Early Hominin Paleoecology

2. Facies Analysis and Plio-Pleistocene Paleoecology

35 2 Facies Analysis and PlioPleistocene Paleoecology Craig S. Feibel Sedimentary rocks are the result of chains of complex genetic processes,including particle formation, transport, and accumulation, along with subsequent histories of in situ modification following deposition. Some processes may produce a variety of sedimentary products with only slight variation in the available components or environmental conditions. And some sedimentary products may result from one of a variety of processes or conditions. Typically, however, the suite of characteristics recognizable in sedimentary rocks provides detailed and often unambiguous clues to the processes and environmental conditions during their formation. These suites of characteristics are what enable us to recognize sedimentary facies. Analysis of associations of facies, along with their vertical and lateral relationships, allows the interpretation of sedimentary environments and the reconstruction of ancient landscapes.The ability to recreate ancient landscapes and the sequence of changes in environmental character through time is a valuable tool in ecological reconstruction from ancient records. Placing fossil or archaeological assemblages into this framework can greatly enhance our understanding of the conditions of site formation, the processes active before, during, and after assemblage, and the broader environmental setting of a particular locality. A facies refers to “the sum of lithologic and paleontologic characteristics of a sedimentary rock from which its origin and the environment of its formation may be DOI: 10.5876/9781607322252:c02 craig s. feibel 36 inferred” (Teichert 1958). The concept is an invaluable tool for the description and interpretation of sedimentary rocks, as it allows the easy integration of lithological characteristics, primary and secondary sedimentary structures, and pedogenic (soil) overprints. A facies is a distinctive rock type, recognizable on the basis of one or more defining characteristics, and it carries with it an interpretation of the processes known to result in that particular assemblage of characteristics .From a practical point of view we can consider facies strictly in terms of rock characters, or we can interpret them in terms of the related environmental conditions. In application, it is often more convenient to discuss the environmental interpretations that result from facies recognition, but it is essential to bear in mind that the interpretation rests upon recognition of characteristics , and that in some cases multiple processes can result in similar or identical characters.The basic units characterized as facies have fundamental significance in terms of environmental dynamics, and when considered sequentially within sedimentary packages, reflect discrete components of ancient landscapes. Using Walther’s Law, that sequential facies packages reflect lateral relationships upon the landscape, the broader character of ancient landscapes can be reconstructed in considerable detail (Reading 1978). SEDIMENTARY ROCK CHARACTERISTICS The formation of sedimentary rocks results from the assemblage of components in a sedimentary basin or trap, or on an available surface. The components of sedimentary rocks are typically distinguished in three groups: particles (clasts or grains and their coatings), voids (empty space), and plasma (fluids filling void space or coating particles). The particles themselves are of primary interest, but in certain cases the voids (determining porosity and permeability) and plasma (often mediating cementation and coherence) can be significant. A common sedimentary analysis involves granulometry, the investigation of particle size (texture), shape (sphericity), and population distribution (sorting). Perhaps more significant, however, is the rock fabric, the arrangement or organization of those particles. The sorting, packing, and arrangement of particles, along with the accumulation of secondary cements, strongly controls rock properties and is highly reflective of processes involved in the history of that rock. It is also important to consider that some sedimentary particles are what may be termed superparticles, formed by the amalgamation of individual particulate components. This is a common process in soil development and in the behavior of very small particles (clays) or biogenic materials. facies analysis and plio-pleistocene paleoecology 37 The genesis of individual sedimentary particles may proceed along a variety of pathways, and is often as complex as the rocks they assemble to form. The major groups of sedimentary particles are detrital, comprising weathered or fragmented precursor rocks; chemical, precipitated directly from a solution; biogenic, produced by biological activity; and volcaniclastic, resulting from the explosive fragmentation in volcanic settings. There are some special cases and gray areas in sedimentary particle characterization as well. Most detrital sediment is the product of weathering of silicate rocks, and is commonly termed siliciclastic. But carbonate rocks may also be subjected to weathering and erosion , producing detritus that is not siliciclastic in nature...