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

Majewski W.

Acta Geologica Polonica

2000

Vol. 50, no. 4

431-439

The Częstochowa Clay from south-central Polandcontains 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 early-diagenetic 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.