Practising science: reading the rocks and ecology

1 Earth Sciences: reading the rocks

1.5 The formation of sedimentary rocks

1.5.1 Sedimentary material

The laying down, or deposition, of layers of rock fragments, mineral grains, or biological material, such as the shells or other hard parts of dead organisms, can produce sedimentary rocks. Once deposited, the loose, unconsolidated sediment may be converted into a solid rock by compaction and cementing of the grains together by chemical action deep below the surface. These rocks consist, therefore, of fragments of sedimentary material, bound together by even smaller fragments, or some sort of cementing material, and so display a fragmental texture, as shown in Figure 7.

1.5.2 Sedimentary processes

Sedimentary grains are formed when the rocks at the Earth's surface are slowly broken up physically by exposure to wind and frost, and decomposed (chemically) by rainwater or biological action. These processes are collectively termed weathering. Once a rock has been broken up by weathering, the small rock fragments and individual mineral grains can be eroded from their place of origin by water, wind or glaciers and transported to be deposited elsewhere as roughly horizontal layers of sediment. The resulting sediment reflects the original rock types that were weathered, the efficiency of erosion and transport, the extents of chemical and physical degradation of the sediment grains during transport, and the conditions under which the grains were deposited from the transporting water, wind or ice. For example, sand-sized grains of quartz are one of the main constituents of sandstone, but those grains may have been transported by water in a river, carried by waves on a sea-shore, or blown around in hot desert sandstorms (to give just three possibilities). How might we distinguish which of the many possibilities is the most likely in any given case?

One approach is to use the size and shape of the grains in a sediment or sedimentary rock to reveal quite a lot about the origin of the sediment. For example, a vigorous river transports much larger grains than a gentle current in a lake, so the size of the grains gives an indication of the strength of the currents that could have transported and deposited the grains. In other words, the grain size depends on the energy of the environment in which the sediment was deposited. The general shape of the grains will tell you about the nature of the transporting medium; for example, was it water or air? (See Box 2, A story in a grain of sand).

Any record of ancient life preserved in a rock is known as a fossil – sometimes fossils are rare, whereas a few rocks are composed of virtually nothing else but fossils. In particular, many limestones were formed by the accumulation of the calcite (calcium carbonate – CaCO₃) shells and skeletons of certain marine organisms. Chalk is a well-known type of limestone that outcrops extensively across southern England; it is almost pure calcite, and consists largely of minute calcite plates of countless planktonic algae (phytoplankton) fossils. Other limestones, such as those found in the Peak District of northern England, contain abundant fossils of reef-building corals. Another example of a biologically formed sedimentary rock is coal, which is formed from compressed layers of woody plants.

Fossils are important when reconstructing the geological past because they are records of the environment at the time and place the fossil organisms were living. For instance, limestones rich in corals typically indicate warm shallow seas – the conditions needed for a coral reef ecosystem to thrive.

It is important that as many lines of evidence as possible are used to give a consistent interpretation of a rock's origin. No single feature should be taken as unequivocally diagnostic. For instance, think of a desert sand, composed of well-rounded, red oxide-coated, well-sorted quartz grains. Now imagine that climatic conditions change and these sand grains are swept away by flowing rivers and re-deposited elsewhere. The new sand deposit will be produced by the action of flowing water, but the sand grains may retain many of the characteristics of wind-deposition inherited from their previous history. A misleading interpretation can be reached if other lines of evidence are ignored. Such supplementary evidence could come from any fossils in the rock and the nature of adjacent sedimentary layers. Getting all of the necessary information involves a mixture of making observations and measurements at rock exposures in the field as well as examination and analysis of samples in the laboratory.

1.5.3 Sedimentary strata

We've seen that the detective work of piecing together a part of Earth's history from sedimentary rocks involves detailed investigation of rock samples, but this can give only a partial picture. On the larger scale of a rock exposure, there can be plenty for us to see and to interpret. Sedimentary rocks are usually found as layers referred to as strata (Figure 10), with each stratum (layer) recording the particular conditions at the time of its deposition. (Note: sedimentary layers are often referred to as beds, but strictly speaking the term ‘bed’ is reserved for those strata thicker than 1 cm; thinner layers are known as laminae (singular lamina).) Over time, conditions may have changed, either gradually or quickly, causing the nature of the sediment being deposited to change. In this way, a vertical stack of sedimentary strata is a record of changing conditions during a segment of geological time. The oldest sediments will be at the bottom, with progressively younger strata laid down on top. Geologists refer to this as the principle of superposition – older rocks are overlain by younger rocks; an individual layer is younger than the one beneath it and older than the one above it; the oldest layer lies at the bottom. This provides a relative time-scale. Changes in sedimentary rocks (or in the types of fossil they may contain) up through a sequence of strata provide a record of changing conditions over the passage of time throughout which the rocks were deposited.

