Surficial Geology of the Okoboji (Iowa) 7.5` Quadrangle

Surficial Geology of the Okoboji (Iowa)
7.5' Quadrangle
95°15'0"W
95°12'30"W
43°30'0"N
Qo
Qglhc
Qglhc
Qe
Introduction to the Surficial Geology of the Okoboji 7.5’ Quadrangle
SURFICIAL GEOLOGIC MAP
OF THE 7.5’ OKOBOJI QUADRANGLE
Qglhc
Qo
Iowa Geological and Water Survey
Open File Map OFM-12-10
September 2012
Qglhc
Qoch
Qo
95°7'30"W
43°30'0"N
Qtr_amc
Qglhc
95°10'0"W
Qglhc
Qglhc
Qal
prepared by
Qpglp(ss)
!
Qo
Qe
Qo
!
Deborah Quade, Stephanie Tassier-Surine, Robert McKay,
and James Giglierano
Qglhc
!
Iowa Geological and Water Survey, Iowa City, Iowa
Qpglp(ss)
Qglhc
Qe
Qo
Qal
Qoch
Qe
Qglhc
Qoch
Iowa Department of Natural Resources, Chuck Gipp, Director
Iowa Geological and Water Survey, Robert D. Libra, State Geologist
Qpglp(ss)
Qglhc
Qglhc
Qglhc
Qe
Supported in part by the U.S. Geological Survey
Cooperative Agreement Number G11AC20247
National Cooperative Geologic Mapping Program (STATEMAP)
Qglhc
Qal
Qoch
Qglhc
Qglhc
Qtr_amc
Qglhc
Qal
ACKNOWLEDGMENTS
Special thanks to Gary Gould, Galen Rosenboom and Bernell Walker for allowing us access to their properties; and to Brooks National Golf
Club, Okoboji View Golf Course, the city governments of Arnolds Park, Spirit Lake, West Okoboji and the Iowa Lakeside Laboratory Regents
Resource Center for graciously allowing us access to their properties. Also, thank you to the Dickinson County Conservation Board and to the
Department of Natural Resources (DNR) for allowing IGWS access to numerous public land areas. Thanks to Jason Vogelgesang of the Iowa
Geological and Water Survey (IGWS) for assisting with drilling efforts and geophysical surveys. Dr. Art Bettis (University of Iowa) provided
valued information concerning the surficial geology. Natural Resources Conservation Service staff, Kathy Woida, James Johnson and Joe
Thompson assisted with drilling and well installation as part of a collaborative project to install shallow groundwater monitoring wells in the
Okoboji Quadrangle. A special thanks to Steve Anderson, Urban Conservationist with the Dickinson SWCD/IDALS. Steve’s assistance at the
local level was key to locating numerous drilling and well locations on city government and private lands in the Okoboji area. In addition, Mary
Howes and Casey Kohrt (IGWS) provided GIS mapping technical help and Paul VanDorpe (IGWS) provided proof reading.
Qglhc
!
Qo
Qo
Qe
Qglhc
Qglhc
Qe
43°27'30"N
Qoch
Qglhc
A
Qglhc
LEGEND
!
Qglhc
Qoch
Qo
CENOZOIC
Qglhc
QUATERNARY SYSTEM
!
Qo
Qo
Qglhc
Qglhc
Qglhc
B
Qo - Depres sio ns (DeForest Formation -Wo den Mbr.) Generally 2 .5 to 6 m (8-20 ft) of black to very dark gray ,
calcareous , muck, peat, an d s ilty clay loam colluviu m and o rganic s ed imen ts in drained and undrain ed clos ed and
semi-closed depressions. O verlies gray, calcareous , loam diamicton (Dows Fm. -Morgan/Alden Mb r.) or No ah
Creek Fm. sand. Associated with low relief features that occupy depress ions and low s ags o n the land scape.
Suppo rts wetlan d vegetation and can be permanently covered by water. Hig h water table.
Qo
!
Qglhc
Qal
!
