STATE OF ILLINOIS WILLIAM G. STRATTON Governor THE GENEVA (MIDDLE DEVONIAN) DOLOMITE IN ILLINOIS by Howard R. Schwalb DIVISION OF THE oo^ N c OI ? Ry S E TA JE e ,6EOLOGICAL SURVEY ' Lfl,ef URBANA CIRCULAR 204 1955 OCT 1 CJ Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/genevamiddledevo204schw ; THE GENEVA (MIDDLE DEVONIAN) DOLOMITE IN ILLINOIS by- Howard R. Schwalb ABSTRAC T The Geneva dolomite extends from its type outcrop in Indi fa n t a o WeS f T; int ° S ° Uth - Cent -l ^-is. ^tracing the or^e " lation of Indxana and Illinois Devonian outcrop sequences the mare: Ld'Th 00 ^ "*■ ° rganiC ^ ^ * « S^- Zlil I \ Theref0re ' ltS areal distribution, thickness, litho- logrc characteristics, and relation to surrounding sediments in Illinois were determined and are shown by crosLection d a grams and isopach maps. Samples and electric logs from about 100 wells in seventeen counties were studied. ing westward across the south-central pa t of t^ s^LTlT ^ ^^ ily distinguished bv its rolnr a „^ ( g * '* As lt: 1S eas " Geneva can be traced^tMn HI no r/or th " " T "** ^ U h ™ f " the seance of southern ^^^^E^ * SST? ^ T" graphic problem. The report is b^H Illinois - a major strat samples and core chips Xl^^r" 30 ^ examination * «U WHlm!n h w„o S/"^" Ge0l ° giCal Surve y colleagues D. H. Swann and H B scendtg tder thesVaT^nT T ^^ * —*—•» *«-.. In de- ^ek sLdston;:id%rar L c :^rrneL\ a t n hthT er f iimestone ' Dutch Backbone limestone anrf R=,i , . Cnert - ■ Beneath these formations are the surface to Z north and -ll^T * ° f ^ ^^^ ^ In the "*- of the Grand T, * ° UtCr ° P area ' the CIear Creek and much Crlek and , h 7" f ° rmatl ° nS grade to dolomite. Farther north the Ctear ^o:t:^::zz:z\T mat ir are absent - and doiomite ^zl* related with ,7 Silurian dolomite. The Geneva formation is cor- theTo We ;Tar? f :: e er G L a : Q ° T f the T dle Devonian doiomite and *™*£%* —a. i S brown to very dark ^i^T^ST^^ ILLINOIS STATE GEOLOGICAL SURVEY Fig. 1. - Isopach map showing occurrence of Geneva dolomite in south-centra Illinois. bottom of the formation, though in some areas, as noted by Bieberman (1949), the reverse is true. The color of the Geneva is caused by a minute quantity c disseminated organic material. When the dolomite is dissolved in dilute hydjr chloric acid, the brown organic residue accumulates on the surface of the aci. This test distinguishes the Geneva from other Devonian and Silurian dolomite in Illinois. Differential thermal analyses of the dolomite show an exothermic peak between 400 and 500° C.,* which probably is caused by oxidation of the organic material. Where lighter-colored dolomites lie above the Geneva and are locally oil-stained, they are difficult to distinguish from the Geneva exce, by residues. The distinctive flaky organic residue of the Geneva is easily re, ognized, even in the presence of a film of oil. jui.iv The texture of the Geneva dolomite is granular or sucrose, and the bulK the rock is composed of uniform dolomite rhombohedrons. Analyses of core, from various areas suggest that the Geneva porosity ranges between 10 ana percent, averaging about 17 percent. This is the general porosity range sug- * Analysis by Herbert D. Glass, Illinois State Geological Survey. GENEVA DOLOMITE IN ILLINOIS Fig. 2. - Electric log and lithologic cross five wells, section showing Geneva dolomite in gested also by resistivitv data t*i^ ;„+ with larger dolomite rhombohedrons and here Si? ♦ *" "*** crystals. The grain size is very fine to fine Tht r f f ° SmaU qUartZ is slightly coarser than the overly ng lighter c^ored dT » C ° mm ° nly not obvious in the samnlo ^ . "gnter-colored dolomites. Bedding is not uncommon ? "" SeCtl ° nS ««"***. but stylolite parting are ed ano ir^^.^^,^!"'-"' *« ™* - funded and frost- to be bimodal; it is mostly fnthw -T " * S^ 8 " The Sand tends f ew coarse «r^\r^^Sr^ i ^i^ ,M ? r ^T "*«* * the Geneva, but where OTM . B f •+ ■ i 7 ^ rainS ' Chert ver Y rarely is found in -ter. Mo'st of ^ C^nevatl om^t eTo f^^^TT ' t "* T * ~* material is the only insoluble residue. Y '^ ° f ° rganic The Geneva appears unfossiliferous. However manv cf «,« , ties seem to be very crude m „iH. „* <■ " owe ^ er > m any of the vesicular cav- noderately abundant For! 1 °* f ° SSil debriS - Scolecodonts are esidues. ChitTno 2 oa occurT^" rZ ^^^ **" * ** Sparse -•«»«« Uinois samples! MlM » SampleS but have not been noted in The type locality