Published: Apr 1997
Pages: 85 - 98
Abstract: Pelagic chlorite and chlorite-hematite oncoids occur in red nodular limestones of the Toarcian and Oxfordian age, thought to represent intervals of condensed sedimentation. At a single locality, they were also found in an intraformational limestone breccia with fragments of a calcrete. Intraclasts, fragments of hardgrounds and belemnite rostra form oncoid nuclei. Wrinkled laminae and pseudocolumnar structures characterize their cortices. Encrusting foraminifers are important builders of the studied oncoids. Rare synsedimentary cracking of oncoids is indicated by the presence of neptunic microdykes. Sets of chlorite and hematite concentric laminae responded differently during compaction and tectonic processes. A peculiar phenomenon is the recrystallization (aggrading neomorphism) of the micrite admixture in hematite and chlorite oncoids; new-formed larger calcite grains contain the hematite pigment arranged in dendritic patterns. The hematite pigment also formed characteristic "teeth" in the early diagenetic calcite veinlets. The oncoids are formed by Fe-chlorite, polytype IIb. The origin of the chlorite was connected neither with volcanic activity nor with the initial metamorphism. The condensed sedimentation with mineralized oncoids was associated either with local conditions or with the sea level high stand.
Keywords: Western Carpathians, Jurassic limestones, chlorite oncoids, hematite oncoids, encrusting foraminifersDownload PDF document