The middle dolomite unit of the Cambrian Metaline Formation in the Metaline district, Washington, was deposited in a low-energy, shallow-water environment. Deposition occurred as a complex mosaic of subtidal, intertidal, and supratidal environments in a restricted lagoonal and broad tidal-flat setting.

Where primary depositional features have not been masked by intense diagenesis, the middle dolomite unit is characterized by seven distinctive lithofacies. The vertical succession of these lithofacies differs from locality to locality and is laterally discontinuous. The seven lithofacies and interpreted depositional environments are: (1) black, birdseye dolomite, deposited in the supratidal zone; (2) cryptalgalaminate dolobindstone, deposited in the upper intertidal and supratidal zones; (3) laminated, intraclastic dolofloatstone, deposited in the outer intertidal zone; (4) gray massive to mottled dolomite, deposited in the intertidal and subtidal zone; (5) intraclastic-oncolitic dolofloatstone, deposited as lag deposits in tidal-flat channels; (6) oncolitic dolofloatstone, deposited n shoal areas in the upper subtidal zone; and (7) lenticular-bedded dolomite, deposited in the subtidal zone.

Changes in lithofacies over narrow vertical ranges were due more to changing hydrographic and sediment-supply conditions than to numerous minor eustatic sea level changes. Through time, however, there was a gradual rise in sea level, and subtidal sediments became dominant in the upper middle dolomite unit.

The low-energy nature of the middle dolomite unit was the result of either a very long wave fetch, which extended across many kilometers of shallow water, or a remote barrier, which effectively reduced the hydrokinetic energy below that expected in open-marine conditions.

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