of the formation (Collett lss;i io fr rn Shelby County Ind „ M r «, °n l^ouett, 1882) is at Geneva in southeast- k i94i. P . ^l^J^Z^^^z^T obscure (Daw " Ue formation les^rectiv on th Pr , e r "^ ° f ^ P ° int ' and the Jefferson- *eva occurs in » arcuate belt f *"*" ** *' ° UtCr ° P - In -Surface, the arcuate belt which curves northwest from the outcrop to ILLINOIS STATE GEOLOGICAL SURVEY Fig. 3. - Isopach map showing thickness of post-Dutch Creek Devonian dolomites. It enters Illinois in Clark and Edgar( Trends to the southwest, and extends as far as Montgomery, I west-central Indiana (Bieberman, 1949) counties (fig. Bond, and Clinton counties. The maximum observed thickness of the Geneva in Illinois is 50 feet. ; A rather persistent sandstone zone is at the base of the Geneva dolomite and is included with the dolomite on the thickness map. The sandstone yane from less than an inch to several feet thick. The sand differs in its uniform size, angularity, and lack of frosting from the floating sand grains in the Ge- neva. It is uniformly fine, subrounded to angular, white to brown, and much is quartzitic. In some well cuttings the sandstone is intermixed with the bas few feet of dolomite. In one drill core this sandstone was found two feet abo the base of the Geneva dolomite. The sandstone extends beyond the limits ( the Geneva to the west and south and appears to be equivalent to the Dutch Creek sandstone of the southwestern Illinois outcrop area. The Geneva dolomite thins toward the west, as shown in cross section 1 A' (fig. 2). In well 2 of this cross section, traces of Geneva dolomite interii ger with light-colored dolomite like that overlying the Geneva farther east, and the Geneva can no longer be distinguished as a formation. GENEVA DOLOMITE IN ILLINOI " SdlSon'of r" d Hth0 , l0giC Cr ° SS Sectlon of flve we " s <*°™S gradation of Geneva dolomite to limestone. extn^iriTllTeil ^^^^ *• <«- *w feet of dolomite and ored dolomite, in ^Zl^^?^^?* *»> DutchGree. sandstone is a, oil-producing formal Cr ° SS ^^ "» ^z e x;TnoV s : z iable :rr ce of doiomites - The - ^ **•- Geneva. The/rang from almZ S ^ " gMiC rCSidUe ty P iCal of the eral are finer graTnedthTnth^C ^^t^ t0 Wl > «*«*«. and in gen- i» the upper dolmUes tattted^/^- ^"f"" ^"t^ 5 ° f Sand are found This sand is r«JS frS ^ to ""^ "* * SandSt ° ne lenSes " and grain si Z e, St. Peter said or th e f, T"' r6Sembles ' b °«> » -rting the Geneva As the r" ", , atln§ grainS in a11 but ^ basal part of ju xeet ~ imum thickness of the Geneva T^ ' correspond to the areas of max- ,nd lateral!, ^^S S,^£^r.£J"-" ■ ^ ^ ~ 3- County, the strata overling the Oene^ ^^^^^^1- :ree c k 7a;Lr:ir!dfn; ( ;^:^: t :t d r mite section — *• — t the dolomite section was penelat "d fcwelT *4 S i * ^ ' ^ ^ tOP mailable for studv but ft, Li ■? Sample cuttin g« were not hich also indicates bv the d ° l0mit y e * uen « is ^own by the electric log, l of the tololTl'ZZl " s -«vity the greatly decreased poros- etely to lime" one fJ^StSTn ^ ^^ *" ^^ C ° m " k is present at the base of the stu n ' ^^ »** C " ek ^ S ^ ILLINOIS STATE GEOLOGICAL SURVEY In two Illinois wells studied, in Cumberland and Bond counties, the basal *. f «,- r^neva has no trace of organic material and is pale yellow or Cf rLt P ?ac n es t h h e a Dutc h GreeU sandstone identifies the base of the Grand Tower or Geneva equivalents, but in areas beyond the »«*«™ ^ * *" Geleva where the Dutch Creek is missing, the Devonian dolomite is diffacult fo d stingSsh from the underlying Silurian dolomite. Some of the features that are locaUy useful in identifying the Devonian dolomites in microscopic exam- ination of well cuttings are: the presence of sand in the Devonian dolom tes andTts absence in the Silurian in this region; the occurrence of glaucomte or "ert in the SUurian dolomites, both rare in the Devonian; and the generally coarser texture and lighter color of the Silurian dolomites. GENEVA DOLOMITE IN l LLINOls REFERENCES Bieberman, D. F 1Q4Q c+^^4.- Collett, J., 1882, Geology of Shelbv ro,,r, t r j- P ' Hth Ana. Kept., p. 55-88 ^ "*""* DepU Geo1 ' and Nat. Hist. Dawson, T. A. 1941 Th« r*~ sou'thern lLaLi: wSTSS ^T" 9 "" ■"' ^^ ^ *' ° Ut "°P «» inaiana Dept. Conservation Geol. Div., 48 p Sutton, D. G., and Sutton, AH 1Q^ 7 »,',„ ^ Jour. Geol., v. 45, pt 3 pV 320 JssT ^ ° f SOUthern *"*«* ILLINOIS £a/td 'ofj&ncoHh, \ CIRCULAR 204 ILLINOIS STATE GEOLOGICAL SURVEY URBANA