HUDSON EPISODE
Qglhc
Qglhc
Qo
Qal
43°27'30"N
Qo
Qo
Qe
Qglhc
Qglhc
Qtr_amc
Qo
Qtr_amc
Qo
Qo
Qo
Qoch
Qal
Qo
Qpglp(ss)
Qo
Qoch
Qal - Alluvium (DeFo rest Formatio n-U ndifferentiated) Variable th ickness of less than 1 to 5 m (3-16 ft) of very
dark gray to b rown, no ncalcareous to calcareou s, mas sive to stratified silty clay loam, clay loam, loam to sandy
loam allu vium and colluviu m in stream valleys, o n hillslopes, and in clos ed d ep res sio ns. M ay overlie Dows
Formation (M organ or Alden M brs.) or Noah Creek Formatio n. As sociated with low-relief modern flood plain,
closed dep ress ions , mo dern d rainageways , or toeslo pe p ositio ns on the land scap e. Seas onal hig h water tab le and
potential fo r frequent flood ing .
Qal
!
Qtr_amc
Qpglp(ss)
Qglhc
Qpglp(ss)
Qo
Qo
Qpglp(s s) - La ke Sediment s mall-s ca le la ndform features (Dows Fo rmation-Woden Mbr.) G enerally less than 3
m (1 0 ft) of dark grayis h brown, mass ive, calcareous , silty clay loam o r s ilt loam overlyin g a thin (<1 m) basal
increment o f sand and gravel. U nit overlies yellowish to grayis h b rown calcareous, s tratified loam to silt loam to
sand y loam diamicton ; textures can be quite variable (D ows Fm.-Mo rgan M br.) o r it may o verlie a gray ,
calcareous , mass ive, dense lo am diamicton (Do ws Fm.-Ald en M br.). Small p ostglacial lake beds us ually
asso ciated with very low relief till plain w ith reticulate-linked depress ion systems and occas ionally on hummock y
mo derate relief terrain. Hig h water table.
Qpglp(ss)
Qo
HUDSON and WISCONSIN EPISODE
!
Qe
Qglhc
Qo
Qglhc
Qo
Qglhc
Qe - Sand Dunes a nd Sand Sheets (Peoria Formation -s and facies) Generally les s than 3 m (10 ft) of yello wish
brown, mas sive, calcareous, loamy s an d to fine s and. It may overlie yello wish -b ro wn, coarse-grained sand and
gravel (Noah Creek Fm.), or it may o verlie yellow ish to gray ish brow n, usually calcareous , stratified loam to s ilt
loam to sandy loam diamicto n (Dows Fm.-M organ Mb r. ). Usually restricted to a narro w belt along majo r river
valley bottoms o r ad jacent up lands on the Des Moines Lob e. Off th e Des M oines Lobe this unit is no t restricted to
dunes along valley areas and may occur as s and stringers overlying unnamed eros ion surface loamy sediments.
Qe
!
Qpglipr
Qglhc
43°25'0"N
43°25'0"N
!
Qpglipr – I ce push ridge features (DeForest Fo rmatio n-Tribo ji Mbr.) Gen erally 3 to 5 m (10-16 ft) of dark gray ,
dark gray ish brow n, d ark brown to dark yello wis h brow n, mediu m to coars e s and , g ravelly sand to pebbly g ravel ,
with poss ible inclus ions o f fine-g rained o rganic-rich silt. Unit overlies gray, calcareo us, stratified loam to sand y
loam d iamicton; textures can be q uite variable (Do ws Fm.-Morgan Mb r.) or it may o verlie a g ray, calcareous ,
mass ive, dens e loam diamicto n (D ows Fm. -Alden Mb r. ). Lake sediment ass ociated with extant lak es of the Des
M oines Lo be. High water table.
WISCONSIN EPISODE
!
!
Qal
Qoch
Qglhc
Qglhc - Collapsed la ke sediments -hummocky isolated ice-walled lake plains (flat-topped hummocks ) (Dows
Formation-Lake Mills Mbr./Morgan M br.) Generally less than 3 m (10 ft) of dark grayish brown, mas sive,
calcareous , s ilty clay loam to s ilt lo am; often overlying a th in basal increment of s an d and gravel (<1 m). It
overlies y ellowish to grayis h b rown, us ually calcareous , stratified loam to silt loam to sandy lo am d iamicton;
textures can be q uite variab le (Dows Fm.-M organ M br.). Is olated ice-walled lake p lains comp rise a p ortio n of this
land for m area. Oth er h ummocks consist of less than 15 m (49 ft) of yellowis h to grayis h b ro wn, u sually
calcareous , stratified lo am to s ilt loam to sandy loam diamicton (D ows Fm.-Morgan Mbr.). Overlies gray ,
calcareous , mas siv e, dense loam diamicton (Dows Fm.-Ald en Mb r. ). M oderate to hig h relief h ummocky landform
area with 3 to 1 0 m of local relief.
