Name: GEOL 104 Dinosaurs: A Natural History Geology Assignment DUE: Mon. Sept. 17 Part I: Environments of Deposition Geologists can use various clues in sedimentary rocks to interpret their environment of deposition: that is, the type of conditions that were present when they were laid down. Some aspects of the environment of deposition are revealed by the type of sedimentary rock. Carbonate rocks are composed of calcium carbonate, like limestone (including chalk): these form primarily form in saltwater (marine) environments or along their shores (the white beaches of the Caribbean, for instance, or possible future limestones). Coal, compressed plant material that was buried so fast it did not have time to decay, indicates the presence of swamps. Evaporites (rock salts) show an environment that was very arid; the quick evaporation of the salty lake or sea water results in accumulations of evaporite deposits. On the other hand, detrital sedimentary rocks (breccias, conglomerates, sandstones, siltstones, and mudstones) can form in a variety of environments, so we need additional information. Another aspect of the environment recorded in the sedimentary rocks is the energy of the environment: that is, how fast the water (or wind) was moving. Essentially, the higher the energy, the larger the size of the particles of sediment. Slow moving water can only move extremely fine (small) particle sizes: silt, clay, and mud. This slow moving water might be the still water of lake or lagoon or the slowly moving water of a river at flood stage that has overflowed its banks. Moderately quick moving water (or fast moving wind) can move sand. Only the fastest moving water can move gravel: these conditions are found in mountain streams or the strongest (and deepest) parts of the channels of rivers. Detrital sedimentary rocks are classified mostly on the size of the largest particle, and thus reflect the energy of the environment: • Breccias and conglomerates have gravel-sized particles, and represent the highest energy. These are further distinguished by how continuous the flow of the water was: o Breccias have angular particles, and so were not transported for very long and probably only moved once (for example, an avalanche) o Conglomerates have rounded particles, and were transported more continuously (for example, in a river) • Sandstones represent moderately fast water or wind. This could be in stream systems, in desert winds, at the beach, and so forth. • Siltstones represent more slowly moving water and wind: as sandstone above, but with slower speed. 1Name: • Mudstones generally represent water that is still or moving only very slowly: the middles of lakes, lagoons, deeper water, etc. (One special case is on floodplains: when a river overflows its banks, it deposits lots of mud over everything.) Note that the energy of environment can affect the type of fossils preserved. Higher energy often means that the animal bodies will get buried faster, and so have a greater chance of being fossilized. However, higher energy also means that the bodies (especially of small animals) will tend to be torn apart and the edges of the fossils will be rounded by rolling. In contrast, large bodied animals are unlikely to be buried quickly enough in low energy environments to be fossilized (an exception is a river at flood stage: there is so much mud available that dinosaurs (or today, even houses!) can be buried). However, if a smaller animal falls to the bottom of a low energy environment that is anoxic (low in oxygen) there will be no decay organisms like worms to disturb the body: this can often produce extremely well preserved fossils. One set of clues comes from types of layering or bedding within the sedimentary strata (layers). Laminations (very thin layers) indicate environments where the water is very still (and anoxic): lagoons, the interiors of lakes, and the like. In contrast, cross-bedded layers and ripple marks indicate that there was some form of higher energy environment: the channels of streams and rivers, desert sand dunes, the shores of lakes and rivers. In the case of the shores of lakes and rivers the ripple marks will be symmetrical around their crest, indicating that they are bi-directional (produced by water moving to-and-fro, as the water laps onto shore and drains back again). In contrast, in any given layer of a stream channel or sand dune deposit, the ripples will be unidirectional, indicating a single direction of the current. Because winds keep on shifting in deserts and on sandy beaches, though, these unidirectional cross-beds will often show dramatic shifts in the direction of the current: these are called trough cross-beds. Bedding and ripple marks are just two types of sedimentary structure. There are others that also give clues to the environment of deposition. For example, mud cracks can only form in spots which were wet but were then exposed to the air and sunlight (so that they could dry out): if you find mud cracks in rocks, you know that the environment had to have been occasionally exposed to air. Similarly raindrop marks also imply ground that is wet but exposed to air, at least during that rainfall. Tracks of terrestrial animals (like dinosaurs) would indicate that the ground was soft but was either exposed to air or in very shallow water (obviously if the water was too deep, the feet of the animals wouldn’t touch bottom!). 2Name: In each of the following cases, use the evidence provided to circle the single most likely possible environment of deposition, and give your reasons for your choosing. 1) A limestone with dinosaur tracks, mudcracks, and bidirectional ripple marks. This most likely formed at: a. a stream channel. b. a desert. c. the seashore. d. bottom of a lake or lagoon. Reason for choosing? 2) Laminated mudstone containing the articulated skeletons of many small animals. This most likely formed at: a. a stream channel. b. a desert. c. the seashore. d. bottom of a lake or lagoon. Reason for choosing? 3) Trough cross-bedded siltstone and sandstones, with the articulated skeletons of some small and medium sized dinosaurs, mammals, and lizards. This most likely formed at: a. a stream channel. b. a desert. c. the seashore. d. bottom of a lake or lagoon. Reason for choosing? 4) Conglomerates and sandstones with crossbeds and unidirectional ripple marks; the rocks contain fossils