In the Oriola depression of Southern Portugal near the town of Evora, field
examinations revealed the presence of Tertiary sedimentary deposits that had
the consistency of duricrusts. These duricrusts, occurring close to the land
surface, were examined in the field as well as in the laboratory, with the
objective of establishing their composition and formation. Micromorphological
examinations showed that the duricrusts were composed of clasts of fine sand
to fine gravel sizes cemented together by a matrix dominated by either
palygorskite or carbonate. The matrix : clasts ratio varied from about 1 : 3
when the cement was carbonate, to 3 : 1 or higher when the matrix was
palygorskite. Occasionally the ratio in the latter was even higher. The clasts
consisted of quartz, feldspar, and some Mg-rich metamorphic minerals. The
palygorskite matrix fibres were arranged in mats within which they had
parallel orientation. The mats or ‘domains’, which had a length of
15–25 □m and a width of about 40 □m, showed random
orientation. In analogy to the term ‘calcrete’ the term
‘palycrete’ is used for the palygorskite duricrusts. The
palycrete, of an average thickness of 0.5 m, frequently rested directly on the
Hercynian basement rocks, and was covered by a recent solum. In the B horizons
of the solum, the palygorskite appeared to undergo recent weathering and
transformation into smectite. The duricrusts were proposed to have formed by
authigenic calcite or palygorskite, which filled the interstices between
clastic particles that had been deposited on Hercynian basement rocks
following their peneplanation in the early Tertiary. The proposed environment
of deposition and formation was that of an intermittent playa-lake in a
semi-arid, seasonal climate, where strong evaporative processes had been
active. Alteration and weathering of the mafic minerals contained in some of
the clasts, in addition to interstitial solutions rich in Si and Mg, had
created the chemical environment required for palygorskite neoformation. From
the state of weathering–disintegration of the palygorskite in the soils
formed on the palycrete, it is inferred that the environment for palygorskite
neoformation had ceased to exist after palycrete formation.
Sediment Transport of Fine Sand to Fine Gravel on Transverse Bed Slopes in Rotating Annular Flume Experiments
