Tar sandstone from the Miocene of California, USA.

Sedimentary rocks form by the solidification of loose sediments. Loose sediments become hard rocks by the processes of deposition, burial, compaction, dewatering, and cementation.

There are three categories of sedimentary rocks: 1) Siliciclastic sedimentary rocks form by the solidification of sediments produced by weathering & erosion of any previously existing rocks. 2) Biogenic sedimentary rocks form by the solidification of sediments that were once-living organisms (plants, animals, micro-organisms). 3) Chemical sedimentary rocks form by the solidification of sediments formed by inorganic chemical reactions. Most sedimentary rocks have a clastic texture, but some are crystalline.

Tar sandstones & tar sands are oil-impregnated reservoir rocks that have been subjected to long periods of weathering and petroleum degradation. The largest deposit in the world is the Athabasca Tar Sand of Alberta, Canada. Although oil prices noticeably skyrocketed starting in 2004, Albertan tar sands have been economically exploited for many years, starting in the 1970s. It is expected that Canada will continue mining and processing Athabasca tar sand into crude oil in the future.

The tar sandstone shown above is from the Miocene-aged Monterey Formation of southern California. Many areas in southern California have significant accumulations of petroleum. Natural petroleum seeps can be seen in places (for example, the La Brea Tar Pits in the Los Angeles-Hollywood - translated, it becomes "the the tar tar" pits!).

Stratigraphy: Monterey Formation, Miocene, 10-12 Ma