Interpreting past depositional environments

Sediments accumulate in a wide range of settings that can be defined in terms of their geomorphology, such as rivers, lakes, coasts, shallow seas, and so on. The physical, chemical and biological processes that shape and characterise those environments are well known through studies of physical geography and ecology. Those same processes determine the character of the sediment deposited in these settings. A fundamental part of sedimentology is the interpretation of sedimentary rocks in terms of the transport and depositional processes and then determining the environment in which they were deposited. In doing so a sedimentologist attempts to establish the conditions on the surface of the Earth at different times in different places and hence build up a picture of the history of the surface of the planet.



The concept of facies
The term facies is widely used in geology, particularly in the study of sedimentology in which sedimentary facies refers to the sum of the characteristics of a sedimentary unit. These characteristics include the dimensions, sedimentary structures, grain sizes and types, colour and biogenic content of the sedimentary rock. An example would be crossbedded medium sandstone: this would be a rock consisting mainly of sand grains of medium grade, exhibiting cross-bedding as the primary sedimentary structure. Not all aspects of the rock are necessarily indicated in the facies name and in other instances it may be important to emphasise different characteristics. In other situations the facies name for a very similar rock might be red, micaceous sandstone if the colour and grain types were considered to be more important than the grain size and sedimentary structures. The full range of the characteristics of a rock would be given in the facies description that would form part of any study of sedimentary rocks. If the description is confined to the physical and chemical characteristics of a rock this is referred to as the lithofacies. In cases where the observations concentrate on the fauna and flora present, this is termed a biofacies description, and a study that focuses on the trace fossils in the rock would be a description of the ichnofacies. As an example a single rock unit may be described in terms of its lithofacies as a grey bioclastic packstone, as having a biofacies of echinoid and crinoids and with a Cruziana ichnofacies: the sum of these and other characteristics would constitute the sedimentary facies.

Facies analysis
The facies concept is not just a convenient means of describing rocks and grouping sedimentary rocks seen in the field, it also forms the basis for facies analysis, a rigorous, scientific approach to the interpretation of strata. The lithofacies characteristics are determined by the physical and chemical processes of transport and deposition of the sediments and the biofacies and ichnofacies provide information about the palaeoecology during and after deposition. By interpreting the sediment in terms of the physical, chemical and ecological conditions at the time of deposition it becomes possible to reconstruct palaeoenvironments, i.e. environments of the past. The reconstruction of past sedimentary environments through facies analysis can sometimes be a very simple exercise, but on other occasions it may require a complex consideration of many factors before a tentative deduction can be made. It is a straight forward process where the rock has characteristics that are unique to a particular environment. Hermatypic corals have only ever grown in shallow, clear and fairly warm seawater: the presence of these fossil corals in life position in a sedimentary rock may therefore be used to indicate that the sediments were deposited in shallow, clear, warm, seawater. The analysis is more complicated if the sediments are the products of processes that can occur in a range of settings. For example, crossbedded sandstone can form during deposition in deserts, in rivers, deltas, lakes, beaches and shallow seas: a cross-bedded sandstone lithofacies would therefore not provide us with an indicator of a specific environment. Interpretation of facies should be objective and based only on the recognition of the processes that formed the beds. So, from the presence of symmetrical ripple structures in a fine sandstone it can be deduced that the bed was formed under shallow water with wind over the surface of the water creating waves that stirred the sand to form symmetrical wave ripples. The shallow water interpretation is made because wave ripples do not form in deep water but the presence of ripples alone does not indicate whether the water was in a lake, lagoon or shallow-marine shelf environment. The facies should therefore be referred to as symmetrically rippled sandstone or perhaps wave rippled sandstone, but not lacustrine sandstone because further information is required before that interpretation can be made.

