Francisco López-Aguayo

Empirical model of morphodynamic beachface behaviour for low-energy mesotidal environments

Abstract Four years of monthly monitoring were carried out on a South Atlantic beach in Spain, in a low-energy mesotidal environment where beaches change slowly, from reflective to dissipative states, following a typical seasonal behaviour. In this work a two-dimensional study of beach morphological variations has been made, related to the incident wave variations. After applying several morphodynamic parameters to the field data, poor or null representative results were obtained. It was then necessary to design other type of characterisation of the incident wave energy. In this sense, the intertidal normalised beach slope was compared with the erosive potential of the incident waves, expressed as a combination of the dimensionless grain fall velocity parameter ( Ω ) and the energy density of waves. Median grain size did not vary significantly during the surveys. For this reason, a new parameter, named wave erosivity factor , was introduced by considering the fall velocity of grains as a constant in the Ω parameter. The resulting ratio between normalised beach slope and wave erosivity expresses the equilibrium state of the beach for any given energy level. The departure from the equilibrium curve is largest in the intermediate situations, while at the extremes the points are better adjusted to asymptotic tendencies towards equilibrium: on reflective states, small increases in the wave erosivity will produce important beach changes; on dissipative beaches, important increases in the wave erosivity will not produce significant morphological modifications. The resulting equilibrium curve is presented as a function of the natural range of morphological variation of this beach.

Long-shore distribution of morphodynamic beach states along an apparently homogeneous coast in SW Spain

We present a morphodynamic study of an apparently homogeneous rectilinear coast in SW Spain. The study area covers 14 km of mesotidal sandy beaches, interrupted in some places by rocky-shore platforms. The method used consisted of a monthly monitoring of 12 beach profiles during two years. According to the results obtained from the study, which also include granulometric analyses andin situ determination of the beach disturbance depth, three main beach classes have been differentiated: low-reflective beaches, dissipative beaches and rocky-shore platform beaches. Their longitudinal distribution is not linked to their distance to the main source of sediments in the area (mouth of the river Guadalquivir). Instead, a very irregular long-shore variation of morphodynamic beach states appears. It is deduced that this long-shore variation is mainly linked to local contouring conditions (e.g. the presence of rocky shoals which affect wave-breaking processes), and not to the regional long-shore currents prevailing in the zone.

Polytypism of cookeite in low-grade metapelites of the Cameros Basin, Spain: Lack of correlation of well-ordered polytypes with pressure

Abstract Cookeite occurs in low-grade, chloritoid-bearing metapelites of the Cameros Basin, northern Iberian Range, Spain, as thin coatings on poikiloblastic pyrite crystals <25 cm on edge. Textural relations imply formation via hydrothermal fluids in a syn- to late-regional metamorphic environment. Selectedarea electron diffraction patterns of cookeite display a wide range of stacking order-disorder, from patterns with streaking parallel to c* in k ≠ 3n reflections to sharply defined reflections having one-, two-, and greater than two-layer periodicities. Well-ordered polytypes in crystals up to 0.1 µm thick are of type Ia, as determined by powder X-ray diffraction. Coexisting chamosite likewise displays a range of order in polytypism from complete disorder to the well-ordered IIb polytype which is typical of low-grade metamorphism. Contrary to assertions that ordered cookeite polytypes are indicators of medium- to high-pressure metamorphism, the occurrence of ordered stacking sequences is direct evidence that, although ordered cookeite may occur in medium- to high-P environments, it is equally indicative of low-P environments. It can therefore not be used as an indicator of high pressure. The occurrence of several ordered and disordered polytypes in the same sample is compatible with the generally accepted notion that freeenergy differences between polytypes are too small to be indicators of specific stable-equilibrium P-T conditions. The general occurrence of semirandom stacking is attributed to a lack of equilibrium in structure that is generally characteristic of phyllosilicates in low P-T environments of formation.