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Supply, segregation, successions, and significance of shallow marine conglomeratic deposits

Monterey, California, USA

The accumulation of gravel in the shallow marine environment generally involves special conditions of sourcing, transport, and concentrating mechanism. Accordingly, the correct interpretation of shallow marine conglomerate can not only provide unique insights to tectonics, delivery systems, sedimentary processes, and sequence stratigraphy, but also facilitate prediction of the distribution and quality of hydrocarbon reservoirs within these deposits. Despite their economic and scientific importance, shallow marine conglomerate remains imperfectly understood relative to shallow marine sandstone. Studies of modern shallow marine gravel are relatively few and sporadic; most of our knowledge derives from the stratigraphic record.

The study of shallow marine conglomerate may be facilitated by analysis of the processes whereby gravel is delivered to the marine environment (supply), and concentrated once in place (segregation), as well as the succession in which the conglomerate occurs. Although the source of some shallow marine is obvious for some shallow marine conglomerate, it is less certain for many others. Little is known about the potential extent of longshore gravel transport. Understanding issues of supply may be critical to predicting the distribution of gravel in many ancient shallow marine deposits.

The degree of segregation of gravel and sand in the shallow marine environment can critically influence a deposit’s reservoir character. Waves are particularly effective at winnowing sand from gravel, particularly during storms. Waves also tend to drive gravel shoreward and concentrate it on or near beaches. Shallow marine gravel typically accumulates in any troughs or other depressions on the sea floor.

The occurrence of gravel in the shallow marine environment is particularly sensitive to changes in the rates of accommodation and sediment supply, which in turn determines the nature of the succession in which conglomerate occurs. Coastal gravel can occur in progradational, transgressive and aggradational successions, each of which bears a distinctive signature. Conglomerate in progradational deposits is mostly in the form of beach foreshore/upper shoreface or Gilbert delta accumulations. In transgressive successions, conglomerate may form transgressive lags or accumulate in tidal inlets. Aggradational deposits of shallow marine conglomerate, which seem to require a persistent balance of relative sea level and sediment input that is generally uncommon, can be difficult to explain.