Glacial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 304

FIGURE 5 Simplified drawings based on actual specimens of trace fossils from Connecticut River
valley (USA) varves. (A) Surculichnus bifurcauda in epirelief. Gray area is negative relief; hatched area
is positive relief. (B) Unknown arthropod trace fossil (UATF) 4 in epirelief. Gray area is negative relief;
hatched area is positive relief. (C) UATF 3 in positive hyporelief (cast). (D) UATF 2 in positive hypo-
relief (cast). (E) UATF 1 in positive hyporelief (cast). (F) Undichna unisulca . (G) Five morphotypes of
Broomichnium flirii . (H) Three morphotypes of Undichna britannica .(I)Twomorphotypesof
Undichna simplicitas .(J) Undichna bina . Scale bars in (A), (D), (G-J ¼ 2 cm); (B, C, and E ¼ 1cm);
(F ¼ 10 cm). See Benner et al. (2009) for detailed explanations and interpretations of each trace fossil.

fossils, is coincident with a sedimentologically corroborated shift from mostly
glacial input to the lake to mostly non-glacial inputs ( Benner et al., 2009 ). The
last identified stage in the succession, Stage 4 (NEVC year 7610; 13.51 ka

BP), coincides with a sedimentological shift to higher percentages of sand depo-
sition, probably indicating shallower lake environments as water levels dropped
or as the basin filled. These contain an abundance of fish breeding pits, Piscich-
nus , in addition to arthropod and other fish-produced trace fossils. The apparent
lack of small invertebrate traces (i.e., Cochlichnus ) in Stage 4 varves may be a
result of taphonomic factors (poor preservation in sand) rather than actual
abundance of the tracemaker. It is possible that the rate of stage transition (suc-
cession) increased as time passed in the valley after a significant reservoir of
organisms ready to inhabit newly exposed water ways had accumulated in ice-
free areas. Close observation of species assemblages in modern late-stage glaci-
ated valley lakes may help to elucidate these processes.