Fragipan Horizon - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 158

The maximum expression of fragipans in IL occurred in lower, moderately
well-drained topographic positions (Grossman et al. 1959a ). In their analysis of
fragipan soils in Midwestern USA, Franzmeier et al. ( 1989 ) noted that fragipans
were most common in soils on slopes

12 %. Although our analysis and other
studies (e.g., Ciolkosz et al. 1992 ) suggest that fragipan soils may occur on all slope
classes from 0 % to

<

25 %, drainage is important in their formation. In eastern
USA, the maximum expression of the fragipan occurs in soils of intermediate
wetness, e.g., somewhat poorly drained and poorly drained soils (Grossman
and Carlisle 1969 ). In Pennsylvania fragipan soils could be ranked by drainage
class from most abundant to least abundant: poorly drained > moderately well
drained

>

well drained (Ciolkosz et al. 1992 ). Fragipans appear to require a mini-
mum 18,000 yr to form (Ciolkosz et al. 1992 ). However, the time required and
stages of fragipan formation are not well understood.

>

15.6 Genesis of Fragipan

Several investigators have recognized stages in the development of fragipans
(Grossman et al. 1959c ; Miller et al. 1971b ; Ciolkosz et al. 1992 ; Weisenborn
and Schaetzl 2005b ) and have suggested that fragipans eventually may degrade
(Weisenborn and Schaetzl 2005b ; Szymanski et al. 2011 ). Numerous investigators
have provided descriptive models of the genesis of fragipans and these can be
divided into physical, chemical, and physicochemical models (Table 15.4 ). Some
explanations of strictly regional fragipans include earthflows triggered by earth-
quakes (Scalenghe et al. 2004 ; Certini et al. 2007 ) and periglacial processes from
former permafrost (van Vliet and Langohr 1981 ; Habecker et al. 1990 ; Payton
1992 , 1993a , b ; French et al. 2009 ). One model of fragipan formation is based on
the existence of lithologic discontinuities (Habecker et al. 1990 ; Aide and Marshaus
2002 ; Bockheim 2003 ; Wilson et al. 2010 ) or lithic/paralithic contacts (Lindbo
and Veneman 1993 ; Aide and Marshaus 2002 ) that influence the movement of
weathering products and enable them to accumulate as a fragipan. Other studies
have emphasized the importance of the architecture of soil pores, influenced by the
nature of the soil parent materials and by pedogenesis (Ajmone-Marsan et al. 1994 ;
Lindbo et al. 1994 ; Falsone and Bonifacio 2009 ). Not only was there a decrease in
total porosity with depth in the fragipan, but also residual pores were more abundant
than transmission pores. The “hydro-consolidation” hypothesis (Bryant 1989 )
involves a collapse of soil structure when it is “loaded and wet” (see also Assallay
et al. 1998 ). Repetitive wetting and drying cycles may be critical to enable disper-
sion of clay and precipitation of clay on mineral grains, respectively (Miller
et al. 1971a , b ; Attou and Brua 1998 ).
Although physical processes are certainly involved in the evolution of fragipans,
chemical processes may also be important. Silica may be released from weathering,
translocated into the subsoil, and along with Fe oxides, bind with clays (Steinhardt
and Franzmeier 1979 ; Steinhardt et al. 1982 ; Norfleet and Karathanasis 1996 ;