"Rock of the Month #187, posted for January 2017" ---

Trace fossils

also known as ichnofossils, represent the traces of once-living creatures, but are not the fossil of a creature itself. Common examples of potential trace fossils are footprints and other tracks, burrows and fossil excrement (coprolites). As with the body fossil of the actual organism, the preservation of a trace fossil relies on favourable circumstances, since they are easily destroyed by rain, waves and other processes. They can be considered part of the wider spectrum of features known as sedimentary structures, which include ripple marks, raindrop pits, and cross-bedding. Thalassinoides is recognized widely in strata of Jurassic to Miocene age. Such trace fossils are widespread worldwide, and are well documented in the Jurassic (Patel et al., 2014) and Cretaceous (Saha et al., 2010) of India. The ichnogenus is commonly found to shallow water sedimentary facies, but may also occur in deeper water habitats (Crimes, 1977). Thalassinoides (formerly Spongeliomorpha) is also found in older sediments: an example is the attractive, Ordovician-age Tyndall stone, an ornamental stone quarried northeast of Winnipeg in southern Manitoba, Canada (Coniglio, 1999). In the Tyndall stone, the burrow filling is mostly dolomite, whereas the host limestone is calcitic. The host rock became calcified before the burrow fill, which remained soft longer and was eventually preferentially dolomitized.

A note on the regional geology

The Hunstanton area has notable chalk cliffs (Ellis, 1957). Hunstanton, on the eastern side of the wide coastal inlet known as the Wash, is noted for an unusual red chalk, in contrast to the typical white chalk of the famed White Cliffs of the south coast of England, and elsewhere. The local succession includes units named the Carstone, the Hunstanton Red Rock ("red chalk"), and (above a thin ferruginous layer) the Lower Chalk. A detailed description of the local Cretaceous strata can be found in Chatwin (1961, pp.19-37). The oldest beds of the Lower Cretaceous are found in West Norfolk, from Downham Market north to Hunstanton. From base to top, the sequence at Hunstanton includes:

• Carstone - jointed ferruginous sandstone, used locally as a building stone. At the base are concretions and phosphatic nodules that contain ammonite fossils. It is equivalent to the Lower Greensand of southern England. Circa 35 feet 10.7 m) in thickness, with fossils restricted to the base.
• Hunstanton Red Rock (the Red Chalk) - equivalent to the Gault clay elsewhere. Some 4 feet (1.2 m) thick at Hunstanton. Three bands are recognised. The Hunstanton Red Rock contains fossils similar to the Gault, such as Beudanticeras, Euhoplites and Hoplites. Both Carstone and Red Rock are condensed sequences laid down over an extended period of intermittent deposition, thus the ammonites represent different zones. Belemnites and the bivalve Inoceramus are also found. "Spongia paradoxica" is found at the top of the Red Rock as well as the base of the overlying chalk.
• Lower Chalk - regional subsidence led to the deposition of a much thicker and more widespread carbonate sediment, the chalk. At Hunstanton some 56 feet (17.1 m) of chalk is preserved. Fossils of the topmost Red Rock and the overlying basal chalk (the Sponge Bed) are almost identical. The Sponge Bed is a hard white chalk just 1.5 feet (45 cm) thick.

References

Bell,FG (2002) The geotechnical properties of some till deposits occurring along the coastal areas of eastern England. Engineering Geology 63, 49-68.

Chatwin,CP (1961) British Regional Geology. East Anglia. Institute of Geological Sciences / HMSO, London, 4th edition, 102pp.

Coniglio,M (1999) Manitoba's Tyndall stone. Wat on Earth 12 no.2, University of Waterloo, 15-18.

Crimes,TP (1977) Trace fossils of an Eocene deep-sea fan, northern Spain. In `Trace Fossils 2' (Crimes,TP and Harper,JC editors), Seel House Press, Liverpool, 351pp., 71-90.

Ellis,C (1957) The Pebbles on the Beach. Faber and Faber Limited, London, 2nd edition, 163pp.

Patel,SJ, Joseph,JK and Bhatt,NY (2014) Ichnology of the Goradongar Formation, Goradongar Hill Range, Patcham Island, Kachchh, western India. J.Geol.Soc.India 84, 129-154.

Purcell,D (1967) Cambridge Stone. Faber and Faber Ltd, London, 115pp. plus 48 plates.

Reid,C and Woodward,HB (1882) The Geology of the Country Around Cromer, Explanation of Sheet 68E. Geol.Surv. G.B. Memoir, reprinted text, 56pp.

Saha,O, Shukla,UK and Rani,R (2010) Trace fossils from the late Cretaceous Lameta Formation, Jabalpur area, Madhya Pradesh: paleoenvironmental implications. J.Geol.Soc.India 76, 607-620.

Wells,AK and Kirkaldy,JF (1966) Outline of Historical Geology. Thomas Murby & Co., London, 6th edition, 533pp.