Sedimentary Depositional Environments

Continental Deposition

On the continents, sedimentation might be thought to begin with clastic materials shed from the flanks of mountain ranges. These alluvial fans are characterized by poorly sorted, boulder and gravel dominated, debris flow conglomerates. Fluvial (river) facies include cross-bedded and rippled river sandstones and parallel or cross-bedded floodplain mudstones (siltstones and clay shales). Lacustrine (lake) facies include sands deposited at the mouths of rivers which empty into the lake and along the shoreline as well as muddy facies on the deep lake bottom. Swamps often form in low-lying areas (for example, the area near sea level behind the shore environment) in which parallel layered, organic-rich black shales and coal form. In arid regions with little vegetation and few rivers, aeolian (wind deposited - sand dunes) environments may dominate. Aeolian sandstones frequently display large scale (1 to 3 meter) crossbed sets.

Marine Deposition

Deltas form at the mouths of rivers where large volumes of siliclastics are dumped into the ocean (and lakes also). Thick accumulations of sand, silt and mud form in several subenvironments, including stream channels, flood plain, beaches, tidal flats, and sand bars. From the beach outward, well sorted, clean, shoreface sandstones accumulate in the wave-dominated near shore environment. These become gradually finer from fine sand through silt and into quiet water offshore mud facies, including parallel layered shales. In mid and high latitudes, these muds continue out onto the continental shelf. In tropical latitudes coral reefs or carbonate platforms often form offshore from the interlinked skeletons of carbonate secreting corals, molluscs, etc. Coral requires sunlight and warm clear water. Therefore limestone reefs do not form in the deep oceans (too dark) or in siliclastic, wave-dominated, turbid, near-shore environments. Farther offshore, at the edge of the continental shelf, is the continental slope and rise, down which gravity flows or turbidites move poorly-sorted sands and muds down into the deep ocean basins. On the deep abyssal plains, far from the influence of turbidite transported continental materials, organic muds or marine oozes are the result of a fine rain of the shells of microorganisms filtering down from near the surface.