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Material name: Bj4a
Synonyms: N/A
Material (geologic): Middle Jurassic (Late Bajocian) flint/chert.

Detail of Bj4a wackestone
Foto: Jehanne Féblot-Augustins, 2005.

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Material description

Mode of occurrence: Occurring in continuous beds 5 to 6 cm thick, highly jointed, jutting out from the compact light brownish grey to light reddish tan limestone. Upper layers of an outcrop at the top of a cliff face.
Cortex: Fresh, rough, grey-white, 1 cm thick, transition sharp.
Colour: Blue-grey (N6).
Pattern: Featureless, rare pale-coloured specks.
Appearance: Homogeneous material, although highly jointed, dull and opaque, moderately smooth, medium to fine-grained.
Structure: No particular structure.
Texture: Wackestone.
Matrix: Opaque.
Grains: 30 to 40 %, white, pale yellow, ochre, black, rounded with high to low sphericity; main grain size 100 µ, with a few grains ranging between 250 and 1000 µ.
Grain composition: Very numerous sponge spicules and foraminifera (frequent black "ghosts" outlined by impurities), pelecypod fragments, serpulids, dispersed entrochi and ooliths, rare pellets, rare Cyclostoma bryozoans.
Further information: Click here for the original French description (opens in a new window), or here for more information on the attributes used in the description (use your browser's "BACK"-button to return here).
Citing: Information on citing this page can be found here.

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Balme à Juliette 1
Locality: Ambérieu-en-Bugey / Les Balmeaux - Balme à Juliette 1, France, Ain department
Synonyms: Sampling site Bugey 4 after Féblot-Augustins
Geographical description: The sample was taken from the upper flint-layers at the top of a cliff approx. 1 kilometre to the east of Vareille, a hamlet at the eastern side of Ambérieu-en-Bugey. For a more precise explanation of the local situation, see the description under Bt4
Geographical co-ordinates: east/x 836.4
north/y 2111
(Lambert zone II, NTF)

This corresponds roughly with
Lat. 45° 57' 29" N
Long. 005° 23' 13" E

Co-ordinate precision: The coordinates given were taken from a large-scale map, and are within 50 metres of the actual exposure.
The latitude/longitude coordinates are a direct transformation of these data on WGS84 and are accurate to the same level. The high precision to the second is given to prevent the introduction of a rounding-error.

The location can be found on the 1:25 000 topographical map sheet 3130 E of the French Institut Géographique National.

Geology: According to the geological map, the parent-rock belongs to unit J1c of late Bajocian age.

Map of all sampled localities of Bajocian flint in the Bugey (PDF, 190 KB, opens in new window).

Type of source: Primary.
Other types of flint: In the same exposure the middle and lower banks of flint consist of the coarser Late Bajocian varieties Bj4b and Bj4c.
  Bj4a chert with cortex
Foto: Jehanne Féblot-Augustins, 2005
Flake of Late Bajocian flint
Foto: Jehanne Féblot-Augustins, 2005


Last modified on:
November 15, 2005
Contents primarily by:
Jehanne Féblot-Augustins
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