Ancient Braided River System in Spain - Essay Example

Morphological Study of a Selected Stretch of an Ancient Braided River System in South-Eastern Spain Introduction: The 'Urra Field Centre', incumbent within the dryland system of the Tabernas region in the Spanish province of Almeria, is a well-known site for geological study (Urra Field Centre, 2005). The field centre includes a substantial part of the Sorbas Gypsum karst. The centre lies about 20 km from the provincial capital Almeria in a semi-arid region characterised by a mean annual precipitation of 218mm ranging from lowest 115mm to highest 431mm per annum (Prakash, 2005). The mean annual temperature is (Prakash, 2005). Landcover is mostly low bush and grasses with or without trees (Prakash, 2005). Land usage includes agriculture (barley and others under irrigated conditions), grazing, mining (gypsum, gravel and sand) and tourism (movie-making and others) (Prakash, 2005). This particular study is being conducted to assess sorting/fining patterns in accreted materials in sections of an old braided river system incumbent within the Urra Field Centre. The Sorbas Gypsum Karst: The gypsum karst of the Tabernas-Sorbas basin of SE Spain is a Neogene intramontane one in the Betic mountain range and is composed of significant deposits of evaporates from the Messinian period (Calaforra and Pulido-Bosch, 2003). The semi-arid environment in the region has assisted in conserving the large numbers of surface and underground gypsum karstic forms and the speleogenetic processes characteristic of such a region (Calaforra and Pulido-Bosch, 2003). In essence, this implies that, at least for this field study at the Western edge of this karstic formation, a semi-confined status existed upon the gypsum strata (Calaforra and Pulido-Bosch, 2003) through which the old braided river section flowed and it is possible that only those speleogenetic processes characteristic of a karst region such as this have been influential in the accretion process of sediment evident today. Braided River Systems: In study of fluvial systems such as rivers the term 'braided' implies a low proportion of flood-plain mudstones and a high one of sedimentary sand elements like sandstone (Adams and Bhattacharya, 2005). There is characteristic distinction which states that 'meandering' channels have high mudstone-to-sandstone ratios and tend to have high sinuosity combined with single channel flows (Adams and Bhattacharya, 2005). In contrast, 'braided' channels have a high sandstone-to-mudstone ratio and tend to have multiple channel flows (Adams and Bhattacharya, 2005). This distinction is blurred at times by continuity of variables (Adams and Bhattacharya, 2005) but, for the purpose of this field study, it is accepted that the study area comprised of a 'braided' channel portion. Of eminent interest to this study are the depositional and erosional patterns of the river at the meandering stage and the braided one. It has single-layered laterally accreting patterns with similar eroding ones in the meandering stage while accretional patterns in the braided stage are multi-layered and vertical (Adams and Bhattacharya, 2005). Field Work Rationale: To test whether 'braided' stream sections have lateral and/or vertical sorting/fining tendencies. Methodology: Six different randomly selected channels of the known old braided river system in the Urra Field Centre were chosen. From each channel, three samples from the left and right banks and the centre of the stream were taken. Sample bags were used to collect the pebbles from the sample-taking areas. A tape measure was also used to measure the channel sections. A pebble template and a grain size chart were used to size up the collected pebbles. Additionally, a camera was used to collect photos of the sample-taking areas to provide rough assessment of the pebble characteristics at each site. The pebble sizes were measured and the data tabulated as per each site characteristic. In this part of the study the data shall be analysed through multiple statistical descriptive tests such as mean size and standard deviation. It is noted that the pebbles were interbedded with silts, the banks were well vegetated and there were small shrubs on the streambed. This may suggest, as the current literature on the geology of the region does, that the streambed does not support any flows presently. This presence of the small shrubs on the bed strongly suggests this and it precludes any present accretion process. It is notable that the parametric measurement of the pebbles (mm) has been converted to the basis of an entirely arbitrarily chosen weight system as per the following table. Parameters Conversion Range Measures (mm) >4 4 -5.6 5.6 - 8 8 - 11 11 - 16 16 - 22 22 - 32 32 - 45 45 - 64 64 - 90 >90 Weights 1 2 3 4 5 6 7 8 9 10 11 Instead of the actual measurements the weights have been used to conduct the final statistical analyses. From each location - the left and right banks and the centre, exactly 50 pebbles have been collected from each site in the whole sample area. Results: Table 1: Mean Values (Weights) Site No. Left Bank Centre Right Bank 1 9 9 4 2 8 8 9 3 7 11 7 4 8 10 8 5 8 10 7 6 5 9 6 Note: 1. The mean values are actually the weights that most closely correspond with the actual means (actual mean = weighted mean + standard deviation for normal distributions). 