The Lower Ordovician oolitic ironstones of Valongo anticline have abundant chamosite
and/or siderite. True oolites are rare in the studied levels. Some pseudo-oolites are related to the
presence of algae, as evidenced by cathodoluminescence. Iron came from emerged areas and deposited
during transgression of Early Ordovician, in a shallow platform protected by barriers, with exhalativesedimentary
vent in reducing environment rich in organic matte
