Cambrian floods

The thickness of the sands (Tapeats Sandstone, 30-100 m) deposited by the ocean waters as they lapped and flooded eastwards onto the Laurentian continent was partly controlled by the residual Precambrian topography and also their relationship with the overlying muds of the Bright Angel Shale (82-137 m thick). Again, we encounter an important geological concept, that of a cross-time relationship known technically as 'diachronism' (see box). The two types of strata interfinger and the time boundaries cut through them. There is a problem in dating the strata because the basal sands only preserve what are known as 'trace fossils' - tracks, trails and burrows left in the sediment by mobile organisms, mostly various kinds of worms but also at times by trilobites, snails and other shelled creatures. It is generally (but not always) impossible to put a relative date to these, but they can be very useful in aiding our understanding of the depositional environments.

Luckily, the transitional strata with the overlying shales do contain some rare shells (body fossils) that establish a late Early Cambrian age (perhaps around 520 million years ago) for some of the sandstones and a mid-Cambrian age (perhaps around 510 million years ago) for others. Near the top of the lower Cambrian in California there is a famous sand deposit, known as the Zabriskie Quartzite, which is riddled with small vertical, sediment-filled fossil burrows, not unlike those found on modern intertidal beaches. However, the distinguishing feature of these burrows is that they are simple single vertical tubes (several centimetres long and between 0.5 and 0.25 mm in diameter) known as Skolithos. They occur almost worldwide at the base of the Cambrian and are generally regarded as the dwelling or resting burrows of some unknown suspension feeding organism. The varying age reinforces the view that these are diachronous facies. The other implication is that some of the lower sands are perhaps of early Cambrian age. However, to get an idea of what life was like at the beginning of Cambrian times we have to look elsewhere.


Basically, the concept of diachronism (meaning 'across time') is very simple and readily understandable, as it still potentially occurs in any sedimentary environment. For instance, marine shoreline deposits typically include pebbly beach conglomerates that pass seawards into contemporary intertidal sands and offshore muds. The offshore decrease in sediment size is directly related to the decreasing energy in the depositional environment from wave and surf power to quieter water below wave base (apart from storm-generated events). All these different deposits (sedimentary facies) can be laid down at the same time over a distance from beach to offshore of maybe no more than a kilometre.

When sea level is rising and flooding in over a landscape, the beach and other zones migrate both inland and overlap on top of one another through time as they move inland. The long-term net result is a basal layer of conglomerate extending inland, covered by a layer of sand and then mud. When that is turned into hardened rock strata, the geologist is confronted with a succession of three types of strata that may have taken hundreds of thousands, if not millions, of years to accumulate. It will not be immediately evident that the boundaries between the conglomerate, sandstones and mudstones are not time planes. The true time planes cut through all three deposits, which are thus diachronous facies. Resolving the time planes can be very difficult and require detailed examination of any fossils and other data.

The palaeontological evidence can be hard to come by and differentiate. Typically conglomerates do not preserve any fossils and intertidal beach sands are difficult environments for life. Because they are constantly being reworked by waves and tidal currents, such sands tend to be inhabited by organisms that live in the sand and can rebury themselves if disturbed, such as worms, crustaceans (shrimps and crabs) and active clams such as razor and wedge-shells. The offshore quieter muds are host to yet different organisms, so that each facies tends to have its characteristic biota. Historically, it took some time for palaeontologists to realise that this is what happens.

0 0

Post a comment