Sequence stratigraphy and cyclic development of Basal Zechstein carbonate-evaporite deposits with emphasis on Zechstein 2 off-platform carbonates (Upper Permian, Northeast Germany)

Abstract

Although the Basal Zechstein is classically subdivided into two cycles (Z1 and Z2, lower part) it can, alternatively, be subdivided into three complete third-order depositional sequences (ZS1–ZS3) and the associated lowstand systems tract/transgressive systems tract of a fourth sequence (ZS4). Third- and higher-order sea-level fluctuations influenced the depositional and diagenetic history of the carbonate-evaporite alternations within the Basal Zechstein. The main subject of the present study, the Zechstein 2 Carbonate (Stassfurt Carbonate, or Ca2), is interpreted to encompass both transgressive (predominantly in slope and basinal settings) and highstand systems tracts of Zechstein sequence ZS3 as well as part of the lowstand systems tract of the fourth Zechstein sequence (ZS4). In northeastern Germany, hydrocarbon reservoirs of the Ca2 developed along both northern and southern margins of the Southern Permian basin on top of the topography of the Zechstein 1 cycle (Z1). This Z1 topography resulted from deposition of the Werra Anhydrite (A1), the highstand systems tract of the second Zechstein sequence (ZS2). The northern Ca2 carbonate platform is consistently narrow and fringed by coated-grain and composite-grain bars and shoals. It is characterized by a steep, fault-controlled slope which precluded the formation of in situ shallow-water, grain-dominated carbonates during relative sea-level lowstands. Displaced shallow-water coated-grain and composite-grain packstones/wackestones may, however, occur as slope-fan deposits (turbidites). In contrast, the southern carbonate platform is wide, tectonically dissected and fringed by grain-dominated shallow-water carbonates (coated-grain and composite-grain bars and shoals). Zechstein 2 shallow-water carbonates, detectable by Basal Zechstein seismic anomalies, also developed on pre-existing topographical highs in off-platform positions. Three depositional cycles (Ca2 cycles I, II and III), most probably reflecting high-amplitude, fourth-order sea-level fluctuations, can be identified within the Stassfurt Carbonate. Our observations suggest that sea-level variations, possibly reflecting cyclicity in the Milankovitch frequency band ( 0.1 0.4 Ma ?), and the pre-Zechstein topography (pronounced during formation of the Werra Anhydrite) were the most important factors in controlling the depositional as well as the diagenetic history of the Zechstein 2 Carbonate in off-platform positions.