Material and Methods

Detailed sedimentological approach of the Serra do Caeté Formation from outcrops and cores data has led to the development of more conspicuous stratigraphic and depositional model for the study area. All samples were described and/or collected on Caete hill near to Mirassol d’Oeste and Porto Esperidião towns. There outcrop description was performed by vertical profiles along the Caeté hill that allowed the exposed stratigraphic control.

18 4. Results

Serra do Caeté Formation

The Serra do Caeté Formation is outcropped along the Caeté hill. There, this unit is 120m thick usually be covered by soil and vegetation that contribute to the scarcity of good expositions. The sedimentary rock strata formation of the Caete hills presents edge craggy (cuesta) and the relatively flat on top, oriented according to NE-SW and dip varying from 5º to 20º SE.

The succession rocks of the study area allow us to describe three distinct siliciclastic facies association, which are grouped as: bituminous fine-grained, conglomerate and rhythmites of sandstone and mudstone (Fig. 3.2).

Figure3.2 - Stratigraphic column of the Serra do Caeté Formation from Porto Esperidião (PE) to Mirassol d’Oeste (MO).

19 4.1. Bituminous fine-grained facies

The bituminous fine-grained facies is a siliciclastic rock only observed at the subsurface, about 45 m depth. It is composed of layers up to two centimeters of bituminous shale with layers of sandstones or even conglomerates and breccias. The most common structure found was parallel bedding, but we can also find low angle cross-lamination and deformations caused by dropstones (up to 5 cm length) (Fig.3.3). There are also secondary structures like small calcite-filled fractures which are oblique to the bedding.

Curiously, the lenses of sandstone in the bituminous shale could be show a marked presence of sulfides (i.e. pyrite) which are disseminated throughout the lenses. These sandstones are fine- to medium-grained size, gray and black in color, moderately sorted.

Added to, the presence of pyrite might also be observed in the contact of the sandstone and shale and thin beds of conglomerates. The conglomerate is matrix-supported with no orientation of the clasts, often gray and green in color and polymictic. The clasts vary from granules to pebbles reaching up to 10 cm length. There is a strong odor of hydrocarbon when the bituminous shale is placed in direct contact with fire. This fact could be suggestive of some fossil association. However, there is no trace of any fossiliferous association observed in bituminous shale and sandstone.

Figure3.3 – Betuminous facies: A) Scour structure (at the top) and dropstone (at the bottom) with deformation of the bedding; B) pyrite crystals along the bedding; C) low-angle cross lamination.

Py

B

C

A

20 4.2. Conglomerate facies

The intermediate portion of the Serra do Caeté Formation is composed by the conglomerate and sandstone beds. The facies of this association are outcropped along exposition up to 20 m height. The conglomerate is polymictic, gray in color, varies up to boulder (40 cm length) into a hard sand-grained matrix (Fig. 3.4). The clasts are supported by the matrix, which are poorly to very poorly sorted, with a subangular to angular roundness and to a great extent faceted, but poorly striated. Their fractions are predominantly in pebble, granule and gravel with a great lithological and erosional diversity features such as rounding the edges of the clasts and low sphericity. When grouped into rocks provenience, their source very likely come from igneous rocks (granite), metamorphic rocks (gneisses) and sedimentary rocks (sandstones) amid an open framework in which the clasts show no orientation.

Figure3.4 – Conglomerate facies: A) polymictic conglomerate outcrop with faceted clasts; B) striated clast; C) faceted pebbles with well-rounded boulder; D) faceted pebbles with grooves.

It was randomly collected 366 clasts that vary from granules to pebbles from Caeté hill and described following the rounding, sphericity, lithology, striated features and the presence of faces and edges. The lithologies of the clasts were grouped in metamorphic 42%, quartz 24%, granite 13%, sandstone 19 and mudstone 2%. The results shows that 92% of the clasts are faceted, but almost do not present striated features and only 4%

A B

C D

Sandstone

21

showed poorly some striated features (Fig 3.4B). The rounding presented 23% angular, 46% sub-rounded, 19% rounded and 12% well rounded.

Sandstone beds could be observed along the conglomerate facies and comprises an fining-upward character to this facies association. The sandstone is fine and presents yellow color, sometimes whitish, moderately sorted, and micaceous. This facies is massive in structure, but sporadically presents ripple marks and some bedding with normal graded character or even might expose the parallel lamination (Fig. 3.5). The gravel-grained clasts eventually occur in these beds. Very friable texture also occurs in the sandstone when the rock is altered by surface weathering, but it is restricted to local portions.

Figure3.5 – Sandstone facies: A) normal graded sandstone with scours at the bottom; B) sandstone with expressive ripple marks.

The sandstone petrography shows fine- to medium-grained sand with the pore space filled by sericite and/or silica cement. There are oriented trend quartz grains and subordinate feldspar amid a matrix composed of clay minerals (<10% modal) and cement. The quartz grains (90%) are monocrystalline and shows wavy extinction, poorly sorted, range from angular to sub-round with a predominance of angular type.

The feldspar crystals are represented by anhedral to occasionally subhedral forms (tabular).

22

4.3. Rhythmites of sandstone and mudstone facies

The rhythmites of sandstone and mudstone are the most representative of the Serra of Caeté Formation and presents greatest vertical extent among the three facies associations added to significant content in iron and phosphate which led previous works to described them as a kind of iron formation (Fig. 3.6). This facies contains layers of sandstones, small layers of conglomeratic sandstones, siltites, shales, diamictites (up to 30cm). The rhythmites of sandstone and mudstone are estimated about 50 m thick in outcrop which shows laminated parallel beds to low-angle cross-laminated set which dips toward SE. The cementation is iron oxide (hematite/goethite) that color ranges from red to yellow.

In terms of structures, the rhythmites of sandstones and mudstone are layered and exhibit normal graded character, cut and fill structures, ripple marks, dropstones, small waves in parallel lamination and cross lamination with low-angle truncation, convoluted folds, as major primary structures. Secondary structures occur as small fractures filled by ferruginous materials, which are crossing obliquely the primary lamination.

The beds organization occurs as millimeter to submillimeter pairs of clear and another dark layers setting the sedimentary layering. The clear pair is fine-grained sand with bedding marked by partial orientation of grains of quartz and feldspar. Overall, the grains are angular while the largest ones are sub-angular. The dark layer is composed of dark clay minerals and poorly oriented grains of quartz and feldspar. Quartz grains are monocrystalline with undulatory extinction, poorly sorted and range from angular to subrounded with predominance of angular ones. The grains of K-feldspar are represented by rounded grains of microcline and orthoclase. The clay minerals represent over 10% occurring between the quartz grains in the form of cement. The accessory minerals such as sericite and biotite (probably detrital particles) occur in both layers.

However crystals of biotite and opaque minerals are very fine and rare generally dispersed throughout the rock.

It was not possible to identify in thin section phosphate minerals indicators such as apatite crystals.

23

Figure3.6 – Rhythmites of sandstone and mudstone facies: A) general aspect of this facies association up to 3 m thick; B) syn-depositional deformation caused by dropstone of granite; C) structures of wave action (undulation, planar cross lamination, cut-and-fill and bidirectional lenses;

D) rare cm-scale hummocky cross-stratification in rythmites of sandstone and mudstone. A and B show the general aspect of the higher iron content portion of this facies association.