Qglhc
!
!
Qoch
Qglhc
Qglhc
Qo
Qoch
!
Qtr_amc
Qe
Qtr_bm
Qal
Qal
Qal
Qtr_amc
Qglhc
Qe
Qoch
Qtr_bm
!
!
Qglhc
!
Qo
Qoch
Qoch
!
43°22'30"N
!
Qoch
95°15'0"W
95°12'30"W
0
0.5
1
0
0.5
1
3
2
GEOLOGIC CROSS-SECTION A-B
Qo
1400
Qglhc
Qglhc
Qglhc
Qo
Qal
Qtr_amc
Qoch
Qal
LAKEFIELD_SW
LAKEFIELD_SE
LAKE_PARK
OKOBOJI
SPIRIT_LAKE
LAKE_PARK_SE
MILFORD
SPIRIT_LAKE_SE
Qtr_amc
Eolian
Deposits
Glacial
Deposits
Qo
Qal
1500
Qal
Location Map
Correlation of Map Units
Alluvial and Outwash
Deposits
B
Hwy 86
Qglhc
SIOUX_VALLEY
Drill Holes
±
4
Miles
Kilometers
4
3
Montgomery
Qal
95°7'30"W
Qo
1400
Qu
1300
1300
1200
1200
Elevat ion (feet above sea level)
Elevation (feet above sea level)
1500
Little Sioux
River
2
Adjacent 7.5' Quadrangles
Wa ter Features Rivers, lakes and small p onds fo rmed b y blo ck age o f drainageways and river channels. Extent
map ped as sh own in county so il s urveys and as identified o n aerial imagery.
95°10'0"W
1:24,000
Baker, R.G., Rhode s, R.S. II, S chwe rt, D.P., Ashw orth, A.C., Frest, T.J., Hallberg, G.R., a nd Janssens, J.A., 1986, A ful l-glacial biot a
from sout hweste rn Iowa, USA: Journal of Quaternary Sc ie nc e, v.1, p. 91-107.
Bettis, E.A. III, Q uade, D.J., and Kemmis, T .J., 1996, in Hogs, bogs, and logs: Q uat ernary deposits and e nvironme nta l geology of the
D es Moines lobe . Edited by: E.A. Be ttis, III, D.J Q uade, a nd T .J Kemmis. Iowa D epa rtment of Natura l Resourc es-Geologica l
Survey Bureau, G uidebook Series No. 18, 170 p.
Bettis, E.A. III, 1997, La te-M iddle and E arly-L ate Wisconsin G laciat ion in Nort h Central Iowa . G eological Society of America NorthCe ntral S ection M eeting Abstra cts with Programs 29 (4):5.
Brevik, E., 2000. L imits of ic e thickness in Iowa during the Late Wisconsia n: Journa l of the Iowa Academy of Science, v.107, p. 4650.
Boellstorff, J., 1978a, North Americ an Pleistocene Stages reconsidere d in l ight of probable Plioc ene -Pleistocene c ontine nta l
glacia tion: Science, v. 202 p.305-307.
Boellstorff, J., 1978b, Chronology of some late Ce nozoic de posits from t he ce ntral Unit ed St ates and the ice ages: Transa ctions of the
N ebraska Academy of Scie nce, v. 6, p. 35-49.
Clark, P.U., 1992, Surfa ce form of the southern L aurent ide ic e she et and its implic ations to ice-sheet dyna mics. Geologic al Society of
A merica Bulletin, v. 104, p. 595-605.
Fullerton, D .S., 1986, St ratigraphy a nd correlation of glacia l deposit s from Indiana to Ne w York and Ne w Jersey, in Ric hmond, G .M.
a nd Fullerton, D.S., eds., Q uate rna ry gla ciations in the U nite d St ates of America, in Sibrava, V., Bowen, D.Q., a nd Richmond,
G .M., eds., Qua ternary Glac iations in the Northern He misphere: Quaternary Sc ie nce Re view s, v. 5, p 23-37.
