Sedimentary
Processes in the Cambrian - Ordovician mudrocks, Famatina Range, northwestern
Argentina
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ESTEBAN, S. B.
Fine-grained
sedimentary rocks occur in the Lower Member of the Volcancito Formation
(Latest Cambrian-Early Ordovician) of the Famatina Range, northwest Argentina.
These deposits are characterized by their finely laminated structure and their
dark color (dark grey to black). This member is 160 m thick and includes
shales and marls interbedded with numerous storm layers deposited in a shelf
environment. The presence of only a few unequivocal hummocky
cross-stratified beds and dominance of shales and marls indicates that most
of the Lower part of the succession (Latest Cambrian) was deposited below,
storm wave base.
Shales are
characterized by alternating light and dark very fine, parallel, ungraded
laminae. Laminae are generally less than 0.5 mm tick. The light layers are
thinner, usually quartz-rich, with scarce clay and organic matter. These
laminae occasionally are undulated or thin laterally. Grain-size of quartz
ranges from very fine- to fine-grained silt. Dark colored laminae are thicker
than light colored laminae. Clay and organic-matter occur as continuous
deposits of dark brown color.
Marls are formed by
parallel to wavy, 0.5 to 1 mm thick, light and dark laminae. Small scale
cross-lamination has been observed in the marly interval. Light layers are
composed of silt-grade carbonate and quartz. Small fossil fragments are
preserved within laminae. The dark colored laminae contain abundant
organic-rich micrite material and minor proportions of very fine-grained silt.
Lamination type, and fabric features have been used
to interpret the dominant depositional processes, Thin, non-graded and
parallel laminated layers record