The basal part of the Hüdai Formation (Early Cambrian) in the Aydıncık (Mersin) area comprises red sandstone beds at different stratigraphic levels, showing ripple cross-lamination. These beds were deposited in the middle to lower part of an intertidal flat environment. Bedding-parallel reddening and its cyclic character suggest syn-sedimentary or early diagenetic red colouration in a marine environment. Both X-ray diffraction and petrographic analyses indicate that the red sandstones are composed of predominantly K-feldspar and quartz grains and an illite matrix. These are dominantly arkosic wacke. A small amount of hematite (<5 wt.%) is present in all the samples, which gives a red colour to the whole rock. Scanning electron microscopy images show that hematite pigment appears as spherical and rod-shaped particles in sizes of 1 &#956;m or less disseminated on the grains and in the matrix. Considering their size and morphology, the hematite particles can be interpreted as fossil bacteria, indicating a bacterially induced precipitation, probably from seawater. A 2-step biomineralisation process could explain iron oxidation during early diagenesis in an intertidal environment: (a) trapping of iron ($Fe^{2+}$) on the bacterial cell wall with oxidation by bacterial activity; (b) inorganic precipitation followed early-stage bacterial nucleation sites resulting in crystal growth. The Fe2+ for reddening is provided by an intrastratal alteration of the iron-bearing minerals.