Hyperpycnite

You may have heard of turbidites. This type of deposit forms from pulse-like flows, where sediment travels rapidly down a slope because of a gravity contrast with the surrounding fluid. Turbidites typically show an upward decrease in gain size and a vertical succession of structures (massive/planar laminated/ripple cross-laminated) indicative of decreasing energy of the flow. There are, however, some deposits that differ from turbidites by showing a symmetric internal organization, with increasing and then decreasing grainsize, and with structures pointing to increasing and then decreasing energy of the flow. Hyperpycnites are such kind of deposit, forming when a river, laden with sediment during its flooding stage, flows into a basin. The flow typically waxes, reaches peak strength, and then wanes. Therefore, the resulting hyperpycnite reflects the waxing and waning cycle in its internal structure. This picture shows a nice hyperpycnite from the Pennsylvanian Minturn Formation, near McCoy (CO, USA). The bed displays reverse grading in its bottom part and a gradual upward transition from ripple laminated, to planar laminated, to massive (waxing phase). The upper part of the bed shows normal grading and hints of ripple-cross lamination (waning phase).

sedimentary structures sediment gravity flows event beds sedimentology sandstone Pennsylvanian