Sedimentological and palaeoenvironmental studies in the Broadford Beds (Hettangian-Sinemurian) of north-west Scotland

Planorbis to Turneri Zone sedimentary rocks were studied at outcrops on Skye, Raasay, Applecross, Ardnamurchan, Morvern and Mull. A proposed lithostratigraphy includes two Groups, four Formations and seventeen Members. Increased faunal diversity in Mull and Skye reflects environmental stability during the Angulata and Semicostatum Zones; low diversity in the Bucklandi and Turneri Zones signifies lower stability. Species abundance in Mull and their decrease in Skye during Angulata-Bucklandi Zone times signifies a northward increase in environmental instability. Smectites are present in the Planorbis-Bucklandi Zone shales. The Semicostatum-Turneri Zones are composed of non expanding clays; the smectite formed by erosion of previously exposed alkaline igneous rocks under warm, alkaline, low rainfall conditions. Progressive transgression covered the source area and a change to more acidic conditions with higher rainfall also promoted the formation of "ironstones" and chlorites. The Angulata and Bucklandi Zones comprise bioclastic, oolitic, carbonate and siliciclastic sediments signifying shallow marginal marine semi-enclosed basins. Thin laterally impersistent beds of coarse,poorly sorted pebbles signify an unstable hinterland; during the Semicostatum and Turneri Zones fully marine conditions were established. Ferruginous beds are oolitic in Skye. They probably formed as "mud balls" and due to acid leaching of feldspars. Phosphatic nodules are calciumhydroxyapatite. The limestone/shale "rhythms" are explained by a combined primary and diagenetic origin. The limestones consist of low-Mg calcite Both ferroan and non ferroan calcite are present. Tectonic stresses and diagenesis deformed crystals and formed veins. Sr+2 values show a decline as the carbonate fraction increases. Three different ranges are distinguished for three facies and ages. Sr concentration is bimodal and probably inherited from the original sediments. The northern and southern basins evolved separately, sedimentation was controlled by differential tilting, uplift and subsidence. Palaeoslopes were gentle, to the west-northwest and south-southwest. Local topography (e.g. Central Strath High) modified the slopes; no movement occurred along the main faults of the area. Sedimentation relates to the "taphrogenic rifting stage" proposed for the North Sea Mesozoic.</p

Sedimentological and palaeoenvironmental studies in the Broadford Beds (Hettangian-Sinemurian) of north-west Scotland

Planorbis to Turneri Zone sedimentary rocks were studied at outcrops on Skye, Raasay, Applecross, Ardnamurchan, Morvern and Mull. A proposed lithostratigraphy includes two Groups, four Formations and seventeen Members. Increased faunal diversity in Mull and Skye reflects environmental stability during the Angulata and Semicostatum Zones; low diversity in the Bucklandi and Turneri Zones signifies lower stability. Species abundance in Mull and their decrease in Skye during Angulata-Bucklandi Zone times signifies a northward increase in environmental instability. Smectites are present in the Planorbis-Bucklandi Zone shales. The Semicostatum-Turneri Zones are composed of non expanding clays; the smectite formed by erosion of previously exposed alkaline igneous rocks under warm, alkaline, low rainfall conditions. Progressive transgression covered the source area and a change to more acidic conditions with higher rainfall also promoted the formation of "ironstones" and chlorites. The Angulata and Bucklandi Zones comprise bioclastic, oolitic, carbonate and siliciclastic sediments signifying shallow marginal marine semi-enclosed basins. Thin laterally impersistent beds of coarse,poorly sorted pebbles signify an unstable hinterland; during the Semicostatum and Turneri Zones fully marine conditions were established. Ferruginous beds are oolitic in Skye. They probably formed as "mud balls" and due to acid leaching of feldspars. Phosphatic nodules are calciumhydroxyapatite. The limestone/shale "rhythms" are explained by a combined primary and diagenetic origin. The limestones consist of low-Mg calcite Both ferroan and non ferroan calcite are present. Tectonic stresses and diagenesis deformed crystals and formed veins. Sr+2 values show a decline as the carbonate fraction increases. Three different ranges are distinguished for three facies and ages. Sr concentration is bimodal and probably inherited from the original sediments. The northern and southern basins evolved separately, sedimentation was controlled by differential tilting, uplift and subsidence. Palaeoslopes were gentle, to the west-northwest and south-southwest. Local topography (e.g. Central Strath High) modified the slopes; no movement occurred along the main faults of the area. Sedimentation relates to the "taphrogenic rifting stage" proposed for the North Sea Mesozoic.</p