Skip to Main Content
Skip Nav Destination

A detailed study of a section of the Pennsylvanian Wamsutta Formation illustrates a fundamental sedimentologic principle: similar processes of sedimentation active in entirely different depositional environments can produce similar assemblages of sedimentary structures in the resulting deposits.

One section south of Boston was selected for study for two specific reasons: diverse structures are available for examination, and the nonmarine and alluvial origin of the Wamsutta Formation has been established by other workers from analysis of primary structures, textures, fossils, and regional stratigraphy. Sedimentary properties observed at this locality are most closely comparable to present-day overbank, flood plain deposits. Prominent characteristics include a repetitive sequence of well-developed mud-cracked horizons within laterally continuous sheets of red sandstones and abundant scour-and-fill and channel bodies.

In addition to nonmarine, fluvial properties, the Wamsutta Formation also displays an assemblage of structures, textures, and sandstone composition similar to those typically associated with turbidites, flysch, and eugeosynclinal deposits. The section includes closely spaced, regular alternations of coarse- and fine-grained beds, some of which are graywackes with sole markings. The grading can be repetitive.

The vertical sequence within a single graded bed is as follows: intraformational conglomerates and graded sandstone form the base of the unit. Horizontally laminated medium-grained sandstone and, in turn, rippled sandstone of medium to fine grain follow. Massive red, mud-cracked siltstone occurs at the top of the unit. A stratum within this sequence is designated as the basic sedimentation unit in the outcrop locality. Each represents an episode of flooding on a flood plain, possibly on levees and low areas adjacent to channels. The repetitive sequences of alternating sandstone sheets and mud-cracked horizons indicate the normal depositional environment at the study locality was subaerial and was periodically submerged and exposed. The prominence of mud cracks and red coloration at the top of the beds and the regularity with which graded strata are repeated suggest that a subaerially exposed top-stratum may have been subjected to floods on a seasonal or annual basis.

Floods and turbidity currents have much in common: (1) the flows appear suddenly in a foreign environment; (2) these flows are characterized by high discharge, velocity, and load; (3) the sediment load contains a wide range of size grades; (4) currents are able to erode the bottom and remove the scoured material; (5) the sediment load is released progressively as the intensity of the flow decreases away from the source. The resulting beds are graded vertically and probably also laterally. Similar depositional processes active in environments as different from each other as continental flood plains and marine basins can produce similar assemblages of sedimentary properties.

You do not currently have access to this chapter.

Figures & Tables




Citing Books via

Related Book Content
Close Modal

or Create an Account

Close Modal
Close Modal