Abstract:
The ooidal ironstones origin preserved in the sedimentary record has been subject of many discussions in the last decades. These deposits form prominent units in the Paleozoic record, and an assessment of their sedimentological and stratigraphic significance is essential for the interpretation of sedimentary successions. The ooidal ironstones from the Ponta Grossa Formation are associated with a major sequence boundary and subsequent transgressive surface. These ooidal ironstones were described in a borehole located in the north part of Paraná Basin. They were interpreted as they had been deposited under conditions of low net sediment accumulation with episodic storm events reworking the sediments, in a shallow-marine environment. Through petrographic studies, X-ray diffraction, and scanning electron microscope (SEM) analyses made possible the depiction of the ooidal ironstones diagenetic evolution. Such analyses indicate that the ooidal ironstone mineralogy was formed during eodiagenesis. The ironstones are dominated by berthierine ooids and grain-rimming and pore-filling siderite, with later ferroan calcite. The ooidal interval observed in the borehole presented wave cross lamination. This faciologic aspect reinforces the idea that the conditions required for the development of the ironstones deposits were intense sediment reworking and slow net sediment accumulation rates.
Keywords:
ironstone; diagenesis; sequence stratigraphy; Paraná Basin