1.5.4 Stratigraphy and geological time

Stratigraphy is the study of how the types of strata have varied over time and also considers how they are distributed geographically. One of the most useful results of stratigraphy is a generalised geological succession – the stratigraphic column – that defines the divisions of geological time. Figure 11 shows the geological time-scale. The broadest division of Earth history is into two intervals (called eons) of very different length: the Cryptozoic Eon and the Phanerozoic Eon. The Cryptozoic is a vast amount of time – from the origin of the Earth, 4 600 million years (Ma, short for mega-annum) ago, to the start of the Phanerozoic, 545 Ma ago. Cryptozoic is derived from Greek words meaning ‘hidden life’. This is the period of time when organisms did not possess hard parts such as shells, so their remains are difficult to find in sedimentary rocks. In contrast, Phanerozoic is derived from Greek words meaning ‘visible life’, reflecting the great abundance of fossils derived from the hard parts of organisms throughout this eon. The Phanerozoic Eon is divided into three eras – the Palaeozoic, Mesozoic and Cenozoic Eras (meaning ‘ancient life’, ‘middle life’, and ‘recent life’, respectively). Each of these eras is divided into a number of periods of unequal length (Figure 11). The current period, which started 1.8 Ma ago, is the Quaternary Period.

Looking at this column, you might think that there is a complete sequence of rocks everywhere, but there is not, in the same way that there are no historical records for certain times from certain areas. In the field, the Earth scientist might find rocks of, say, the Triassic Period lying above and in direct contact with rocks of the Carboniferous Period, so that rock evidence for the Permian Period is not present. This means that either there was no deposition of sediment during the Permian Period, or that these rocks were deposited, and then eroded, before rocks of the Triassic Period were laid down. This type of relationship in the geological rock record, where at a contact there are beds of the intervening age missing, is called an unconformity. The recognition of unconformities in the field is a key part of unravelling the geological history of an area because they represent some sort of hiatus in the conditions at the Earth's surface where the sediments were deposited.

1.5.5 Fossils and ancient environments

An essential component of any environment is the plant and animal life that is adapted to the prevailing conditions. Fossil plants and animals are therefore wonderful sources of information about ancient environments. Plants can leave behind remains ranging from roots, leaves and twigs to seeds and pollen. Leaves and twigs are relatively fragile, and require a comparatively low energy environment (e.g. the mudflats of an estuary) for their preservation. Seeds, pollen and spores are surprisingly robust, and are often the only parts of a plant to survive. Animals can be preserved in one of two ways – either by some part of their body remaining as a fossil, or by some trace, such as the animal's footprints preserved in a muddy sediment, becoming preserved as sedimentary rock (a trace fossil). Even dung can end up fossilised, the resultant fossils being known as coprolites.

Body fossils of animals include shells, skeletal frameworks, bones and teeth. We do not have room here to describe the most common fossil groups. Figure 12 illustrates some of them, with their characteristic features labelled. It is more important that here we consider how an organism gets fossilised, and what they can tell us about the environment in which they lived.

First of all, we have to consider the organism itself. Does it have any hard parts? If not, e.g. a jellyfish, then its chances of fossilisation are very low indeed. However, some soft-bodied organisms burrow into sediment, e.g. the lugworm that leaves the familiar worm casts on the shore, and at least there is the chance that their burrow becomes preserved as a trace fossil. An organism with a one-piece shell, such as a periwinkle or garden snail, stands a better chance of being preserved intact, than if its skeleton is made up of lots of pieces, like a sea urchin or crinoid (Figure 12c).

Fossils of land-dwelling organisms (e.g. Figure 12b) indicate deposition in or at least near a terrestrial environment, whereas marine fossils indicate a marine environment suitable for the fossil organisms to have lived in (e.g. correct temperature range, light levels, salinity, water depth). This evidence can then be added to evidence from the features of sedimentary grains (Section 1.5.2) to help reconstruct the environment.

1.5.6 Sedimentary structures

Consider some of the places where sedimentary materials are moved and deposited. Are the sediments always laid down in perfectly horizontal, perfectly flat layers? No; as often as not, the depositing surface is not perfectly flat. Instead, a system of parallel ridges, or ripple marks, like the ones shown in Figure 13A, form by the action that flowing water has on the erosion, transport and deposition of sand grains. In sedimentary strata, ripples and dunes (which are effectively larger versions of ripple marks) are normally seen in cross-section rather than in plan view, because pristine depositional surfaces are rarely exposed. In cross-section, the thin sedimentary layers that built up each ripple or dune can be visible, inclined at an angle to the top and bottom of the bed as a whole (Figure 13b). These structures are called cross-stratification and are a clue to the strength, direction and setting of the currents that produced them.

Many features associated with either erosion or deposition in a sedimentary environment can be preserved. For instance desert dunes can yield cross-stratification on a scale measured in many metres. All such structures are referred to as sedimentary structures and, as we have just seen, provide evidence with which to interpret ancient sedimentary environments.