Ha llberg, G.R., 1980, Ple ist oce ne stratigraphy in e ast -ce ntral Iowa: Iowa Geologic al Surve y Te chnica l Informa tion Serie s 10, 168 p.
Ha llberg, G.R., 1986, Pre-Wisconsin glacia l stratigraphy of the centra l plains re gion in Iowa, Nebraska, K ansa s, and Missouri: in
Richmond, G.M. a nd Fullerton, D.S., eds., Quaternary glac iations in t he Unite d States of Americ a, Report of t he Internationa l
Correlation Programme-Project 24: in Sibrava, V., Bowen, D .Q., and Richmond, G.M., eds., Quaternary Sc ienc e Reviews,
Q uat ernary Glac ia tions in the Northern Hemisphere, v. 5, p. 11-15.
Ha llberg, G.R., Line bac k, J.A., Mickelson, D.M., Knox, J.C., Goebe l, J.E., Hobbs, H .C., Whitfield, J.W., Ward, R.A., Boellsto rf, J.D.,
a nd S wine hart, J.B., 1991, Quate rnary geologic map of the D es Moines 4° x 6° quadrangle, U nited States: U.S. Geologica l
Survey, M iscella neous Investigations Serie s, M ap I -1420, 1:100,000 sca le map shee t.
Hershe y, H. G., 1969, Geologic ma p of Iowa , Iow a Geological Survey, sca le 1:500,000 (out of print).
Hooyer, T.S. and Ive rson, N.R., 2000, Cla st-fabric development in a she aring granular ma teria l: Implications for subgla cial till and
fault gouge. Geologica l. Societ y of Ame rica Bulletin, 112(5), 683-692.
Hooyer, T.S. a nd Iverson, N.R.., 2002, Flow mechanism of the Des M oines lobe of t he Laure ntide ice she et, Journa l of Glac iology.,
48(163), 575-586.
Johnson, W.H., 1986, Strat igra phy and c orrelat ion of the glacial deposit s of the Lake M ichiga n lobe prior to 14 ka BP, in Ric hmond,
G .M. and Fullert on, D.S., eds., Quaternary G laciat ions in the United State s of America, in Sibrava, V., Bow en, D.Q ., a nd
Richmond, G.M ., e ds., Q uat ernary Glacia tions of t he Nort hern He misphere: Quaterna ry Sc ienc e Reviews, v. 5, p.17-22.
Kemmis, T .J, Be ttis, E.A. III, a nd Quade, D.J., 1994, T he Des Moines lobe in Iowa: a surging Wisconsinan glacier: American
Q uat ernary Assoc iation, Program a nd Abstracts, 13th Bie nnial Me eting,
Mc Bride , T . H., 1899, G eology of Osceola and D ickinson Counties: Iowa Geologica l Surve y, Annual Report, v. 10, p. 185 -239.
Ma thew s, W.H., 1974, Surfac e profile of the Laurent ide ice shee t in its ma rgina l area s: Journal of Glaciology, v. 13, p. 37-43.
M inneapolis, M innesota. p. 112.
Prior, J.C. and Korht, C.J., 2006, The Landform Regions of Iowa, Iowa Geologica l S urvey, digital ma p, available on IDNR GIS
L ibrary- http://www.igsb.uiow a.edu/nrgislibx/
Quade, D.J., Gigliera no, J.D ., and Bett is, E.A., III, 2004, Surfic ial ge ologic mate rials of the Des Moine s L obe of Iowa , Phase 6:
D ickinson and E mmet Counties: Iowa Geologica l Surve y Open Fi le M ap OFM-04-2, 1:100,000 scale map sheet.
Quade, D.J. and Vogelgesa ng, J.A., 2011, East Okoboji Tec hnical Assessment for IDNR Fisheries Bureau, Iowa G eological and
Wate r Survey, 11 p.
Ruhe, R.V., 1950, Rec la ssification and correlation of the glac ial drifts of nort hweste rn Iowa and adjacent a reas: University of Iow a
D epa rtment of G eology, Iowa City, unpublished Ph.D. thesis, 124 p.
