Calculations of salt basin depth and deposition time of salts, exemplified by the Oldest Halite (Na1) unit from the Zechstein PZ1 cycle at the Puck Bay (northern Poland)

Abstract

Defining of the depth of a past evaporite basin is a very disputable problem due to lack of recent environmental equivalents of fossil evaporite giants. A simple geometric method of calculation of the initial depth of chloride basin - applied for low tectonized stratiform successions - was proposed based on facies analysis of both chloride (halite) and sulphate (anhydrite) deposits, being the major products of each evaporite cycle. Estimated depths of several minor basins, composing the marginal part of the large Zechstein (Upper Permian) salt basin of the Oldest Halite (Na1) in northern Poland (at Puck Bay) varied from 110-112 m in the southern (Lisewo) and north-eastern (Jastrzebia Góra) basins to 60 m for the landward, western (Debki) one. Deposition time for both sulphate and halite deposits of the Oldest Halite (Na1) unit from the study area was calculated using their estimated deposition rates and counting of "annual cycles" reflected by rhythmic sulphate intercalations in halites (the concept of Richter- Bernburg, 1957, 1959). Estimated values of deposition time - with the annual rhythms - for the most complete successions of deep-water salts were variable for any of the evaporate basins under study and they varied from the minimum of 933 years (Debki basin) to the maximum of 2771 years (Lisewo basin). This last value is comparable to the time value (2266 years) calculated for the thickest deep-water salt profile (Bia3ogarda IG 1 well) at the minimum deposition rate, so the deposition of the preserved Oldest Halite complex could probably last > 2700 years. Analogous calculations of deposition time for main evaporite (sulphate and salt) units of the Polish Zechstein basinal succession (PZ1, PZ2 and PZ3 cycles) evidenced that these deposits, constituting ca. 72% of the whole Zechstein profile, presumably required from 0.28-0.39% (at maximum precipitation rate) to 2.5-3.6% (at minimum precipitation rate) of the estimated Zechstein duration. Consequently, the most (> 96%) of Zechstein time was probably dedicated to accumulation of 1/3 of the Zechstein sequence (dominantly clastics) and periods of erosion and/or non-deposition