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Amalgamated beds

In sedimentary sequences, a bed generally represents a single deposition event: a period of time in which the conditions of sedimentation in a basin remained constant, resulting in a layer with characteristic composition and structures that can be distinguished from the underlying and the overlying beds. Amalgamated beds are an exception. Amalgamation occurs in high-energy environments characterized by alternating phases of erosion and deposition, for example alluvial plains or turbidite fans. Amalgamated beds form when two or more layers with similar composition are merged together due to the presence of one (or more) erosional surfaces. The resulting amalgamated bed may apparently look as the result of a single deposition event, since it shows similar sedimentary structures, lithology, and grain size along its thickness. Amalgamated beds can be distinguished based on the recognition of erosional surfaces, sharp and sudden changes in grain size or inconsistent sequences of sedimentary structures within the same bed. Amalgamated beds are relatively common in coarse-grained sandstone and conglomerate sequences which deposit in high-energy environments.

erosional truncation and amalgamation in sandstone

Channel dug into fine-grained sandstone filled by conglomerate to coarse-grained sandstone associated with two subsequent turbidite events. Macigno Sandstone. Cala del Leone, Quercianella, Italy. Photo Samuele Papeschi/GW.

amlgamated beds

Amalgamation of two turbidite beds through an erosional contact. Macigno Sandstone. Cala del Leone, Quercianella, Italy. Photo Samuele Papeschi/GW.

erosional channel in turbidite sandstone
Above: erosional contact, responsible for the amalgamation of two turbidites into a single bed. Note the truncation of the lamination in the bottom layer.

Above: amalgamation between two sandstone layers separated by an erosional contact.

Above: channel fill and complex amalgamation structures affecting several turbidite layers. How many layers are there? Several rip-up clasts are visible.

Pics credits: Samuele Papeschi/GW. Locality: Cala del Leone, Quercianella, Italy. Check out the full story here.

References
Mackenzie, F. T., & Garrels, R. M. (1971). Evolution of sedimentary rocks. New York: Norton.
Pettijohn, F. J. (1975). Sedimentary rocks (Vol. 3). New York: Harper & Row.

        

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