Name: 1 GEOL 104 Dinosaurs: A Natural History Geology Assignment DUE: Mon. Sept. 18 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 almost exclusively in saltwater (marine) environments. Coal, compressed plant material that was buried so fast it did not have time to decay, indicates the presence of swamps. On the other hand, sandstones and mudstones (like shale) 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 fast moving water can move gravel: these conditions are found in mountain streams or the deepest parts of the channels of rivers. 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 (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 singleName: 2 direction of the current. Because winds keep on shifting in deserts, though, these unidirectional cross-beds will often show 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!). Here is a table that includes some common environments of deposition in which dinosaur fossils might be found, with descriptions of the types of features of the sedimentary rocks that would be produced: Environment of Deposition Characteristics of Sedimentary Rocks in that Environment Streams (rivers, creeks, etc.) Some gravel in the fastest part of the channel, otherwise sand, unidirectional ripples and cross-bedded sandstone Edges of streams (including rivers at flood stage) Silt or mud, some unidirectional ripples, some trackways Middle of lake Laminated mud (if anoxic), otherwise mud with burrows of bottom-dwelling worms, clams, etc. Lagoons Laminated mud or laminated limestone (if anoxic), otherwise mud or limestone with burrows of bottom-dwelling worms, clams, etc. Deserts Trough cross-bedded sandstones Shores of lakes Mud with mud cracks, raindrop marks, footprints, bi-directional ripples Shores of seas As above, but mud may also be carbonate mud (limestone) Below are cases of various fossil finds. Based on the description of the fossil and its surrounding sedimentary rocks and structures, indicate which of the above environments of deposition was the most likely setting. 1) Individual isolated dinosaur long bones (leg bones, ribs, etc.) and teeth with their edges rounded, as well as fish scales and turtle bones, in cross-bedded sandstone containing some gravel. Environment: 2) Dinosaur footprints in mud-cracked mudstone with bi-directional ripple marks. Environment:Name: 3 3) A complete 40 cm dinosaur, with skin impressions, in very fine-graded laminated mudstone. The layers also preserve undisturbed impressions of flowers and insects. Environment: 4) Dinosaur footprints in mud-cracked limestone with bi-directional ripple marks. Environment: 5) You discover a deposit formed by a stream. Nearly all the bones you find are from very large dinosaurs, and even the smaller bones of these dinosaurs are lacking. What can you infer about the energy of the environment when these fossils were deposited? 6) You discover a deposit with finely laminated mudstones. There are plentiful remains of tiny dinosaurs, lizards, mammals, insects, and the like, but no fossils of animals bigger than a dog. One colleague infers that there were no large dinosaurs in this environment, but you put forth an alternative hypothesis explaining