The efficacy of travertine as a palaeoenvironmental indicator: palaeomagnetic study of neotectonic examples from Denizli, Turkey

Abstract

This study has aimed to evaluate whether a discernible environmental signature is recorded in tectonic travertine by applying palaeomagnetic study to examples from the Denizli region in western Turkey. Palaeomagnetic sampling in 7 quarry exposures through short stratigraphic intervals of bedded travertine has determined variations in magnetic susceptibility and palaeofield direction; the former is a potential proxy of climatically-controlled atmospheric dust input and the latter is a possible indicator of directional changes resulting from geomagnetic secular variation of the ancient magnetic field and hence a measure of the rate of travertine accretion. Most sites record normal polarity as predicted from emplacement during the Brunhes Chron, although one had reversed polarity evidently imparted during the Matuyama Chron and confirming longer-term preservation of remanence. A few sites with coherent directions widely different from the recent field axis appear to have slumped or tectonically rotated since emplacement. Within-section groupings of palaeomagnetic directions are tight with lack of dispersion indicating that secular variation has been averaged over protracted periods of time. Magnetic remanence is therefore a diagenetic phenomenon, as expected from prolonged infiltration through porous bedded travertine. Magnetic susceptibilities are mostly very weak and dominated by the diamagnetic host, but some positive values record paramagnetic and ferromagnetic constituents. We find that environmental signatures may be revealed in bedded travertine by magnetic susceptibility. Palaeomagnetic directions provide no reliable constraint on incremental growth although fissure emplacements, including an additional example reported in this study, can record a short-term record of secular variation and yield estimates for the duration of fissure activity. In contrast the tufa-like deposits laid down by geothermal waters spilling out at the surface are highly porous and susceptible to later fluid seepage and only atmospheric dust landing on the surface can potentially record environmental effects. Isotopic and palaeomagnetic systems are homogenised over long intervals of time and unable to record short-term near-surface changes.