Rates of deformation of an extensional growth fault/raft system (offshore Congo, West African margin) from combined accommodation measurements and 3-D restoration

The aim of this paper is to quantify the evolution in time and space of the accommodation (space available for sedimentation) in the case of a growth fault structure resulting from gravity‐induced extension comprising a listric fault/raft system located along the West African margin. To achieve this, use was made of an original approach combining two complementary techniques (accommodation variation measurements and 3‐D restoration) in order to quantify vertical and horizontal displacement related to deformation, using a data set made up of a 3‐D seismic survey and well logs. We applied sequence stratigraphic principles to (i) define a detailed stratigraphic framework for the Albo‐Cenomanian and (ii) measure subsidence rates from accommodation variations. 3‐D restoration was used to (iii) reconstruct the evolution of the 3‐D geometry of the fault system. The rates of horizontal displacement of structural units were measured and linked to successive stages in the growth of the fault system. Subsidence of the structural units exhibits three scales of variation: (1) long‐term variation (10 Ma) of c. 80 m Ma^?1 for a total subsidence of about 1400 m, compatible with the general subsidence of a passive margin, and (2) short‐term variations (1–5 Ma) corresponding to two periods of rapid subsidence (about 150–250 m Ma^?1) alternating with periods of moderate subsidence rate (around 30 m Ma^?1). These variations are linked to the development of the fault system during the Albian (with downbuilding of the raft and development of the initial basin located in between). During the Cenomanian, the development of the graben located between the lower raft and the initial basin did not seem to affect the vertical displacements. (3) High‐frequency variations (at the scale of genetic unit sets) range between ?50 and 250 m for periods of 0.2–2 Ma. Accommodation variations governed these cycles of progradation/retrogradation rather than sediment flux variations. In addition, the nine wells display a highly consistent pattern of variation in accommodation. This suggests that the genetic unit sets were controlled at a larger scale than the studied system (larger than 20 km in wavelength), for example, by eustatic variations. Translation rates are between 3 and 30 times higher than subsidence rates. Therefore, in terms of amplitude, the main parameter controlling the space available for sedimentation is the structural development of the fault system, that is to say, the seaward translation of the raft units, itself resulting from a regional gravity‐driven extension.     

Representation of fish communities by scale sub-fossils in shallow lakes: implications for inferring percid–cyprinid shifts

The palaeoecological potential of fish scales was assessed by comparing contemporary population data with scale remains obtained from littoral (n = 10) and open water (n = 10) surface sediment samples in two English shallow lakes, Selbrigg Pond and Cockshoot Broad. Scales and/or scale fragments were present, in low numbers (<20 per 100 cm³ wet sediment) in 34 of 40 sediment samples. In accordance with fish population data, higher densities of scale remains were found in Selbrigg compared to Cockshoot, and in littoral compared to open water samples. Taxonomic difficulties, exacerbated by scale fragmentation, made it impossible to assign the majority of remains to individual species. Most remains could, however, be placed into one of two groups: (i) percids – represented by both scales and scale fragments; and (ii) cyprinids – largely represented by scale fragments. To allow comparison of fish population and sedimentary scale data, both were converted to percentages of the aggregate percid–cyprinid total. Whole scales recovered were almost exclusively percid (45 of 48), thus bore little resemblance to the contemporary fish data. Nevertheless, percentages of scale fragments (Selbrigg: 34 and 66%; Cockshoot: 13 and 87% percid and cyprinid, respectively) and of whole scales and fragments combined (Selbrigg: 54 and 46%; Cockshoot: 46 and 54% percid and cyprinid, respectively) reflected the presence of the numerically dominant fish groups and the broad inter-site differences in their relative abundance (Selbrigg: 36 and 64%; Cockshoot: 10 and 90% percid and cyprinid, respectively). A running mean of scales per sediment volume indicated that some 400 cm³ of sediment was required to accurately characterise the remains present. This study suggests that, with the appropriate methodological considerations (e.g., collection of large sediment samples), fish scale remains may be used to determine the past presence–absence and relative abundance of percid and cyprinid species. As such, this technique may be a valuable supplementary tool for establishing longer-term changes in the fish communities of shallow lakes.     

Monitoring volcanic hazard using eddy covariance at Solfatara volcano, Naples, Italy

An eddy covariance (EC) station was deployed at Solfatara crater, Italy, June 8–25, 2001 to assess if EC could reliably monitor CO₂ fluxes continuously at this site. Deployment at six different locations within the crater allowed areas of focused gas venting to be variably included in the measured flux. Turbulent (EC) fluxes calculated in 30-min averages varied between 950 and 4460 g CO₂ m⁻² d⁻¹; the highest measurements were made downwind of degassing pools. Comparing turbulent fluxes with chamber measurements of surface fluxes using footprint models in diffuse degassing regions yielded an average difference of 0% (±4%), indicating that EC measurements are representative of surface fluxes at this volcanic site. Similar comparisons made downwind of degassing pools yielded emission rates from 12 to 27 t CO₂ d⁻¹ for these features. Reliable EC measurements (i.e. measurements with sufficient and stationary turbulence) were obtained primarily during daytime hours (08:00 and 20:00 local time) when the wind speed exceeded 2 m s⁻¹. Daily average EC fluxes varied by ±50% and variations were likely correlated to changes in atmospheric pressure. Variations in CO₂ emissions due to volcanic processes at depth would have to be on the same order of magnitude as the measured diurnal variability in order to be useful in predicting volcanic hazard. First-order models of magma emplacement suggest that emissions could exceed this rate for reasonable assumptions of magma movement. EC therefore provides a useful method of monitoring volcanic hazard at Solfatara. Further, EC can monitor significantly larger areas than can be monitored by previous methods.     

Hydrogeological Zoning of the Central Minusinsk Basin by the Conditions of Formation of Fresh Groundwater Resources

A detailed hydrogeological zoning of the central part of the Minusinsk artesian basin is performed. Local basins confined to smaller plicated structures, as well as hydrogeological massifs of local scale, are recognized within large block structures. Direction of subsurface streams and their role in the formation of groundwater resources and composition are discussed. Forecast resources of fresh groundwater are estimated for every recognized structure.