Lecture 8 Flashcards
What is meant by a ‘depositional environment’ in sedimentary geology?
A depositional environment is the setting in which sediment is laid down (e.g., rivers, deserts, deltas, reefs). Each environment imparts distinctive sedimentary characteristics—grain size, sorting, sedimentary structures, and fossils—that help geologists interpret where and how the rock formed.
Which major depositional environments are recognized, and how are they subdivided?
Geologists classify depositional settings into three broad categories: 1) Terrestrial (e.g., glaciers, deserts, rivers, lakes), 2) Coastal (e.g., deltas, beaches), and 3) Marine (e.g., shallow shelves, deep basins).
How do glaciers serve as a terrestrial depositional environment?
Glaciers can carry sediment of all sizes, from clay to huge boulders. When the ice melts, it leaves unsorted, unstratified glacial till, reflecting the powerful but indiscriminate carrying capacity of moving ice.
What kind of deposits form in mountain stream environments?
Rapid, turbulent water can carry large clasts, depositing them when flow slows. These coarse, angular to subrounded clasts typically form breccias or conglomerates, reflecting high-energy flow.
How do deserts create distinctive sedimentary deposits?
In desert settings, strong winds sort out fine dust from coarser sand. Windblown sand accumulates in dunes with large cross beds, forming well-sorted, well-rounded sandstone layers.
What happens in river (fluvial) environments?
Coarser sediments (sand, gravel) settle in channels, forming sandstone or conglomerate. Finer sediments (silt, clay) deposit on floodplains or point bars, creating siltstone or shale. Features like ripple marks, cross-bedding, and mud cracks can be preserved.
How do lakes produce sedimentary rocks?
Low-energy lakes allow fine clay and silt to settle, forming thinly layered shale upon lithification. In some lakes, chemical precipitation occurs when evaporation concentrates dissolved minerals.
What characterizes coastal delta deposits?
At deltas, a river enters a standing body of water and loses velocity. Sediments settle out, with coarser material near channels and finer offshore. Deltas can feature shale, sandstone, and even coal (in swampy parts).
What distinguishes a beach depositional environment?
Beaches have high-energy wave action that rounds and sorts sand or pebbles. The well-rounded, well-sorted medium-to-coarse grains often show cross-bedding and ripple marks.
How do shallow-marine carbonate environments differ from siliciclastic ones?
Warm, shallow, clear water fosters marine life that secretes carbonate shells (e.g., corals). Over time, these accumulate into limestone. In siliciclastic settings with abundant sand or mud, you get sandstone or shale instead.
What types of sediments dominate in deep-marine deposits?
Fine clay settles far from shore, along with skeletal remains of plankton, forming shale, chert, or chalk. Coarser sediments rarely reach deep water unless carried by turbidity currents.
How do geologists use sedimentary rocks to reconstruct past environments?
By examining grain size, sorting, sedimentary structures (e.g., ripple marks, cross-beds, mud cracks), fossil content, and mineralogy, geologists interpret whether a rock formed in a terrestrial, coastal, or marine setting—and the specific sub-environment.
Why is understanding depositional environments important for resource exploration?
Sedimentary processes can concentrate minerals (e.g., gold in placer deposits) and hydrocarbons (e.g., coal in swamps, oil in certain marine or deltaic settings). Recognizing the depositional environment guides where to explore for these resources.
