Coastal morphology and heavy mineral accumulation in an upper-macrotidal environment – A geological-mineralogical approach from source to trap site in a natural placer laboratory (Channel Islands, Great Britain)

Abstract

The Isles of Jersey and Guernsey, GB, are situated in the Bight of St. Malo which ranks among the upper-macrotidal environments with one of the highest tidal ranges ever measured on the globe. They are built up of metasedimentary and –magmatic basement rocks followed by a suite of gabbro-diorite-granite intrusive rocks. Based on sedimentological, geomorphological and mineralogical investigations focused on heavy minerals (HM) the coastal environment has been subdivided into three principal coastal types headland-, headland-bay and bay types. Chemical and physical weathering, neotectonics, eustatic sea level changes, wave action, land-derived sediment supply and aeolian processes had not only a strong impact on the various coastal landform series (CLFS) established but also on the variegated HM assemblages casting them into the role of markers for the coastal environment and some kind of an ore guide for marine placer deposits. Tidal-dominated coastal environments under study sculptured exclusively out of crystalline bedrocks are conservative with regard to the economic geology and potential of HM placer-type deposits provided while physical weathering predominates over the chemical one. The tidal processes in context with the other land-forming processes are special importance for the diversity of coastal features within the environment hosting HM on a small- and large-scale. Neotectonics may accentuated or suppress tidally-controlled features in that strong uplift and provokes typical tidal features to get suppressed so that vast tidal flats are missing in a great deal of coastal areas and rocky coasts formed instead with consequences for the accumulation of HM. The “GMS tool (granulometry-morphology-situmetry) has methodologically proved to be very effective for the study of the coastal zones hosting HM in two principal ways. Granulometric and the morphological variation of HM can successfully be used for the host rock environment analysis (“ore guide”). Morphometric studies and situmetric measurements of gravel clasts in the coastal sediments bearing HM accompanied by similar measurement of joints and faults cutting through the bedrock can assist in the localization of HM trap sites in shoreline platforms and their rock veneer. The HM associations in upper-macrotidal environments are variegated in terms of morphology, mineralogical composition and their geomorphological trap sites so that they constitute a natural laboratory for environment analysis in terms of hydrodynamic and (paleo) climate along marine terrigenous shorelines and help better understand the way of HM from source to trap site in modern and paleoplacers.