2. The weight that coincides most closely with the weighted mean has been accepted. Table 2: Location Mean Standard Deviation Distribution (P) Left Bank 7.48 2.09 0.72 Centre 9.51 1.39 0.95 Right Bank 6.81 2.40 0.94 Note: 1. The above-tabulated data has been generated from that of Table 1 above. 2. The data has been analysed at 95% confidence levels. 3. The tabulated data conforms closely with the actual pebble measurements taken. Except for the right bank, pebble measurements tended to exceed the 16mm mark. There was wider measurement range for the left bank than for the other two locations. Graph 1: Note: 1. For locations, 0 = Left bank; 2 = Centre; and 4 = Right Bank. Discussion: It immediately becomes apparent from Table 2 & Graph 1 that there are larger sized pebbles at the centre of the river. This also implies that the sorting/fining process has been more intense in a lateral progression away from the centre towards the two banks. It seems that the fining process has been more intense at the right bank than the left. The lower mean (Table 2) implies this. Also, the wider range of pebble sizes at the left bank is implicate in the lower P value (Table 2) there. In contrast, the right bank and centre have more close association in pebble sizes (P values, Table 2). Pebbles at the right bank and centre tended to exceed the 16mm mark. The closest association in pebble sizes is available for the centre (Table 2) with standard deviation at 1.39 together with P value at 0.95. In contrast, the two banks tended to have pebble sizes with a wider distribution: Std. Dev (Left = 2.09, P = 0.72; Right = 2.40, P = 0.94). There is implication that the deviation from modal values for the right bank is greater than for the left though the left bank tended to have a wider pebble size range. Conclusion: The study believes that it has been successful. There is evidence that in braided streams the accretion is multilayered and most intense at the centre (Adams and Bhattacharya, 2005). There is also evidence that the fining process in such streams works upwards at the centre while a more lateral fining process is at work also (Macfarlane and Hecox, 2003). It is evident from the results and the discussion that the largest sized pebbles are at the centre. Thus, it cannot be construed whether there has been a fining/sorting process at work at the centre (Adams and Bhattacharya, 2005) since layered sampling has not been conducted but it is evident from the samples that there has been a lateral fining/sorting process. This last is consistent with evidence collected elsewhere (Macfarlane and Hecox, 2003) where a fanning out of the sorting/fining process has been found to be at work in braided streams such as the Ogallala in Eastern New Mexico and the Texas Panhandle (Macfarlane and Hecox, 2003). In the fanned lithofacies the distal parts are finer compared to coarser grains at the proximal regions (Macfarlane and Hecox, 2003). From this it is construable that, in a semi-arid environment such as the study area where ephemeral stream dynamics tend to have lower energies, the larger grains tend to be deposited at the proximal regions - the centre in this study - as the sorting process works to develop a pebble deposition system where the larger ones accrete at the centre while the finer ones do so at the edges - the two banks. Further study can be conducted at the same location with layered sampling, especially at the centre, to assess whether a more characteristic dynamic process of braided streams, layered sorting/fining, has been at work in this old river system with the finer pebbles accreted at the top in the centre. The pebble sizes available from the centre, though largest, may still be smaller than pebbles available underneath them in bottom layers at the centre. The types of pebbles - whether mudstone or gypsum/sandstone-derived - is another scope for study as it is predictable that a braided stream such as the one in study should have a higher ratio of mudstone-to-gypsum/sandstone (Adams and Bhattacharya, 2005). References: Adams, Michael M, and Bhattacharya, Janok P., No Change in Fluvial Style Across a Sequence Boundary, Cretaceous Blackhawk and Castlegate Formations of Central Utah, U.S.A., Journal of Sedimentary Research, November 2005, Vol. 75, No. 6, 1038-1051. Calaforra, Jose M., and Pulido-Bosch, A., Evolution of the gypsum karst of Sorbas (SE Spain), Geomorphology 50 (2003) 173-180. Macfarlane, P. Allen, and Hecox, Gary, Analysis of the High Plains (Ogallala) Aquifer Framework in Two Small Areas of the Northwest Kansas Groundwater Management District No. 4, Kansas Geological Survey, Open File Report 2003-14. Accessed on 5th May, 2008, at: http://www.kgs.ku.edu/Hydro/Publications/2003/OFR03_14/index.html Prakash, Anupma, 2005. Accessed on 4th May, 2008, at: http://www.gi.alaska.edu/prakash/fieldwork/tabernas/index.html Statistical Software Accessed on 4th & 5th May, 2008, at: http://www.physics.csbsju.edu/stats/cstats_NROW_form.html Urra Field Centre, 2005. Accessed on 4th May, 2006, at: http://www.urra-enterprises.com/en/fieldcentre/index.html Read More