cloud occurs over the warm pool region of the Pacific Ocean, with secondary maxima over the South American and Central African landmasses, where the percentage of cloud occurrence in the upper troposphere can exceed 75%. Cloud occurrence at all altitudes within the Tropics, over both land and ocean, increases with the underlying surface temperature at a rate of approximately 13%°C−1. Extrapolated threshold temperatures for the formation of cloud are about 5°C lower than those found from nadir viewing observations. This difference is believed to be a consequence of the averaging process and the inclusion of outliers in the dataset. ENSO cycle changes in cloud occurrence are observed, not only over the Tropics but also over the subtropics, indicating a difference in the meridional Hadley circulation between ENSO warm and cold years. Sunrise–sunset cloud differences indicate that large-scale variations, whose form resembles that of the Hadley and Walker circulations, are present, with a timescale of 1 day or less. The global distribution of upper-tropospheric ice and its positive correlation with surface temperature on all timescales are generally consistent with the behavior of lightning and the global electrical circuit.

Evolution of two troughs in the tropical Indian Ocean and their characteristic features

Evolution and characteristic features of double trough systems in the tropical Indian Ocean have been studied with the help of Climatological Atlas (Part I andIl) ~f the Tropical Indian Oc.ean (Hastenrath and Lamb 1979). It is confirmed that there are two troughs (Northern Hemisphere EquatorIal Trough and Southern Hemisphere Equatorial Trough) in this region (including south Asian landmass) all the year round, one in northern hemisphere and the other in southern. Both are migratory in nature and, perhaps, thermal in origin. In the convergent zones of the two troughs, there is extensive cloudiness. The migration of these trough systems during their respective summer seasons appear to be related to the extensive heating of the south Asian/ African land masses surrounding the Indian Ocean in north and west.

High Waves in the Benguela Current

Abstract Analysis of merchant ship observations reveals a sharp ridge of high wave frequency along the axis of the Benguela Current, nearly coincident with the major shipping lane in that area. The authors could find no other reference to this wave regime in the literature. The majority of these waves are found to be southerly or southeasterly seas reinforced by swells of similar origin and direction. The steady winds within the stable subsiding air on the eastern edge of the semi-permanent subtropical South Atlantic Oceanic high pressure area are channeled to the northwest by the South African land mass, forming the driving mechanism that generates the waves. The thermal low over the continent further reinforces this regime, as does coastal upwelling. Resultant sharp local anomalies in air temperature, cloud cover and other elements are also present.

Geology of Tunisia and Adjacent Parts of Algeria and Libya

The southern part of the study area is part of the stable Saharan platform which, since the beginning of the Paleozoic, has been affected only by epeirogenic movements. Pre-Carboniferous Paleozoic strata are largely clastic, both continental and marine. Early invasions of the sea came from the west, but Carboniferous and younger transgressions came from a northerly (Tethyan) direction. Caledonian tectonic movements produced structural alignments and important regressions, but general uplift resulting from the Hercynian orogeny caused the major withdrawal of the sea. The dominant feature of post-Hercynian erosion in Algeria is a great T-shaped anticlinorium which projects into Tunisia. On the east, a projection of the Hercynian Nefusa uplift of Libya extends westward and c nnects with the anticlinorium. Just south of these positive areas is the Erg Oriental basin, which was connected during most of the Paleozoic with the Homra basin of western Libya. The former is bounded in the western part of the study area by the north-northeast-trending Hassi Touareg anticlinorium. Restriction of the western Tethys basin and post-Hercynian subsidence along the margin of the African landmass led to deposition of a thick series of Triassic and Liassic evaporites. Younger strata of the Saharan platform are relatively thin, and although continental sands were important during certain stages, lagoonal dolomite, evaporite, and shale are the dominant rock types. North of the platform was a subsident zone, the Jeffara flexure, where considerably thicker and more marine strata are characteristic. Associated with the flexure and extending beyond it is a complex series of faults. On the north and west, the principal later Mesozoic events consisted largely of epeirogenic uplifts with regression and local erosion, and transgressive downwarps. Frequent and more intense moveme ts characterized the early Cenozoic, and local deformation, marking the onset of Alpine orogeny, began in the Oligocene. Stronger orogenic pulses occurred at the end of the Miocene, but the last and most violent phase of the Tunisian Atlasic orogeny took place during the Pliocene-Pleistocene. In addition to the Saharan platform and Jeffara flexure, Mesozoic-Cenozoic tectonic features include stable shelves of the Pelagian craton in eastern and offshore Tunisia, and of Kasserine Island in central Tunisia. The latter was exposed during latest Cretaceous time and is bounded on the east by a series of north-south-trending faults and Jurassic horsts marking the eastern boundary of the Atlas zones. Between this axis and the Pelagian shelf and between the Saharan platform and Kasserine Island is a series of Cretaceous and Tertiary intracratonic basins, and in northern Tunisia are similar ones containing thick sections of Miocene-Pliocene clastic rocks. In northwestern Tunisia are many Triassic evaporite diapirs, and some thrust faults are present in a zone of very thick Oligocene flysch. Three hydrocarbon-productive trends are present in the study area: (1) Ypresian (early Eocene) carbonate rocks: facies change regionally southwestward from globigerinid mudstone (source beds) to dolomitized nummulitic calcarenite (reservoir) to evaporite (updip seal). Recent discoveries seem to be associated with small structures, but regional extent of the trend and attendant possibilities for stratigraphic entrapment suggest the potential for large reserves. (2) Paleozoic and Triassic sandstones: this trend contains the only proved, major reserves of Tunisia in the Triassic reservoir at El Borma field. Cambrian-Ordovician strata are productive at Rhourde El Baguel in Algeria, and sandstones of the Silurian-Lower Devonian Acacus Formation are capable of production in many Libyan well . Major fields are structurally controlled, but distribution of reservoir rocks is related to Hercynian uplift, which influenced erosion of Cambrian-Ordovician clastic strata and deposition of presalt Triassic detritus. Hydrocarbon source beds for both systems are believed to be Silurian euxinic shales. (3) Cretaceous shoreline: several small oil fields and one gas field currently are productive, mostly from bioclastic carbonate reservoirs of Aptian to early Senonian age. These accumulations are on surface anticlines in a foothills belt which is not deformed so severely as the main Atlas ranges.

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.