Miocene gypcretes from the Calama Basin, northern Chile

Adrian J. Hartley*, Geoffrey May

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

33 Citations (Scopus)


Gypcretes of Miocene age are preserved beneath a 9·53 ± 0·36 Ma ignimbrite along the eastern margin of the Oligo-Pleistocene Calama Basin, northern Chile. They are restricted to a single stratigraphic horizon developed within laterally extensive (>35 km) coalesced alluvial fan deposits, developed along the margin of an endorheic basin. Two types of gypcrete are recognized. Type 1 comprises almost completely gypsum-cemented sandstones containing alabastrine nodules and columns, subvertical and horizontal veins of fibrous gypsum and 'v-shaped' cracks infilled by clastic material, and are interpreted as surface weathered gypsic crusts. Type 2 gypcretes are composed of massive, reddened poikilitic and mesocrystalline gypsum (up to 80% of the rock) with isolated bedding-parallel, clast-rich lenses (200 × 30 cm) and sub-vertical veins of fibrous gypsum. The massive texture resembles that of well developed B horizons in Quaternary alluvial desert soils. The crystal forms suggest an origin as a subsurface gypsic crust formed by a combination of hydromorphic (poikilitic) and illuvial (mesocrystalline) processes with the fibrous gypsum veins suggestive of periodic surface exposure. Gypcrete horizons are up to 25 m thick and composed of both gypcrete types. They represent superimposed phases of surface and subsurface gypcrete development. Quaternary gypcretes are developed in arid climatic regimes, but are not considered to develop under hyper-arid climates. An arid climate is considered to have prevailed in the study area up to 9·5 Ma after which a change to hyper-aridity favoured gypcrete preservation.

Original languageEnglish
Pages (from-to)351-364
Number of pages14
Issue number2
Publication statusPublished - 1 Apr 1998

Bibliographical note

We thank the Natural Environmental Research Council (GM), the Nuffield Foundation and the University of Aberdeen (AJH) for funding fieldwork in northern Chile. G. May was supported by a NERC studentship (GT4/93/2/G). We would like to thank Peter Turner (University of Birmingham) for extensive discussion and B. Valero‐Garcés and B. Schütt for comments on the manuscript. We gratefully acknowledge Guillermo Chong Diaz (Universidad Católica del Norte, Antofagasta) for invaluable logistical and scientific support.


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