Sedimentation of Lower Toarcian (Lower Jurassic) brackish deposits from the Częstochowa-Wieluń region (SW Poland)

The Lower Toarcian Ciechocinek Formation from the Czestochowa-Wielun region is composed of poorly consolidated mudstones, claystones and siltstones with subordinate sandstone intercalations. The succession is represented by twelve lithofacies grouped into seven facies associations of a shallow brackish marine basin with an embayment character. The sedimentation was dominated by clay and silt deposition from low-density suspension flows, spreading out from river mouths, and from suspension clouds whirled up during storms. Sand intercalations mark episodes of storms, removing coarser material from nearshore to more distal settings, migration of elongated sand bars, as well as sedimentation at river mouths and on sandy shoals. The deposition was affected by three main factors: (1) progradation and displacement of river mouths; (2) river discharge fluctuations; and (3) short-term weather changes.