Middle Jurassic concretions from Częstochowa (Poland) as indicators of sedimentation rates

Abstract

The Częstochowa Clay from south-central Poland contains numerous carbonate (sideritic and calcitic) concretions. Their sedimentary and diagenetic features allowed determination of the relative progress of diagenesis during their formation. Four categories of concretions are identified using such characteristics as: mineral composition, shape, presence of septarian structures, presence and preservation of macrofossils, evidence of bioerosion and benthic colonization. The concretion types recognized are: (1) early-diagenetic calcite concretions with evidence of reworking (hiatus concretions), (2) early-diagenetic fossiliferous calcite-concretions, (3) early-diagenetic siderite-concretions with septaria, and (4) later earlydiagenetic massive siderite-concretions. The reconstruction of early diagenetic environments is based on such factors as: the dynamics of compaction, isolation from oxygenated sea-water, and availability of carbonate ions. All these factors are attributed to changes in the rate of sedimentation. According to the proposed hypothetical mechanisms, the early-diagenetic concretions formed during periods of generally slower sedimentation than the later early-diagenetic bodies.