This short article follows on from How old is the Earth? about the world-wide phenomenon of cyclical bedding in chalk formations – cycles where limestone alternates with marl. In the standard explanation such alternations reflect climate change caused by variation in the tilt of the Earth’s axis: limestones correlate with warmer periods, marls (mixtures of limestone and clay) correlate with cooler periods, and each cycle lasts around 20,000 years. The data speak otherwise. Experiments show that the worms and crustaceans which left burrow traces in these strata can destroy any original layering within months, so that the lighter and darker sediments would have been homogenised after a year or so into a uniform grey. In reality, the burrows are still visible, and the orderly alternation of limestone and marl still preserved. Given that an astronomical cause is required to explain such regularity, the couplets are better understood as reflecting alternations of summer and winter. Limestones formed when the seas were warm, marls when winter rains washed clays into the sea and productivity in the water column dropped off.

Chalk consists of tiny platelets shed by prolific species of algal plankton. It is characteristic of the Cretaceous period (the name means ‘chalky’). However, there is no reason why the geological expression of the annual cycle should be confined only to that period. Here we look at examples from the late Triassic and early Jurassic, earlier time intervals that, like the Cretaceous, are well represented along the Dorset coast of southern Britain (a UNESCO World Heritage Site).

Mercia Mudstone Group, Haven Cliff, Seaton

During the late Triassic age Britain was part of a supercontinent called Pangaea and lay at tropical latitudes. Mudstones deposited in the period (top photograph) formed in a hot sabkha environment, where coastal mudflats were impregnated by salts from the sea. The rusty colour of the darker bands comes from oxidised iron in the sediments. With less oxygen in the water, the iron oxide assumes a paler form. Evaporitic sulphates and silty dolomites also occur. Fossils – mostly spores – are common in the paler units; not much lived in the sediments themselves. The darker bands probably represent the cooler months of late autumn to early spring, the lighter bands the warmer months of late spring to early autumn.

The Blue Lias, Lyme Regis

A fashionable seaside resort made famous by the novelist Jane Austen, Lyme Regis is now celebrated for its fossils and fossil shops. To the west of the town the beach passes along a succession of alternating limestones and marls or dark shales, deposited in the early Jurassic as Pangaea rifted apart into separate land areas and the margins of these areas became flooded. The gentle dip of the strata has now brought one limestone bed level with the beach to form a wave-washed pavement. It is littered with ammonites.

The concentration of ammonites takes some explaining. Although some researchers have attributed it to an exceptionally low rate of sedimentation, the most natural explanation is an environmental disturbance, such as a toxic algal bloom or an episode of anoxia. Ammonites propelled themselves through the water much as the modern nautilus does, but unlike some other species, the ammonites fossilised here lived close to the sea bottom. Clearly they did not die all at the same instant. They are much more densely congregated than they could have been in life. Some overlap each other, and when directly superimposed they can be separated by sediment several centimetres thick. On the other hand, the sheer number of fossils in this bed is extraordinary. Since the enclosing limestone is not unusually thin, an exceptionally low rate of sedimentation seems improbable.

The shells are uncrushed. Cross-sectional views show that, in contrast to the flattened state of the ammonites within the marls, most retained their shape and were only partly filled with sediment. The surrounding carbonate must have hardened into rock quickly, before the weight of the sediments above could have compressed them and before their chemically unstable aragonitic shells could have dissolved back into the water. Since any mass-mortality event must have been brief, lasting, at most, months rather than thousands of years, the rate of sedimentation between the lower and upper ammonites must have been correspondingly rapid.

The time problem presented by these beds has not gone unremarked. As Paul et al. remark, the deposition rate implied by the conventional timescale is a mere 6 mm per thousand years, whereas

It is impossible to imagine an aragonitic shell exposed on the sea floor for even 5000 years without gaining any encrusting epifauna or indeed the shell dissolving away completely. The bivalve Pinna had a [chemically stable] calcitic shell and was semi-infaunal in life. We have seen examples of Pinna in life position in Bed H42, but truncated at the inferred top of the limestone bed, suggesting that even exposed calcite fossils did not survive long unless buried within the sediments, which in turn were probably rapidly cemented. Ammonites with encrusting epifauna that could have grown while the ammonite shell was exposed on the sea floor are rare. … The simplest explanation of the lack of encrusting organisms is that most large cephalopod shells were buried rapidly, before any encrusting epifauna could become established. … The conclusion that limestone beds with large ammonites or nautiloids in them were deposited very rapidly seems inescapable.

C R C Paul et al (2008), Palaeogeography, Palaeoclimatology, Palaeoecology 270.

The direct evidence therefore indicates that the radioactivity-based geological timescale is grossly inflated.

Although, as in the Cretaceous, much of the limestone may have derived from algal plankton, its lumpy texture suggests that inorganic precipitation was also involved, accelerated by sulphur-reducing bacteria that here and there left patches of iron disulphide (pyrite). Another difference is that in this interval the warmer seasons are represented by the darker beds – the marls and shales. With Britain in the early Jurassic lying at a lower latitude than in the late Cretaceous, summer temperatures in the upper water column were uncomfortably high for carbonate-producing algae. Temperatures at the sea bottom were cooler, allowing molluscs, sea-lilies, and various burrowing animals to thrive. On land, the wettest season – the period of enhanced runoff – was now the warmer months. Occasionally, an algal bloom consumed most of the oxygen and the laminated shales were hardly burrowed at all.

Belemnite Marls, east of Charmouth

The Belemnite Marls are named after the dart- or torpedo-like cephalopods that inhabited the near-shore and mid-shelf waters of this time, the early Pliensbachian. Their nearest living relatives are squid. They had a sharp beak, an ink sac, and ten arms projecting from their head. The arms were studded with hooks rather than suckers for catching prey. Calcitic bullet-shaped guards acted as a counterweight at the rear of the body, and after their bodies decayed, these alone remained as fossils of the whole animal. They are especially numerous in these marls.

As in the Blue Lias, these blue-grey muds alternate between dark and light, but the amount of carbonate (the lighter-coloured component) is smaller, and the boundaries between one bed and another more diffuse. Belemnite guards occur throughout, but burrows occur mostly in the lighter beds. Eighty-nine couplets have been counted in this 25-metre thick formation, not counting intervals at the top and bottom when sedimentation was too slow to produce clearly differentiated couplets. On the basis that the cycles were annual, this translates into an average sedimentation rate (after compaction) of about 25 cm a year – not an unreasonable figure for sediments that are only partially burrowed. On the basis that they reflect a 38,000-year cycle in the tilt of the Earth’s axis, the sedimentation rate would have been 0.007 mm per year.

Bridport Sands, West Bay

These are of Toarcian age and thus somewhat younger than the Blue Lias. On a sunny day the cliffs east of West Bay present a majestic series of siltstones coloured deep gold. Projecting strata honeycombed with well-cemented burrows alternate with thicker, softer layers in which burrowing is scarce. The original environment was a relatively deep-water body of sand migrating gradually southwards. Again, the cyclicity is probably annual, with the more carbonate-rich beds forming in the summer. Few remains of seafloor-dwelling organisms occur in the poorly consolidated silts because the daily rate of sedimentation was simply too rapid.