The epicontinental Lower Jurassic of Poland

During the Early Jurassic times terrigenous, continental, marginal-marine and marine sediments up to 1400 m in thickness were deposited in a large epeiric basin extending across Poland. These strata are defined herein as the Kamienna Group, which is subdivided into 12 newly distinguished or re-defined lithoformations (Fm.). Two new members (Mb.) are also defined. Detailed study of exposures and 35 fully cored boreholes has integrated data from lithology, sedimentary structures, trace fossils, body fossils, boron content, clay minerals and palynology. This has allowed lithofacies description, recognition of depositional systems and subsystems and determination of their fluctuation in space and time leading to a high-resolution sequence stratigraphic analysis. Sedimentation in the shallow, epeiric Early Jurassic basin of Poland was particularly sensitive to reflect changes in sea level. Analyses of accommodation space within regular progradational successions associated with highstand systems tracts shows that the Early Jurassic basin in Poland was generally not deeper than some tens of meters, most frequently it was less than 20 m deep. Except for the ammonite-bearing Pliensbachian deposits in Western Poland, biostratigraphical resolution in marginal-marine and continental deposits is usually of a lesser precision. However, an internally consistent sequence stratigraphic scheme of Poland can be compared with fossiliferous marine sediments of the Ligurian cycle of United Kingdom and France. The minor sequences identified within the Ligurian cycle play very important role in correlation as they can be recognised in the Polish Basin, although they may show some differences in dating and range. In the Polish Basin, lowstand (LST) and falling stage systems tract (FSST) correspond with erosion/non deposition stages at the sequence boundaries. Concerning the range of sea level changes, the Exxon model was also adopted and ten of the Exxon Early Jurassic depositional sequences were identified in the Polish Lower Jurassic and are labelled I–X. Transgressive systems tracts prevail in sedimentary record and are represented either by retrogradational or aggradational facies architecture, and high-stand systems tracts are represented by progradational facies architecture. Parasequence boundaries (flooding surfaces) were defined based on careful regional/spatial facies analysis. A marginal-marine parasequence is usually more complex than a simple “quick floodinggradual prograding” model, conventionally interpreted in the marine basins. Due to common preservation of transgressive deposits within marginal-marine parasequences, besides the well-defined flooding surfaces the “parasequence maximum flooding surfaces” were distinguished. The best correlative horizons, particularly in the basin centre, are represented by sequence boundaries, while the maximum flooding surfaces or their correlatives are well identifiable in the marginal parts of the basin. Intensity and frequency of erosional processes in the marginal parts of the basin mean that depositional sequence boundaries are difficult to recognise because of “amalgamation” of stacked and highly reduced depositional sequences. Correlative significance is of transgressive surfaces is enhanced when they are coupled with their nonmarine correlative surfaces. Development of transgression with its coeval effects in continental deposits is discussed and non-marine correlative surfaces of the transgressive surfaces are documented. Once recognised and arranged, sequences and parasequences in the marginal basins (like the Polish Basin) can help to solve some problems concerning identification and range of major bounding surfaces and systems tracts in open marine basins in Europe, particularly in the Early Toarcian. The regional cross sections and cross sections of the whole Polish Basin showing dominant depositional systems and sequence stratigraphic correlation, as well as “time-tuned” palaeogeographical maps of the Polish Basin in Early Jurassic are presented. Subsidence varied through time along the Mid-Polish Trough, between the Holy Cross Mts and Pomerania. In Hettangian and Late Sinemurian the subsidence rate was higher in the Holy Cross Mts and lower Polish Geological Institute Special Papers, 12 (2004): 1–152 I dedicate this work to the Memory of Dr W3adys3aw Karaszewski (1911–2003), an outstanding scientist and a worthy Man. Pracê tê dedykujê Pamiêci Docenta Doktora W3adys3awa Karaszewskiego (1911–2003), wybitnego badacza i zacnego Cz3owieka. in Pomerania region, while in Early Sinemurian and Early Pliensbachian times the situation was opposite. Despite existence of some regional dislocation zones occurring along the edges of the Mid-Polish Trough, which shaped the sedimentation and sediment thickness contrast (for example the Nowe Miasto–I3¿a fault), a gradual decrease of sediment thickness outwards the axis of the Mid-Polish Trough prevails. Additionally, occurrence of conspicuous zones of increased subsidence, which are actually perpendicular to the Mid-Polish Trough (for example the “Mazurian Way”), there is no reason to regard the Mid-Polish Trough in any respect as a “rift basin”, at least in the Early Jurassic times.