%0 Journal Article
%A Gómez Gras, D.
%A Alonso-Zarza, Ana María
%T Reworked calcretes: their significance in the reconstruction ofalluvial sequences (Permian and Triassic, Minorca,Balearic Islands, Spain)
%D 2003
%@ 0037-0738
%U https://hdl.handle.net/20.500.14352/59027
%X The Permian and Triassic of Minorca (Balearic Islands) consists of a 670-m-thick, red, alluvial succession that includes insitu calcrete profiles and reworked calcrete material. In the Permian succession, the calcretes vary from laminar formsdeveloped on the Carboniferous basement to weakly developed nodular calcretes in fluvial sediments. The palaeosols in theTriassic 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 mosaicsof calcite/dolomite crystals, brecciated clasts, rhizolith fragments, and aggregates of clay and/or silt. These clasts appear inthree different types of deposits. Type 1 corresponds to lenticular bodies that fill small scour surfaces, and consists only ofintraformational conglomerates. These deposits are interpreted as ephemeral channels and sheet-floods that represent theinterfluvial drainage systems that captured only the precipitation falling on the alluvial plain. Type 2 includes sand dune 3-Dbodies with flat bottoms and convex tops. These bodies are about 20 cm high and 2 m wide, and were formed byfloodwaters that flowed down the levees of the major streams. Type 3 channel deposits contain reworked calcretes andextrabasinal 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 thealluvial plain.Variations in the aggradation rates of the floodplain resulted in five different infill stages. In the lowstand to earlytransgressive interval, as in stages I (P1) and IV (B1), the fluvial deposits filled palaeovalleys; calcretes and reworkedcalcrete deposits were of difficult formation (apart from terraces) and preservation. Accommodation space was at itsgreatest in the transgressive, stages II (P2) and V (B2). This caused the greatest aggradation of the floodplains, which areformed 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 stageIII (P3), highstand interval, because of the reduction of accommodation space, this could favour the formation ofcomposite 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 basinsand the construction of floodplains.
%~