The inhibiting effect that siliciclastic material has on carbonate-secreting organisms has lead to the generalization that sediments composed of mixtures of carbonate and siliciclastic material should rarely form. However, many modern and ancient shelf deposits contain a spectrum of sediments that are of “mixed” composition. The processes responsible for this mixing can be grouped into four categories: (1) punctuated mixing, where sporadic storms and other extreme periodic events transfer sediments from one depositional environment to another; (2) facies mixing, where sediments are mixed along the diffuse boundaries between contrasting facies; (3) in situ mixing, where the carbonate fraction consists of the autochthonous or parautochthonous death assemblages of calcareous organisms that accumulated on or within siliciclastic substrates; and (4) source mixing, where admixtures are formed by the uplift and erosion of nearby carbonate source terranes. The allochemical constituents of mixed sediments are both coralgal and foram-mollusc in composition. The foram-mollusc assemblage is the most common because of the effects of increased turbidity, unstable substrates, and the clogging of filter-feeding mechanisms associated with a siliciclastic influx.
- Geological Society of America