Subaqueous sediment gravity flow deposits: practical criteria for their field description and classification



A new method for the description and classification of subaqueous sediment gravity flow deposits is proposed. The classification scheme employs a convenient letter code and divides deposits (individual beds) into descriptive categories of two hierarchical levels: facies and subfacies. Facies, as the higher rank categories, are distinguished chiefly on the basis of sediment type (i.e. bed grain size/texture). A total of 13 facies have been distinguished: G= gravel; GS = gravel-sand couplet; GyS = gravelly sand; S = sand; SM = sand—mud couplet; MS = mud—sand couplet; TM = silt—mud couplet; MT = mud—silt couplet; M = mud; MyS = muddy sand; SyM = sandy mud; MyG = muddy gravel; GyM = gravelly mud. Subfacies, as the lower rank categories, are distinguished within the individual facies on the basis of the bed's internal structures. The number of subfacies is unlimited, and their labelling code includes particular facies symbols (see above) preceded by lower—case letters denoting specific sedimentary structures and their vertical arrangement. Subfacies thus refer to the bed's intervals, or divisions, which are labelled as follows: m = massive (unstratified and ungraded); g = graded (unstratified and graded); s = plane-stratified; x=cross-stratified; 1 = parallel– and/or cross-laminated; q = liquefied. For example, subfacies gsG (graded to plane-stratified gravel) are gravel beds that have a lower graded interval and an upper plane-stratified interval; subfacies xG (cross-stratified gravel) are gravel beds that are cross-stratified throughout; subfacies slS (plane-stratified to laminated sand) are sand beds that have a lower plane-stratified interval and an upper laminated (parallel- and/or cross-laminated) interval.