engleski

Nozret “kettles” - sand boils caused by fluidization and preserved by freezing (Novigrad Sea, Croatia)

Nozret section of Novigrad Sea Pleistocene clastic terrace is unique for proglacial lacustrine sediments with dropstones and peculiar “kettles” which occur in the upper part of the section. “Kettles” are 1-5 m wide, circular, and commonly 0.5-1 m deep with linguloid flow structures along their margins which indicate outward flow of sediment-rich water. In the central part of some “kettles” occurs a “plug” of resistant cemented sand, and some sections also display “feeder” channel cutting the underlying lacustrine siltstones. Some “kettles” are partly infilled with alternating siltstones and cemented finegrained sandstone, but their pre-erosional topography is poorly known, but seems to be dome-shaped. Some “kettles” in cross section look like collapsed domes, whereas some are funnel-shaped. Outward-directed flow structures are also developed along some fissures cutting lacustrine sediment, and are composed of porous, fine-grained sandstone with numerous micro-voids. Pleistocene “kettles” are located in SE corner of Novigrad Sea, at the location of modern fresh-water spring Nozret. They stratigraphically post-date deposition of lacustrine sediments, and mark cessation of the lake and exposure of the lake bed. The forms are attributed to fluidization and hydrofracturation which formed two types of “kettles” ; funnel-shaped and collapsed dome-shaped. The pressurized silt-rich water abruptly flushed out, forming linguloid siltstone flow structures which seem to be “fossilized” by rapid freezing as the slurry reached the surface. The formation of “kettles” is attributed to strong seismic shock which initiated hydrofracturation of lacustrine sediments.

proglacial lake, flow structures, hydrofracturation

nije evidentirano