New magnetostratigraphic evidence for the age of Acheulean tools at the archaeo-palaeontological site "Solana del Zamborino" (Guadix - Baza Basin, S Spain)

Abstract

The sedimentary record in the Guadix-Baza Basin (southern Spain) has proved to be a great source of information for the Miocene through the Pleistocene periods, due to the abundant faunal remains preserved, in some cases associated with lithic tools. The Solana del Zamborino (SZ) section has been the subject of controversy ever since a magnetostratigraphic analysis resulted in an age of 750-770 Kyr for Acheulean tools, a chronology significantly older than the ~600 Kyr established chronology for the first Acheulean record in Europe. Although recent findings at the "Barranc de la Boella" site (north-east of the Iberian Peninsula) seem to indicate that an earlier introduction of such technique in Europe around 0.96-0.781 Ma is possible, the precise age of the classical site at SZ is still controversial. The aim of this paper is to constrain the chronology of the site by developing a longer magnetostratigraphic record. For this purpose, we carried out an exhaustive sampling in a new succession at SZ. Our results provide a ~65 m magnetostratigraphic record in which 4 magnetozones of normal polarity are found. Our new magnetostratigraphic data suggest an age range between 300-480 Kyr for the lithic tools, closer to the age of traditional Acheulean sites in Europe.

Figure 1

Location of the paleontological sites of Solana del Zamborino (SZ), Mencal (M-9), and Fonelas (FP-1) using Google Earth free program (version 7.1.8.3036. Map data: Google, Digital Globe). The lithostratigraphic map of the Fonelas-Mencal area was created by Pla-Pueyo and digitalised using ArcGis 9.1. (http://www.esri.com/arcgis/about-arcgis). An online version of the map can be found in http://hdl.handle.net/10481/2373. Legend: TR, recent terraces; TA, ancient terraces; G2, glacis 2; G1, glacis 1; 5Ad, luddites, sands and gravels (locally palustrine carbonates), Detrital Axial System; 5Ac, palustrine-lacustrine limestones (locally tufa), Axial System carbonates; 5Ep, blocks and gravels, Proximal External Transverse System; 5Ed, sands, shales and gravels, distal External Transverse System; 5I, gravel, sand and shale (locally palustrine-calcrete carbonates), External Transverse System; 6Ad, sands and shales (locally palustrine-lacustrine carbonates), Detrital Axial System; 6Ac, palustrine-lacustrine carbonates (locally shale), Carbonate Axial System; 6E, breccia, gravel, sand and shale, External Transverse System, 6I, gravel, sand and shale, External Transverse System; B, basement.

Figure 2

Current state of the paleontological site of Solana del Zamborino. Photograph taken by Claudia Álvarez-Posada. The walls of the quarry are approximately 12 meters in height.

Figure 3

Stratigraphic succession obtained in this study, resulting from combining two parallel sequences measured in SZ in the years 2014 and 2015.

Figure 4

Example of isothermal remanent magnetization (IRM) acquisition curves and their respective thermal demagnetisation for some representative specimens. Each symbol corresponds a different sample. Notice a rapid increase in the IRM at fields lower than 1 T (between 300 and 400 mT) and unblocking temperatures just below 600 °C, suggesting magnetite as the main carrier of stable magnetization.

Figure 5

Representative orthogonal demagnetisation diagrams (Zijderveld). Black and white dots represent horizontal and vertical components respectively. Data in geographic coordinates.

Figure 6

Graphical representation of the virtual geomagnetic pole (VGP) latitude against stratigraphic height, and the resultant local magnetostratigraphy. (MAD: maximum angular deviation, Inc/Dec: inclination and declination of each ChRM direction). Four magnetozones with normal polarity, named N1, N2, N3 and N4 and three magnetozones with reversed polarity named R1, R2 have been defined. At least two magnetostratigraphically consecutive sites of the same polarity were required to define a change in polarity, otherwise a grey bar represents a single site.

Figure 7

Resultant magnetostratigraphy and suggested correlations to the Global chrono-stratigraphical correlation table for the last 2.7 million years (Cohen and Gibbard). Option II is favoured due to its consistency with the nearby Fonelas magnetostratigraphic section (see text for discussion).

Figure 8

Correlation between Mencal (M9), Fonelas (FP-1) and Solana del Zamborino (SZ) magnetostratigraphies and the Geomagnetic Polarity Time Scale (GPTS 2004) modified from Pla-Pueyo.