Geology The Pimenteiro Formation is made up of dense dark-grey shale sections interlayered with thick to thin sandstone, commonly found are also hummocky cross-stratification. The formation is interpreted as a transgressive system, deposited under a shallow marine environment with the influence of storm waves. It possesses two sub-units, the lower Picos Member and the upper Passagem Member. The Picos Member is composed by sandstone and intercalated siltstone and shale, with common hummocky cross-stratification, representing the first great Devonian transgression into the Parnaíba Basin. The sandstone is well defined in the old strata, of Middle Devonian origin, but it gradually fizzles out as it approaches the upper layers, of Late Devonian origin. The Passagem Member is composed by thin and micaceous sandstone, with asymptotic and hummocky stratification. This unit represents a system of fluvio-deltaic floods, in which turbulent, episodic flows went beyond the coastal limit. The formation has outcrops on the states of
Piauí and
Tocantins. Sediments on the eastern border of the formation are dated be from the
Late Eifelian to the
Early Givetian, while the central and western portions were dated to be from the
Late Eifelian to the
Early Famennian.
Paleoenvironment When it was deposited, the Pimenteiro Formation was a shallow marine environment under the influence of currents and storm waves, as well as fluvio-deltaic influences. The Passagem Member has been proposed to be influenced by episodic floods. During the Mid to Late Devonian, the formation was situated below the
antarctic circle, its fauna being part of the endemic
Malvinokaffric Realm. It haboured a diverse fauna of invertebrates such as
trilobites,
brachiopods,
ostracods,
bivalves,
gastropods,
hyolithids, and
tentaculids, as well as fish, like
elamobranches and
acanthodians. Plant remains are also found, as well as
ichnofossils. == Fossil Content ==