Sedimentary records of various three- to four-dimensional patterns of
nature have recently attracted the special attention of many geologist
s. The dramatic development of rhythmic sequence stratigraphy is a con
spicuous recent instance. The significance of event sedimentation in g
eohistory, however, has not been examined sufficiently. The problem of
how to read the records of triggers or thresholds of sedimentary even
ts has been ignored. The difficulty of solving this problem concerns t
he difference in patterns between natural sedimentary processes and th
eir resulting sediments. Terminological confusion of event deposits is
another cause which hinders our taking note of this problem. A few ex
amples of endeavours to discriminate the threshold of sedimentary even
ts are shown by the analysis of two kinds of turbidites of the Mid-Gua
temala Trench, the investigation of slump-derived turbidites of the Su
ruga Trough, and the study of the seismoturbidites in Lake Biwa. Well-
focused investigations of features of trigger-known sediments is the b
est strategy to find the criteria for discriminating the triggers or t
hresholds based on sedimentary records of various sedimentary events.
From this point of view, more detailed studies of the Santorini Erupti
on-induced homogenites, the K-T boundary impact-induced tsunamiites, t
ogether with the Lake Biwa seismic turbidites, are very important. Fur
ther efforts should be made to find and investigate as many trigger-id
entifiable sediments as possible.