Final answer:
Deserts are the least likely environment to deposit siltstone or shale as they lack the still water necessary for fine sediments to settle. Rivers, oceans, and glaciers are more suitable for such deposition. The presence of oysters indicates a marine depositional environment for fossiliferous shale.
Step-by-step explanation:
The environment that is least likely to deposit siltstone or shale is a desert. Siltstone and shale are common sedimentary rocks formed by the compaction of silt- and clay-sized particles, typically in still water environments where fine sediments can settle and accumulate. Deserts are known for their lack of water and thus have conditions that are not conducive to the deposition of these fine-grained materials. On the contrary, rivers, oceans, and glaciers are depositional environments that can transport and eventually deposit silt and clay particles that can form siltstone and shale.
Oysters, which are mentioned in Figure 5.29, are commonly found in marine environments, specifically in shallow marine settings with low oxygen levels. This information, along with color indications from rock, helps geologists determine the depositional environments of sedimentary rocks. While siltstone and shale can form in various environments, the presence of fossiliferous shale and color indicators can provide clues about whether conditions were oxidized or reduced during deposition.