The Upper Devonian to Lower Carboniferous deposits of southwestern Parnaíba
Basin, represented by the Cabeças and Longá formations, occur excellent outcrops in the
Pedro Afonso region, state of Tocantins, northern Brazil. The facies analysis of these deposits
allowed the identification of nine lithofacies divided into three distinct facies associations
(FA): FA1) lodgment subglacial deposits and FA2) delta front, both of Cabeças Formation,
and FA3) lower shoreface to offshore from Longá Formation. The FA1 consists of: massive
diamictite with granules and pebbles (sandstone, quartzite and granite) disseminated in a
muddy-sandy matrix; sheared diamictite with fragments of sandstone (1 cm to 5 m in
diameter), into of a muddy-sandy matrix, and sometimes cutted by normal faults. In some
cases, fragments are contorted and internally exhibit thrust faults and subhorizontal foliation.
The FA2 is composed of massive bedded sandstone with lobed geometry and ball-and-pillow
structure. The FA3 consists in shales and subordinate fine to coarse-grain and pebbly
sandstones. The pebbly sandstones exhibits massive bedding has been interpreted as
transgressive lag. The shales are interbedded with sandstones thin beds with ripples marks,
wavy-linsen bedding, cross lamination, hummocky cross-stratification and intense
bioturbation; starved megaripples occur in coarse to pebbly sandstones. Particulate organic
components in the massive diamictite of Cabeças Formation and shale of Longá Formation,
are represented by prasinophytes algae, acritarchs, spores, chitinozoans, opaque and nonopaque
(non-biostructured, biostructured and cuticles) phytoclasts and amorphous organic
material (AOM and resin). These components, named Palynofacies A, corroborate the coastal
environment of Cabeças Formation. Likewise the offshore environment (distal shelf) disoxicanoxic
is confirmed to Longá deposits corresponding to Palynofacies B. Trace fossils of
Longá Formation are represented by ichnospecies Bergaueria isp., Cruziana isp.,
Helminthopsis abeli, Lockeia isp. and Palaeophycus isp., all of them been icnofacies
Cruziana. The studied succession recorded that during the Neodevonian-Eocarboniferous in
southwestern Parnaíba Basin, a fluvial-deltaic system implemented by ice-melting was
sucessively reworked by advance and retreat of coastal glaciers (Cabeças Formation). After
this the coastal glacial succession was succeeded by post-glacial transgression of (Longá
Formation).
|
