TEM INVESTIGATION OF THE SHELL OF THE BRACHIOPOD <em>THECOSPIRA TYROLENSIS</em> (LORETZ): A CLUE TO UNDERSTANDING GROWTH AND REPLACEMENT OF PRISMATIC AND/OR FIBROUS LOW MG-CALCITE?

The shell of Thecospira tyrolensis (Loretz) is characterized by three distinct layers consisting of low Mg-calcite. The primary layer consists of microcrystalline calcite and it is often diagenetically altered. The secondary layer is composed of imbricate fibres subparallel to the valve surface. The tertiary layer (prismatic) is made of calcite prisms perpendicular to the outer epithelium of the mantle and was formed by ephitelial cells that reverted to inorganic secretion. For this reason, as well as its morphology, the prismatic layer can be com­pared to some pore-filling low Mg-calcites, therefore we would expect it to follow inorganic nucleation and growth theory. Transmission electron microscopy allowed to recognize crystal defects (sets of microtwins and dislocations) which, by creating abutting surfaces and kinks, represent favourable sites where new "calcite seeds" attach themselves to the crystal surface. Stacking of (hkil) planes where growth is faster (in particular the basal piane), favoured by twinning, accounts for the prismatic habit

Quantitative compositional analysis of a Triasic carbonate platform (Southern Alps, Italy)

We have estimated abundance and distribution of automicrite, marine cements and skeletal grains in the Triassic Sella massif, an isolated platform flanked by steep (25-35°) clinoforms. 108 samples were taken at constant intervals from measured sections of the major zones of the platform edifice: the platform top, margin-upper slope, and lower slope. In a first step, carried out in the field and on hand specimen, purely detrital deposits were separated from automicrite facies, i.e. beds with automicrite, cement-filled, primary vugs and admixtures of skeletal carbonate and lithoclasts. In the second step, samples with automicrite facies were thin-sectioned and point counted. The categories used for point counting were (a) automicrite, (b) vugs and cement, (c) microspar or neomorphic spar, (d) skeletal grains and (e) internal sediments. At the platform top 46% of samples are pure detrital deposits, 27% consist of automicrite facies and 27% are too strongly altered by dolomitization to allow classification. At the margin-upper slope 68% of samples consist of automicrite facies, 22% are pure detrital sediments and 10% are strongly altered. At the lower slope 63% are detrital deposits, 10% automicrite facies and 27% are extensively dolomitized. The most important contributors to the automicrite facies are automicrite (41% on the platform top, 29% on the margin-upper slope, 28% on the lower slope) and early marine cement (35% on the platform top, 48% on the margin-upper slope, 27% on the lower slope). The amount of skeletal grains is less than 10%. The automicrite facies stabilized the platform margin and upper slope. Automicrite, abundant early marine cements and micro-organisms such as Tubiphytes, formed a rigid framework, thus substituting for the lack of a metazoan reef. On the upper slopes, the framework of automicrite facies stabilized the slope but intermittently. The automicrite layers are frequently dissected by sediment-filled fractures or are broken into clasts. We assume that they slid on the layers of loose detritus. Bigger slides turned into rubbly debris flows that formed metre-thick breccias at the lower slope and the proximal basin floor. The planar shape and steep angle of the clinoforms indicate that the large-scale geometry of the slope was not controlled by the automicrite but rather by non-cohesive layers of sand and rubble piled up to the angle of repose. The production mode of the Sella is comparable of that of a (mud) mound factory. This factory was highly productive: in 1 Ma, the platform aggraded over 300 m and prograded over 2000 m in all directions. © 2001 Elsevier Science B.V. All rights reserved