Alalade B.,Newcastle University |
Journal of African Earth Sciences | Year: 2013
This study evaluates the influence of igneous intrusions on the thermal maturity of Late Cretaceous (Turonian-Santonian) potential source rock shales within the Gongila and Fika formations penetrated by the Tuma well. Twenty representative shale samples of the two formations between 1000 m and 3195 m were analyzed by bulk organic geochemical and palynofacies methods. A positive excursion in Rock-Eval Tmax values and an increase in the percent of non-fluorescent amorphous organic matter (AOM) were observed between 1500 m and 2900 m. These anomalies are moderately correlated with the abundance of igneous intrusive fragments. At depths greater than 2900 m where igneous intrusive fragments are absent, Rock-Eval Tmax values return to the same trend as observed above 1500 m and the percentage of fluorescent AOM also increases. These observations suggest that the thermal maturity of the shales between 1500 m and 2900 m was elevated by the heating due to the emplacement of the intrusives. Further studies should be undertaken to differentiate reliably the effects of regional burial and those of more localized contact metamorphism, and to determine the effects of each on the maturation of the Late Cretaceous potential source rocks. © 2012 Elsevier Ltd.
Fedi M.,University of Naples Federico II |
Journal of Geophysical Research: Solid Earth | Year: 2011
In potential field problems, the continuous wavelet transform (CWT) has allowed the estimation of the source properties, such as the depth to the source and the structural index (N). The natural choice for the analyzing wavelets has been the set belonging to the Poisson kernel. However, a much larger set of analyzing wavelets has been used for analyzing signals other than potential fields. Here we extend the CWT of potential fields to other wavelet families. Since the field is intrinsically dilated with Poissonian wavelets from the source depth to the measurement level, distortions are unavoidably introduced when CWT uses a different wavelet from the measurement level to other scales. To fix the problem, we define a new form for the continuous wavelet transform convolution product, called "composite continuous wavelet transform" (CCWT). CCWT removes the field dilations with Poisson wavelets, intrinsically contained at the measurement level and replaces them with dilations performed with any other kind of wavelet. The method is applied to synthetic and real cases, involving sources as poles, dipoles, intrusions in complex magnetized basement topography and buried steel drums, from measurements taken at the Stanford University test site. CCWT takes advantage from the special features of the several considered wavelets, e.g., the Gaussian wavelet is useful for its low pass filtering characteristic and Morlet wavelet for its localization property. Hence, depending on the case, an important parameter for the choice of the analyzing wavelet is its central frequency. Copyright 2011 by the American Geophysical Union.
Liu W.-N.,Tongji University |
Li C.-F.,Tongji University |
Li J.,State Oceanic Administration |
Fairhead D.,GETECH |
Zhou Z.,Tongji University
Marine and Petroleum Geology | Year: 2014
Compared to the northern South China Sea continental margin, the deep structures and tectonic evolution of the Palawan and Sulu Sea and ambient regions are not well understood so far. However, this part of the southern continental margin and adjacent areas embed critical information on the opening of the South China Sea (SCS). In this paper, we carry out geophysical investigations using regional magnetic, gravity and reflection seismic data. Analytical signal amplitudes (ASA) of magnetic anomalies are calculated to depict the boundaries of different tectonic units. Curie-point depths are estimated from magnetic anomalies using a windowed wavenumber-domain algorithm. Application of the Parker-Oldenburg algorithm to Bouguer gravity anomalies yields a 3D Moho topography. The Palawan Continental Block (PCB) is defined by quiet magnetic anomalies, low ASA, moderate depths to the top and bottom of the magnetic layer, and its northern boundary is further constrained by reflection seismic data and Moho interpretation. The PCB is found to be a favorable area for hydrocarbon exploration. However, the continent-ocean transition zone between the PCB and the SCS is characterized by hyper-extended continental crust intruded with magmatic bodies. The NW Sulu Sea is interpreted as a relict oceanic slice and the geometry and position of extinct trench of the Proto South China Sea (PSCS) is further constrained. With additional age constraints from inverted Moho and Curie-point depths, we confirm that the spreading of the SE Sulu Sea started in the Early Oligocene/Late Eocene due to the subduction of the PSCS, and terminated in the Middle Miocene by the obduction of the NW Sulu Sea onto the PCB. © 2014 Elsevier Ltd.
Tindall J.,University of Bristol |
Flecker R.,University of Bristol |
Valdes P.,University of Bristol |
Schmidt D.N.,University of Bristol |
And 2 more authors.
Earth and Planetary Science Letters | Year: 2010
One of the motivations for studying warm climates of the past such as the early Eocene, is the enhanced understanding this brings of possible future greenhouse conditions. Traditionally, climate information deduced from biological or chemical proxies have been used to "test" computer model simulations of past climatic conditions and hence establish some of the uncertainties associated with model-based predictions. However, extracting climate information from proxies is itself an interpretative process and discrepancies between climate information inferred from different types of proxy undermines the assumption that model-data conflicts automatically mean that the model is inherently flawed. A new approach which both acknowledges and reduces the uncertainties associated with both model and data is required.Although the oxygen isotopic ratio (δ18O) preserved in calcareous marine fossils has been used to reconstruct past seawater temperature for several decades, significant uncertainties associated with this method persist. These include assumptions about past seawater δ18O for which no proxy exists and which is a key control on the temperature inferred from fossil carbonate. Here we present the results of an early Eocene simulation made using a state-of-the-art General Circulation Model (GCM; HadCM3) with CO2 set at six times pre-industrial values and which has oxygen isotopes incorporated into the full hydrological cycle and hence simulates the δ18O of past seawater. This allows us to explore the implications of the different seawater δ18O correction factors commonly used for δ18O-based temperature reconstruction. It also allows us to focus model-data comparison on δ18O rather than interpret ocean temperature, an approach that reduces uncertainties in model-data comparison since the effects of both the temperature and the isotopic composition of ocean water on δ18O of carbonate are accounted for. The good agreement between model and data for both modern and well-preserved early Eocene carbonate increases confidence in climate reconstructions of this time. © 2009 Elsevier B.V.
Geological Society Special Publication | Year: 2015
This paper presents a reconstruction of the palaeodrainage evolution of the Niger River in West Africa in order to contribute to the understanding of sediment supply to the Niger Delta. It has been covered extensively in literature that the Niger River has undergone changes along its course in the Holocene, as implied by the large bend it makes in Mali. However, other enigmatic bends further downstream are indicative of an older and more complicated history that has yet to be understood, and is the focus of this paper. Until now, sediment supply from the Niger River has been considered as being negligible compared to that of the Benue River. The results of this study imply that the contribution from the Niger River was more important than previously thought. The Niger River obtained its present-day geometry in three phases: a Bida Basin phase (Maastrichtian-Miocene); a Iullemmeden Basin phase (Miocene-Pleistocene); and a presentday Niger River phase (Holocene). In the Miocene, an important capture event occurred, increasing the incipient drainage basin by 106 km2, thereby changing the provenance of the sediment supplied to the Niger Delta from mainly crystalline basement to mixed lithologies including sandstone, shale, limestone and volcanic outcrops. © The Geological Society of London 2014.