Witzke, B.J., Anderson, R.R. and Pope , J.P., 2010, Bedrock G eologic Map of Iow a, sca le: 1:500,000, Iowa Geological and Water
Survey, O pe n File Digital M ap OF M-10-1.
Other Mapping Units
43°22'30"N
Qpglipr
A
Qtr_bm - Till ridge (Do ws Fo rmatio n-Morgan Mb r.) Generally 3 to 15 m (10-49 ft) o f yellowis h to grayish
brown, us ually calcareo us and fractu red, stratified loam to silt loam; stratified sands and gravels to sandy loam
diamicto n; textures can be quite variable. Overlies gray, calcareo us, mass ive, dense lo am diamicton (Dows Fm.Ald en Mb r. ). The Alden M br. in th is mapp ing unit rarely extend s to depths greater than 15 meters (4 9 ft) and
overlies the Sheld on Creek Formatio n d iamicton. At the DM L margin, this lan dform may be mantled w ith a thin
layer o f Peoria Fo rmation s ilt. Low to mo derate relief hummo ck y lan dform features exceed 3 to 5 m (10 -16 ft) of
local relief. This landform is as sociated with the Bemis Mo raine. Seas onal high water tab le.
Qu – Glacia l Till (Do ws Formation and Sheldo n Creek Formation-und iff.) Generally 3 to 50 m (10-17 0 ft) of
yellowis h brown to gray, calcareous , fractu red to mass ive, clay loam; at d ep th this unit can b e variab ly textured
and co ntain significant sand and g ravel bod ies. It is not uncommo n to see few Pierre Shale clasts in core s amples .
This unit o verlies the much older Pre-Illinois ag e diamicto n. Pre Illinois s ediments are generally not more than 9
to 50 m (27 -170 ft) of very d ense, mas siv e, fractured, loamy glacial till of th e Wolf Creek or Alburnett formations.
These formatio ns are undifferentiated an d sho wn on ly in the cros s-sectio n.
Qu
Qal
Qal
Qoch
Qtr_a mc - Till ridge (Do ws Formatio n-Morgan, Pilo t K nob, or Lake Mills Mb rs .) Generally 9 to 15 m (2 7-49 ft)
of yellowis h to grayish brow n, us ually calcareo us and fractured , stratified loam to s ilt loam; stratified sands and
gravels to sandy loam diamicto n; textures can b e q uite variable. Overlies gray, calcareous, massive, dense loam
diamicto n (Do ws Fm.-Ald en M br.). The A lden Mbr. in this map ping unit can extend to dep ths in excess of 25 m
(85 ft). This s ediment package overlies the Sheldo n Creek Formation diamicton or Pre-Illinois age undifferniated
diamicto n. M oderate to hig h relief hummo cky landform features ex ceed 3 to 10 m (10-33 ft) of local relief. This
land for m is as sociated w ith the Altamon t M oraine Co mplex and is characterized by mod erate to hig h relief
hu mmo ck s, esk er and kame features, and ice-walled lakes. Seasonal high water table.
WISCONSIN AND PRE-ILLINOIS EPISODE
Qoch
References
Qoch - Valley train outwas h (No ah Creek Formation) Generally 3 m (1 0 ft) to less than 8 m (26 ft) o f dark gray,
dark gray ish brow n, d ark brown to dark yello wis h brow n, mediu m to coars e s and , g ravelly sand to pebbly g ravel.
Overlies gray, calcareou s, massive, dense, loam diamicton (Dows Fm.-Alden Mb r.). In valley pos itions , it is at the
land su rface of o lder terraces . On the mod ern floo dplain , it is buried by DeFo res t Fm. alluvium. Low-relief
land for ms expres sed as broad terraces; lo ng, narrow lo ngitudinal terraces ; or cus pate-s haped p oint terraces .
Terraces as sociated with the major valley s are b enched on a gray, calcareous , mass ive, dense loam diamicton
(Dows Fm.-Alden Mbr.).
