Publication: Reworked calcretes: their significance in the reconstruction of
alluvial sequences (Permian and Triassic, Minorca,
Balearic Islands, Spain)
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Publication Date
2003
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Elsevier
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Abstract
The Permian and Triassic of Minorca (Balearic Islands) consists of a 670-m-thick, red, alluvial succession that includes in
situ calcrete profiles and reworked calcrete material. In the Permian succession, the calcretes vary from laminar forms
developed on the Carboniferous basement to weakly developed nodular calcretes in fluvial sediments. The palaeosols in the
Triassic are mostly dolomitic, and the profiles reach up to Stage III of soil development (Spec. Pap.-Geol. Surv. Am. 203,
(1995) 1). The clasts, formed through reworking of the palaeosol profiles, are about 0.5–10 cm across and include mosaics
of calcite/dolomite crystals, brecciated clasts, rhizolith fragments, and aggregates of clay and/or silt. These clasts appear in
three different types of deposits. Type 1 corresponds to lenticular bodies that fill small scour surfaces, and consists only of
intraformational conglomerates. These deposits are interpreted as ephemeral channels and sheet-floods that represent the
interfluvial drainage systems that captured only the precipitation falling on the alluvial plain. Type 2 includes sand dune 3-D
bodies with flat bottoms and convex tops. These bodies are about 20 cm high and 2 m wide, and were formed by
floodwaters that flowed down the levees of the major streams. Type 3 channel deposits contain reworked calcretes and
extrabasinal clasts, which overlie erosive surfaces and are found in layers within cross-bedded sandstones and conglomerates.
These are interpreted as channel-floor lag deposits of major channels that entered from distant uplands and drained the
alluvial plain.
Variations in the aggradation rates of the floodplain resulted in five different infill stages. In the lowstand to early
transgressive interval, as in stages I (P1) and IV (B1), the fluvial deposits filled palaeovalleys; calcretes and reworked
calcrete deposits were of difficult formation (apart from terraces) and preservation. Accommodation space was at its
greatest in the transgressive, stages II (P2) and V (B2). This caused the greatest aggradation of the floodplains, which are
formed of thick sequences of fine-grained sediments, isolated meandering channels, weakly developed calcretes
(compound) and reworked calcrete deposits, mostly of types 1 and 2. The density of channels notably increased in stage
III (P3), highstand interval, because of the reduction of accommodation space, this could favour the formation of
composite or even cumulative palaeosols, but of difficult preservation. Reworked calcrete deposits are mostly of type 3,
but types 1 and 2 are also recognised. The reworked calcrete deposits are an important part of the Permian and Triassic fluvial sediments and their occurrence and characteristics are important in order to interpret the infill of terrestrial basins
and the construction of floodplains.