Stable and ultrastable heavy minerals of alluvial to nearshore marine sediments from Central Portugal: Facies related trends Pedro A. Dinis ⁎ , António F. Soares Geosciences Center, Department of Earth Sciences, University of Coimbra, Largo Marques de Pombal, 3000-272 Coimbra, Portugal Received 17 July 2006; received in revised form 16 April 2007; accepted 18 April 2007 Abstract The Pliocene to Pleistocene sedimentary record from the western Iberia Coastal Margin is a thin succession (maximum thickness around 70 m) derived from the metamorphic and igneous Variscan Iberian Massif and from older sedimentary deposits. It comprises a wide variety of facies representative of deposition in environments ranging from inner shelf to alluvial fan. The facies are stacked in an overall regressive sequence. This sequence may be divided into lower order sequences with thin transgressive portions (not always present) and thicker prograding portions. Given the limited burial diagenetic transformation, the differences in heavy mineral (HM) assemblage can be explained by provenance, hydraulic segregation and chemical and mechanical selection. Transparent heavy minerals susceptible to acid treatment were not considered in this heavy mineral analysis. After the acid treatment, the HM signal is relatively monotonous. Except for the mica-rich assemblages in alluvial facies fed from the Variscan Iberian massif by short streams, the HM assemblages are dominated by tourmaline, andalusite, zircon and staurolite. However, it is still possible to recognize two opposite trends in HM composition linked to facies evolution. The first trend is widespread along the coastal margin, although typical of the earlier deposits. It is characterized by a vertical increase in staurolite and andalusite proportions coincident with prograding and regressive facies evolution. Relative sediment starvation during retrogradation, when compared to progradation, favoured the recycling of previous deposits, leading to higher proportions of ultrastable minerals (tourmaline, zircon and rutile). Hydraulic segregation in the nearshore zone also contributes to increased zircon content in the very fine sand fraction. The second trend is observed in stratigraphically higher sediments, deposited when environments were displaced basinward. This trend is common in relatively uplifted western sectors of the coastal margin, where sand and gravel dominate the record. It is characterized by a vertical increase in the content of ultrastable minerals (mostly tourmaline and zircon) throughout the regressive sequence. It may be explained by alluvial storage and the recycling of previous nearshore and coastal plain facies. Recycling, favoured by relative sediment starvation or low accommodation conditions, plays a major role in these HM assemblages. © 2007 Elsevier B.V. All rights reserved. Keywords: Heavy mineral; Facies; Provenance; Hydraulic segregation 1. Introduction The mineralogy of siliciclastic deposits is controlled by numerous factors, such as provenance, sedimentary envi- ronment, diagenesis etc. Heavy mineral (HM) assem- blages can be used to evaluate weathering and diagenetic evolution, approximate the paleogeographic conditions and help with stratigraphic correlations (Parfenoff et al., 1970; Mange and Maurer, 1992; Dill, 1998). HM asso- ciations of sedimentary deposits do not necessarily re- flect their provenance, as they tend to be changed by Sedimentary Geology 201 (2007) 1 – 20 www.elsevier.com/locate/sedgeo ⁎ Corresponding author. E-mail address: pdinis@dct.uc.pt (P.A. Dinis). 0037-0738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2007.04.004