Qoch
Qpglp(ss)
Qoch
The Okobo ji Quadrangle is located in Dick ins on Co unty in north western Io wa. Th e map ping area
encomp as ses the western marg in of the Late Wisco nsin -age Des Mo ines Lob e (DM L) landform region, the most
recently glaciated reg ion of the s tate (Prior an d Ko hrt, 2006 ). The DML is the prod uct o f a Late Wiscons in lob ate
extens ion of the Laurentide Ice Sheet that flo wed dow n a reg ional topo grap hic lo w into Iowa app ro ximately 1 5,000
years before pres ent and reached th e termin al pos ition at Des Mo ines abo ut 13 ,800 RCYB P. The DM L landform is
bo unded b y p re-Wis co nsin topog raph ic highs on th e eas t (M iss iss ippian bedro ck ) and wes t (pre-Wisco nsin glacial
depos its comp rising the Prairie Co teau). In the map area, the DM L consists of a co mplex suite of d ep ositional
land for ms and sediment seq uences related to s upraglacial, sub glacial, and prog lacial sedimentation. The D ML is
characterized b y hu mmock y terrain that fo rms arcuate belts o f mo raine comp lexes and undu latin g plains w ith th ick
increments o f sup rag lacial sed iment (>3 m). In the map area, the Bemis M oraine (terminal en d moraine) occupies
a narro w s liver of the map area in the extreme so uthwest portion of the map (west of th e Little Sio ux River valley).
The res t of the map is dominated b y the y ounger Altamont M oraine Co mp lex (AM C) wh ich is mapped as a
complex this far north on the DM L. The land scape is characterized b y a co mplex s ed iment p ackage and a hos t of
geomorphic features ranging fro m kames, esk ers, clos ed and semi-lin ked d epres sio ns, and ice-walled lak es o n the
mo raine surface; and ka me-type terraces on the eastern wall o f the Little Sioux River Valley in the sou thern part of
the map area. A dominan t feature o n the map is Wes t Okobo ji Lake. The lake b ottom sediments ass ociated with
th is and area lak es are categorized in to pos t glacial age depo sits k nown as the West Okob oji M ember of the
DeFores t Fo rmatio n (Bettis et al., 19 96). The West Okob oji M ember represents the organ ic-rich silt depos its
asso ciated w ith lake sedimentation but do interfing er with co lluv ial and organic sediments ass ociated w ith s lough
and b ackwater areas as well as interfingerin g with u nnamed sandy an d gravelly sediments (Tribo ji Memb er, Quade
and Vogelgesang, 20 11) associated w ith ice-pu sh ridges (rid ges res ulting from the pus hing action of a lake’s ice
sheet against the s horelin e). The map pin g reveals that yo unger advances o f ice re-advanced ov er p rev ious
pos itions; the Altamont I and II moraines reco gnized in central Iowa are not dis cernable in this area. The Bemis
and Altamo nt ice advances are well dated with an ap proximate 1,00 0 year time span between the two ad vances
(Bettis et al., 1 996).
McBride (18 99) d es cribed and map ped the Quaternary geolo gy of the county and no ted “D ickins on Co unty
is pre-eminently the county o f lakes and well isolated kames ” in h is co unty rep ort. T he only regio nal s urficial map
of the area consists of the Des Moines 4 o x 6 o Q uad ran gle at a scale of 1 :1,00 0,000 (Hallberg et al., 199 1). Recent
map ping includ es the Su rficial Geo log ic Map of Dickinson and Emmet Co unties (Quade et al., 2004 ). Statew ide
bedrock geologic maps by Hershey (1969 ), and mos t recently by Witzke, Anderson, an d Po pe (20 10), depict the
increased un ders tanding o f the complex d istribution of geologic u nits at the bedrock s urface acro ss this region,
including Dickinso n County.
The map area has a rich and complex geolog ic his tory p unctuated by at least s even periods of glaciation
between 2.2 million to 500, 000 years ago (Boellsto rff, 197 8a, b; Hallberg, 1980, 198 6). In this area, Pre-Illinoian
Episode glacial deposits and ass ociated buried soils are overlain by much you nger Wisconsin-age glacial depos its .
During earlier and mid W iscons in-age, glacial materials asso ciated with ice adv an ces d ating from ap proximately
40, 000 to 26,0 00 years before pres en t were dep osited thro ughou t the map area. The d iamicton has very s imilar
properties to Dows Formation Alden Mbr. diamicton (s ame source area), s o identification in the field is d ifficult.
