FOSSILS IN THE FIELD
C: Diagenesis of fossil skeletons
6.2 Fossil Distribution and Preservation
Pay attention to the distribution of the fossils while studying or logging a succession of sediments. Fossils can be evenly distributed throughout a rock unit without any preferential concentration in certain beds; this gener- ally occurs only where the sediment is homogeneous throughout. Use the percentage charts in Fig. 3.3 to estimate the proportions of the various fos- sils present.
In many cases, fossils are not evenly distributed but occur preferentially in certain beds or lenses, or in a buildup (a reef). The term coquina (and lumachelle) is often used for an accumulation of shells. Always examine the types of fossil present and determine their relative distribution. See whether there is a correlation between fossil type and lithofacies. Concentrations of fossils at particular levels can arise from current activity or preferential growth through favourable environmental conditions.
Fossils can be arranged in several different ways within a bed: concordant, oblique, perpendicular, imbricated, stacked and nested (see Fig. 6.2, and see Fig. 6.3 for an example). Examine the fossiliferous bed in cross-section and
Bedding cross-section
Bedding plane view
concordant oblique perpendicular
pavement stringer preferred orientation
imbricated current
stacked nested
Figure 6.2 The various arrangements of fossils in sedimentary rocks as seen on bedding planes and in bed cross-sections.
determine the orientation of the fossils; this depends on the levels of current activity and sorting (see below).
On the bedding planes, fossils may be arranged as a carpet or pavement a few valves thick, all lying flat upon the surface, or they may form a linear feature on the surface, a stringer or low ridge (Fig. 6.2), concentrated there by currents.
6.2.1 In situfossil accumulations
Where favourable conditions existed, some fossils will be preserved intact, with little breakage or disarticulation, and at least some of the organisms which lived on or in the sediment will be in their life position. Fossils which are commonly found in their growth position include brachiopods (Fig. 6.4), some bivalves (especially the rudists), corals (Fig. 6.5), bryozoans and stro- matoporoids. It is worth noting the general level of infaunal activity (the amount of bioturbation and the ichnofabric, see Section 5.6.1, Fig. 5.68) in conjunction with the body fossils present.
Figure 6.3 Non-marine bivalves in sideritic mudrock; notice varied orientations of shells. Millimetre scale. Carboniferous, N.E. England.
Figure 6.4 Brachiopods (productids) in growth position. Carboniferous, Wales.
Figure 6.5 Colonial corals in growth position. Carboniferous, Wales.
Fossils can be organised into reefs (the more general term is buildup) (Section 3.5.3). In these the majority of the fossils are in their growth position, perhaps with some organisms growing over and upon each other. Colonial organisms can dominate and the rock characteristically has a massive, unbed- ded appearance (Figs 3.11, 3.12, 5.83 and 5.84). The fossils commonly have a variety of growth forms (shapes), depending on the environmental con- ditions (Fig. 3.13), and there may be an upward change through the reef.
Cavities, both large and small, perhaps filled with internal sediment and cal- cite cement, are common in reefal limestones (e.g., Fig. 3.9).
Hiatal concentrations are fossil accumulations as a result of minimal deposition of mud and sand. The fossils may be bored, encrusted, discoloured or phosphatised, and authigenic minerals such as chamosite and glauconite may be present. These beds are usually highly condensed and biozones may be missing.
6.2.2 Current accumulations of fossils
Concentrations of skeletal material arising from current activity can form in a number of ways. Transportation of skeletal debris by storm currents leads to the deposition ofstorm beds(also calledtempestites: see Section 5.3.3.9, Fig. 5.29). Theseevent beds tend to be quite laterally persistent and possess sharp, usually scoured bases. Internally, they vary from laminated beds with
normal size-grading and good sorting of fossils, to beds and lenses with no apparent sorting of fossils, the deposit having a ‘dumped’ appearance with a wide range of grain-sizes and no internal structure. Examine the arrangement of fossils in the storm beds by reference to Fig. 6.2.
Fossil accumulations can also form through the winnowing action of weaker currents which remove finer sediment and skeletal grains. Suchfossil lag deposits are usually impersistent lenses; they too occur in shallow-shelf successions, especially at transgressive surfaces. Fossil-rich beds can also be formed through reworking of sediment by laterally migrating tidal channels.
The degree of current activity and reworking affects the proportion of bro- ken and disarticulated carbonate skeletons within a fossil concentration. With an increasing level of agitation, fossils grade from perfect preservation, with all delicate structures intact and adjoined, to poor preservation where fossils are abraded and broken. Points to look for with specific fossils are as follows.
• Crinoids: the length of the stems and whether all ossicles are separated;
if the calyx is present whether this is attached to the stem or not
• Bivalved shells(bivalves, brachiopods and ostracods): whether the valves are disarticulated or articulated; if the former, whether there are equal numbers of each valve; if the latter, whether the valves are open or closed
• Some brachiopods and bivalves: whether the spines for anchorage are still attached
• Trilobites: whether the exoskeleton is whole or incomplete (e.g., only tails).
6.2.3 Preferred orientations of fossils
Elongate shells or skeletons affected by currents commonly have a preferred orientation of their long axes (Figs 6.6 and 6.7). This alignment is generally parallel to the current, or less commonly normal to the current if the skeletal fragments were subject to rolling. Preferred orientations are commonly found with crinoid stems, graptolites, cricoconarids (tentaculitids), elongate bivalve shells, solitary corals, belemnites, orthoconic nautiloids and plant fragments.
Some fossil orientations may reflect current directions during life. In a col- lection of elongate fossils on a bedding plane, measure their orientation.
Chapter 7 deals with palaeocurrent analysis.
6.3 Fossil Associations and Diversity