Zechstein 1 and 2 Anhydrites: facts and problems of sedimentation

Abstract

Summary The distribution and lithology of the anhydrite rocks of Zechstein 1 and Zechstein 2 were significantly influenced by the contemporary geography of the basin. Thick ‘pletholitic’ shallow-water deposits with large-scale synsedimentary derangements lie near the margins and pass laterally, with steep boundaries, into thin laminated deep-water rocks on the basin floor. Five sub-cycles (I–V) have been recognized in cored boreholes in the Z1 Werraanhydrit throughout the German Zechstein basin and permit correlation with the Zechstein 1 sequence of the southern North Sea Basin; although not complete in every place, each sub-cycle generally comprises flaser anhydrite overlain by laminated anhydrite. The abrupt changes of facies at the contacts between the sub-cycles cannot be attributed to eustatic sea-level changes but are caused by changes of the climate, water currents and related effects; the periodicity must have resulted from factors originating outside the basin. The following new adjectives are proposed, primarily for varieties of anhydrite although they may prove to be equally applicable to other sedimentary rock types: ‘Pletholitic’—from the Greek ‘plethos’, an indefinable mass—applicable to thick anhydrite rock with a diffuse, cloudy or chicken-wire structure and fabric but with little overall stratification. ‘Stratobolic’—from the Greek ‘bolos’, lump—anhydrite rock with extremely irregular small-scale and medium-scale bedding but with a strong sense of stratification en masse . This term includes ‘flaser’, which is mainly applied to stratobolic rocks with bedding of an exceptionally anastomosing character. ‘Lamellitic’—from the German ‘Lamellenanhydrit’—unusually uniformly and evenly laminated anhydrite, including some varved anhydrite. This term is proposed because the word ‘laminated’ is used in a range of senses and does not adequately stress the uniformity and continuity of bedding in some thinly and evenly layered anhydrite rocks.