Facies associations
The characteristics of an environment are determined by the combination of processes which occur there. A lagoon, for example, is an area of low energy, shallow water with periodic influxes of sand from the sea, and is a specific ecological niche where only certain organisms live due to enhanced or reduced salinity. The facies produced by these processes will be muds deposited from standing water, sands with wave ripples formed by wind over shallow water and a biofacies of restricted fauna. These different facies form a facies association that reflects the depositional environment. When a succession of beds are analysed in this way, it is usually evident that there are patterns in the distribution of facies. For example, beds of the bioturbated mudstone occur more commonly with (above or below) the laminated siltstone or the wave rippled medium sandstone? Which of these three occurs with the coal facies? When attempting to establish associations of facies it is useful to bear in mind the processes of formation of each. Of the four examples of facies just mentioned the bioturbated mudstone and the wave rippled medium sandstone both probably represent deposition in a subaqueous, possibly marine, environment whereas medium sandstone with rootlets and coal would both have formed in a subaerial setting. Two facies associations may therefore be established if, as would be expected, the pair of subaqueously deposited facies tend to occur together, as do the pair of subaerially formed facies. The procedure of facies analysis therefore can be thought of as a two-stage process. First, there is the recognition of facies that can be interpreted in terms of processes. Second, the facies are grouped into facies associations that reflect combinations of processes and therefore environments of deposition. The temporal and spatial relationships between depositional facies as observed in the present day and recorded in sedimentary rocks were recognised by Walther. Walther’s Law can be simply summarised as stating that if one facies is found superimposed on another without a break in a stratigraphic succession those two facies would have been deposited adjacent to each other at any one time. This means that sandstone beds formed in a desert by aeolian dunes might be expected to be found over or under layers of evaporates deposited in an ephemeral desert lake because these deposits may be found adjacent to each other in a desert environment. However, it would be surprising to find sandstones formed in a desert setting overlain by mudstones deposited in deep seas: if such is found, it would indicate that there was a break in the stratigraphic succession, i.e. an unconformity representing a period of time when erosion occurred and/or sea level changed.

Facies sequences/successions
A facies sequence or facies succession is a facies association in which the facies occur in a particular order. They occur when there is a repetition of a series of processes as a response to regular changes in conditions. If, for example, a bioclastic wackestone facies is always overlain by a bioclastic packstone facies, which is in turn always overlain by a bioclastic grainstone, these three facies may be considered to be a facies sequence. Such a pattern may result from repeated shallowing-up due to deposition on shoals of bioclastic sands and muds in a shallow marine environment. Recognition of patterns of facies can be on the basis of visual inspection of graphic sedimentary logs or by using a statistical approach to determining the order in which facies occur in a succession, such as a Markov analysis. This technique requires a transition grid to be set up with all the facies along both the horizontal and vertical axis of a table: each time a transition occurs from one facies to another (e.g. from bioclastic wackestone to bioclastic packstone facies) in a vertical succession this is entered on to the grid. Facies sequences/sucessions show up as higher than average transitions from one facies to another.

Facies names and facies codes
Once facies have been defined then they are given a name. There are no rules for naming facies, but it makessense touse namesthatare more-or-lessdescriptive, such as bioturbated mudstone, trough crossbedded sandstone or foraminiferal wackestone. This is preferable to Facies A, Facies B, Facies C, and so on, because these letters provide no clue as to the nature of the facies. A compromise has to be reached between having a name that adequately describes the facies but which is not too cumbersome. A general rule would be to provide sufficient adjectives to distinguish the facies from each other but no more. For example, mudstone facies is perfectly adequate if only one mudrock facies is recognised in the succession. On the other hand, the distinction between trough crossbedded coarse sandstone facies and planar crossbedded medium sandstone facies may be important in the analysis of successions of shallow marine sandstone. Facies schemes are therefore variable, with definitions and names depending on the circumstances demanded by the rocks being examined. The names for facies should normally be purely descriptive but it is quite acceptable to refer to facies associations in terms of the interpreted environment of deposition. An association of facies such as symmetrically rippled fine sandstone, black laminated mudstone and grey graded siltstone may have been interpreted as having been deposited in a lake on the basis of the facies characteristics, and perhaps some biofacies information indicating that the fauna are freshwater. This association of facies may therefore be referred to as a lacustrine facies association and be distinguished from other continental facies associations deposited in river channels (fluvial channel facies association) and as overbank deposits (floodplain facies association). It can be convenient to have shortened versions of the facies names, for example for annotating sedimentary logs. Miall suggested a scheme of letter codes for fluvial sediments that can be adapted for any type of deposit. In this scheme the first letter indicates the grain size (S for sand, G for gravel, for example), and one or two suffix letters to reflect other features such as sedimentary structures: Sxl is cross laminated sandstone, for example. There are no rules for the code letters used, and there are many variants on this theme (some workers use the letter ‘Z’ for silts, for example) including similar schemes for carbonate rocks based on the Dunham classification. As a general guideline it is best to develop a system that is consistent, with all sandstone facies starting with the letter ‘S’ for example, and which uses abbreviations that can be readily interpreted. There is an additional graphical scheme for displaying facies on sedimentary logs: columns alongside the log are used for each facies to indicate their vertical extent. An advantage of this form of presentation is that if the order of the columns is chosen carefully, for example with more shallow marine to the left and deeper marine on the right for shelf environments, trends through time can be identified on the logs.
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