Record of Late Pleistocene (Oxygen Isotopic Stage 5) climate changes during episodes of karst development on the Northern coast of Crete: Sequence stratigraphic implications

Abstract

In Late Pleistocene times, during Oxygen Isotopic Stage (OIS) 5, aeolian calcareous sand dunes were deposited along the northern coast of Crete, where they now discontinuously crop out wedged between thin shallow marine Tyrrhenian deposits. Late Pleistocene marine sediments are scarce and amalgamated because the combination of slow uplift movement and low elevation of OIS 5 highstand shorelines, close to present mean sea level, promoted their erosion in the shallow marine and subaerial environments. The studied succession is organized in transgressive–regressive (T–R) sequences bounded below and above by transgressive surfaces of erosion superimposed on subaerial unconformities. The vertical stacking of sedimentary facies between consecutive key surfaces demonstrates that the preservation potential of marine transgressive deposits (TST) was higher than that of regressive sediments. Similarly, aeolian strata and palaeosols located landward were well preserved. Detailed sedimentological and petrographic studies of the aeolianites and their framing units from several sites revealed a complex microstratigraphy of diagenetic features that relate to: (1) the succession of highstand shorelines during OIS 5; and (2) episodes of subsurface karstification interpreted to result from soil cover development on limestone substrate during relative sea-level falls. A complex sequence of climate changes during karst formation is suggested by solution pipes that cross-cut or contain calcified root systems, and whose inner walls are covered with carbonate crusts interpreted as calcretes. The repetition during OIS 5 of this transition from rhizocretion formation to karstification to calcretization in the studied deposits is argued to occur in response to contrasting climate conditions at times of sea-level lowstand. It is proposed that palaeosols and rhizocretions formed during the falling limb of the relative sea-level curve under semi-arid conditions, karstification developed at the turn between falling and rising sea level when climate was at its wettest, and calcretization occurred during the rising limb of the relative sea-level curve under arid conditions. Identification of wave-cut surfaces of erosion on top of transgressive marine sediments and below aeolian deposits, suggest that the latter are contemporaneous with falling sea-level marine units deposited basinward below the lowstand shoreline, namely forced regressive systems tracts (FRST), otherwise not recognized in the field. The complex suite of alteration processes recorded within the palaeosols above aeolian deposits are interpreted to represent periods extending from the latest stage of sea-level fall to the earliest stage of sea-level rise, including lowstand systems tracts (LST). This proves a satisfactory complement to reconstruct palaeosea-level fluctuations in settings where the shallow-marine nature of sedimentary environments means that most regressive and lowstand deposits are not recognized in the stratigraphic record.