Identification is aided by the preservation of weathered Sheldon Creek surfaces (oxidized and no ncalero us) and in
so me ins tances th e p res ence o f a pro glacial s and and gravel u nit. In Iowa, this glacial depo sit is fo rmally
reco gnized as the Sheld on Creek Formation (Bettis et al., 1996 ; Bettis, 1997), and in earlier literature is referred to
as the “Tazewell till” (Ru he, 1950 ). Appro ximately 15 ,000 years ag o the s outhern edge of th e Laurentide Ice Sheet
sp lit into s everal lobes that each flowed down regio nal topograph ic lo ws. T he DM L extend ed from cen tral Canada
th ro ugh the Dakotas and Minnesota into Iowa, terminating at what is now the City of Des Mo ines. The D ML was
active in Iowa b etween about 15 ,000 and 1 2,000 RCYBP, abo ut 5, 000 to 8,000 years later than g lacial lobes to the
east made their s outhern mos t max imum ad vance (Jo hnso n, 19 86; Fu llerto n, 198 6). The DM L advance occurred
during a period of regio nal warming and was thus climatically o ut of equilib rium (Kemmis et al., 1994). Ice
th ickness reco nstructions indicate that the D ML was p ro bably th in an d g en tly sl oping (M athews, 197 4; Clark,
199 2; Brevik , 20 00; Hooy er and Iverson, 200 2). Clark (1992 ) recons tructed the DM L thick nes s n ear A mes, Io wa,
at ~8 0 m. M ore recently, ice recons tructions by Ho oyer and Iverso n (20 00) were based on a mo del as suming the
Bemis Mo raine was ice-cored, w hich yielded ice th ick ness es timates o f ~250 m. Despite these variatio ns, all agree
that the D ML ice sheet was extremely thin an d gently s lop ing. T his ice advance was rapid and ep isod ic and was
mo st likely fueled b y basal lub rication; in other wo rd s, a warm -based, non -d eforming b ed glacier. These
assu mptions are b acked up b y evid en ce o f numerou s plants (Baker et al., 1986 ) and trees (Bettis et al. , 199 6) found
near th e base of th e DM L package. Fu rthermore, the co mplex land form sediment assemb lages fo und on the DM L
in Io wa seem more ind icative of regio nal stagnatio n, by a surging-type glacier, not rapid reces sio n.
Surficial depo sits o f the map area are co mp osed o f four fo rmatio ns: DeForest, Dows , Noah Creek, and
Peo ria formations . Huds on age depos its as sociated with fin e-grained alluvial, organic and co llu vial sediments
include the DeForest Formation , which is s ubdivided into the Camp Creek, Roberts Creek , Gunder, Corring ton,
Flack, an d Woden members . The D ows Formation cons ists of u pland glacial d ep osits and is s ubdivided into the
Ald en, Lake M ills, Morgan, and Pilot Knob members . T he Noah Creek Formation includ es coarse sand and gravel
asso ciated with o utwas h from the DM L. Areas of Peo ria Fo rmatio n eo lian materials are p resent alo ng the L ittle
Siou x River valley.
In the map area, bedrock cons ists of mid-Cretaceous age D ak ota For matio n, co nsisting p rimarily of the
lo wer d ominated s andstone Nis hnabo tna M ember and an upper s hale and muds tone dominated Woodbu ry M ember
(Witzke et al., 201 0).
Qpglp(ss)
Qoch
Qe
Hudson
Holocene
Qpglipr
QUATERNARY
Qglhc
Qtr_amc
Wisconsin Pleistocene
Qtr_bm
Qu
Pre-Illinois
Base map from USGS Okoboji 7.5' Digital Raster Graphic (IGS GIS file DRGB13.TIF) which was
scanned from the Okoboji 7.5' Topographic Quadrangle map, published by US Geological Survey
in 1970, photorevised 1980. Topographic contours and land features based on 1966 aerial photography,
field checked in 1970. Land elevation contours (10' interval).Lidar shaded relief from 2010 data.
Iowa Geological and Water Survey digital cartographic file Okoboji_SurficialGeology.mxd, version 10/28/12 (ArcGIS 10.0)
Map projection and coordinate system based on Universal Transverse Mercator (UTM) Zone 15, datum NAD83.
The map and cross section are based on interpretations of the best available information at the time of
mapping. Map interpretations are not a substitute for detailed site specific studies.