The melt-bearing impactites of the Ritland structure, Norway-Implications for melt formation in small impact craters

摘要

A melt-bearing impactite unit is preserved in the 2.7 km diameter shallow marine Ritland impact structure. The main exposure of the melt-bearing unit is in an approximately 100 m long cliff about 700 m southwest of the center of the structure. The melt and clast content vary through this maximum 2 m thick unit, so that lithology ranges from impact melt rock to suevite. Stratigraphic variations with respect to the melt content, texture, mineralogy, and geochemistry have been studied in the field, and by laboratory analysis, including thin section microscopy. The base of the melt-bearing unit marks the transition from the underlying lithic basement breccia, and the unit may have been emplaced by an outward flow during the excavation stage. There is an upward development from a melt matrix-dominated lower part, that commonly shows flow structures, to an upper part characterized by more particulate matrix with patchy melt matrix domains, commonly as deformed melt slivers intermingled with small lithic clasts. Melt and lithic fragments in the upper part display a variety of shapes and compositions, some of which possibly represent fallback material from the ejecta cloud. The upper boundary of the melt-bearing impactite unit has been placed where the deposits are mainly clastic, probably representing slump and avalanche deposits from the modification stage. These deposits are therefore considered sedimentary and not impactites, despite the component of small melt fragments and shocked minerals within the lowermost part, which was probably incorporated as the debris moved down the steep crater walls.