Umber

Umber is a marine sediment associated with black smokers. It is perhaps best known as a pigment but I want to explain what it is as a rock type. It is important in Cyprus-type massive sulfide deposits and the examples shown below are also from Cyprus.

Umber is superbly preserved and very well exposed in Cyprus.

Black smokers are hot springs on the sea floor near spreading ridges. They emit hot water (more than 350 °C) that is mixed with various compounds, some of them metallic. Acidic water is rich in hydrogen sulfide and poor in oxygen. It is black in color because of sulfide precipitates, mostly pyrite.

Hot water usually also contains manganese, copper, and zinc in addition to iron. These sulfides precipitate and build chimney like structures on the sea floor. However, some of the material gets carried away into colder water where it will be precipitated as fine-grained sediment of iron and manganese oxides. This metalliferous fine-grained sediment, which may sometimes reach considerable thicknesses (tens of meters), is known as umber.

It sticks surprisingly strongly to the tongue if you are willing to conduct this test. It is an indication of high porosity of the rock.

Pieces of the rock are relatively lightweight although they now form, because of compaction, considerably thinner layer than originally.

The extent of compaction can be calculated because nearby are brown silicified rocks that are composed of umber if we subtract the silica.

The amount of umber in these rocks may be only one tenth of the pure layers in comparable volume of rocks. So it is clear that the silicification took place before the compaction.

Silica is hydrothermal: introduced to the rocks after the deposition.

Boulder of silicified umber.

Outcrop of umber.