Page 5 - i1052-5173-28-3-4
P. 5

A     78˚W    76˚W                            B                           C
            Hickory Run                                   C
43˚N

42˚N                                                 D
                                                       F
      Last Glacial Maximum (26 ka)  100 km        E       250 m
      Illinoian (130 ka?)                     E
      Pre-Illinoian (>770 ka)

D                                                                                             F

Figure 1. Study site. (A) Hickory Run location in relation to the extent of the Last Glacial Maximum (LGM) (26 ka, Corbett et al., 2017b), Illinoian (130 ka?),
and pre-Illinoian glaciations, after Sevon and Braun (2000). Hickory Run is 2 km south of the LGM boundary and is mapped within the Illinoian and pre-
Illinoian glaciations. (B) Locations of photographs; (C) tors on a ridgeline 700 m NE of the field; (D) elongate, angular, large boulders upslope; (E) small,
rounded boulders downslope; and (F) massive, angular conglomeritic boulders in the SE sub-field.

mostly subrounded and underlain by small,     Peters, 1967). Nuclides build up over time      production rates before they were
polished clasts with a red weathering rind    and can be used to provide age control for      exhumed, when they were covered by
(Fig. 1E). There is a distinct subsection of  surficial deposits; however, such dating        other boulders, and/or when they flipped
the field to the southeast with boulders      requires that at the time of initial surface    during transport.
mostly >5 m long; these appear to be bed-     exposure, rock contained few if any
rock shattered along bedding planes (Fig.     nuclides (Lal, 1991). This is not the case for    If rock surfaces experience burial before,
1F). The field is surrounded by coniferous    boulder fields because both models of devel-    during, or after exposure, by flipping or
forest with stony loam soils (NRCS,           opment (see Introduction) include initial       cover with soil, snow, ice, or other boulders,
2014).                                        cosmic-ray exposure before incorporation of     such complex histories can be detected by
                                              blocks into the field (on cliffs or below a     measuring two isotopes with different
  Glacial erratics are found south of         weathered regolith mantle).                     half-lives in the same sample (Bierman et
Hickory Run (Pazzaglia et al., 2006;                                                          al., 1999; Nishiizumi et al., 1991). Such
Sevon and Braun, 2000), indicating that it      The pertinent question becomes,               analyses most commonly employ 26Al and
was covered by ice at least once, although    “Where were the sampled boulders when           10Be, which are produced in quartz at a
the timing of ice advances is not well        they received the cosmic ray dosing that        ratio of ~7:1 (Argento et al., 2013; Corbett
known (Braun, 2004), and we found no          accounts for the 10Be and 26Al concentra-       et al., 2017a). Because the 26Al half-life,
obvious erratics in the field. The last       tions they contain today?” This question        0.71 m.y. (Nishiizumi et al., 1991), is about
glaciation to override Hickory Run is         arises because there is no unique and           half that of 10Be, 1.38 m.y. (Chmeleff et al.,
mapped as Illinoian (ca. 150 ka; Fig. 1A),    agreed upon process model for boulder           2009; Korschinek et al., 2010), if an
though it is possible that it was 400 ka      field development. If boulders were             exposed sample is buried, the 26Al/10Be
(Braun, 2004). South of the boulder field,    sourced from outcrops upslope of the field      ratio will decrease; if that sample is re-
reversed magnetic polarity deposits indi-     and moved downfield, they inherited             exposed, production of nuclides begins
cate that the oldest, most extensive glacia-  nuclides from exposure on the outcrops. If      again and the ratio increases. Because of
tion was in the early Pleistocene (likely     boulders originated in place, they inherited    the relatively long half-lives of 26Al and
>900 ka); there is another event mapped       nuclides from subsurface exposure. In           10Be, the 26Al/10Be ratio is only sensitive to
between the Illinoian event and the >900      either case, measured nuclide concentra-        burial by meters of material for >100 k.y.
ka event, distinguished by proglacial lake    tions do not allow direct dating of the time    (Lal, 1991).
sediments of normal polarity, likely <740     any boulder became exposed as part of the
ka (Braun, 2004).                             boulder field; rather, they allow for the         Published measurements of cosmogenic
                                              calculation of minimum total near-surface       nuclides, made on samples collected from
APPLICATION OF COSMOGENIC                     histories for each sampled boulder. Such        rock surfaces in high-latitude boulder
NUCLIDES TO BOULDER FIELDS                    histories integrate cosmic-ray exposure         fields, suggest that some blocks were
                                              and express it as the equivalent of uninter-    exposed to cosmic rays relatively recently,
  Cosmogenic nuclides are produced pre-       rupted surface exposure. These times are        while others have concentrations consis-
dominately in the uppermost meters of         minima because we know boulders eroded          tent with near-surface histories extending
Earth’s surface by cosmic ray bombard-        and also experienced less than surface          over hundreds of thousands of years.
ment (Gosse and Phillips, 2001; Lal and                                                       For example, 36Cl concentrations in 18

                                                 www.geosociety.org/gsatoday                                                                                     5
   1   2   3   4   5   6   7   8   9   10