437:. However, in Iowa the Scales, Elgin, and Clermont are all members of the Maquoketa Formation (the Maquoketa is not considered a group in Iowa). Over the years, this has slowly led to the Scales being used in Eastern Iowa for the dark shale and the Elgin used in Western Iowa to refer to the dolomite that grades laterally into the Scales. The difference in terms between Iowa and Illinois has made the present nomenclature all but impractical. There is an active effort to revise the nomenclature. Present studies suggest that the Maquoketa should remain a group and not a formation for practical purposes.
511:
565:
454:
483:
441:
25:
324:
461:
Overlying the Scales
Formation is the Fort Atkinson Formation, which has been informally called the Middle or Divine Limestone by drillers. The Fort Atkinson is a dominantly a red, gray, and white coarse crystalline dolomite with thin beds of green and gray shale. It contains many
551:
Its origin is still debated. We do know that it was deposited near or on an ancient shore and is a sedimentary rock. However, iron oolites are not being actively deposited anywhere in the world today. It is possible that it was originally deposited as a calcareous oolite and
547:
are black and made of iron oxides instead of the typical calcareous oolites. They are about 0.5 millimeters in diameter. There are no known fossils. In
Illinois, it can be as much as 16 feet thick but it is typically no more than 8 feet thick.
433:. In Illinois the Formation is divided into two members, the lower Elgin Member (mostly dark gray to brown and black shale with dolomite) and the upper Clermont Member (upper gray pure shale) both with type sections in
370:
in Ohio. Illinois and
Indiana are the only states where the Maquoketa is considered a group. In other states it is a formation. The Maquoketa was deposited in a shallow intercontinental sea. Most of the
522:
The top and youngest formation is the Neda
Formation. It exists in small patchy areas and was extensively eroded at the end of the Ordovician. It exists in large isolated patches in the subsurface in
310:
595:. In Illinois the Neda is too thin and patchy to be economic. The Brainard is too soft to be used as anything other than fill soil. The Fort Atkinson is used to a limited degree as a
613:. This means that it is very impermeable, and therefore water only measurably flows through it on geologic time-scales. For this reason, it cannot serve as a source of groundwater.
426:
are known from Elmer-Lason Quarry. It is typically 75 to 100 feet thick. It is the only formation of the
Maquoketa Group that is not exposed in northeastern Illinois.
490:. The greenish rock under the concrete block at top center is the base of the Brainard and is dominantly a sticky green shale with thin beds of purplish gray dolomite.
303:
296:
494:
Above the Fort
Atkinson is the Brainard Formation. The Brainard is generally a greenish gray to gray dolomitic soft to hard shale. It contains abundant
198:
606:
The unit has been identified as a massive hydrocarbon reservoir. Recently legislation was passed in
Illinois allowing horizontal drilling in the shale.
641:
1983, D.R. Kolata & A.M. Graese, Facies analysis of the
Ordovician Maquoketa Group and adjacent strata in Kane County, Northeastern Illinois
556:
later replaced the calcite. Or it could have been deposited directly as an iron oolite. There is strong evidence for both theories.
667:
572:
The
Maquoketa has almost no economic use since it is dominantly a weak shale interbedded with thin dolomite, it is not good for
644:
1991, A.M. Graese, Facies analysis of the
Ordovician Maquoketa Group and adjacent strata in Kane County, Northeastern Illinois
89:
498:
near the base. It ranges from 0 to 100 feet thick. The variable thickness is due to an erosional unconformity caused by an
61:
68:
108:
457:
A piece of the Fort Atkinson along Aux Sable Creek in Illinois, showing the coarse texture and multitude of colors.
42:
75:
46:
650:
2007, S.D.J. Baumann & T. Arrospide, Paleozoic Geology of the Fox River from Batavia to Oswego Illinois
57:
677:
662:
399:
In Illinois, the basal formation is called the Scales Formation and consists mainly of dark brown and gray
682:
430:
543:. It is a reddish purple to brown oolitic shale and is sometimes cemented with iron and dolomite. The
540:
515:
672:
622:
536:
588:
518:. The black pencil near the right center of the picture is near the Silurian-Ordovician contact.
35:
406:. In north central Illinois, the bottom half is dominantly a yellowish gray coarse crystalline
580:
647:
2007, S.D.J. Baumann, Unclassified, Unusual, or Problematic Fossils in Northeastern Illinois
532:
528:
524:
282:
174:
82:
434:
275:
268:
8:
258:
190:
158:
411:
400:
335:
248:
482:
564:
510:
263:
243:
223:
152:
627:
487:
367:
339:
233:
218:
474:. It ranges from 0 to 60 feet in thickness but is typically 40 to 50 feet thick.
453:
444:
This table shows equivalent rock units within neighboring states around Illinois.
407:
343:
253:
238:
228:
206:
182:
599:. The Scales is buried too deep and contains too much dolomite to be used for
656:
573:
164:
638:
1975, H.B. Willman & T.C. Buschbach, Handbook of Illinois Stratigraphy
463:
440:
376:
142:
467:
423:
380:
600:
596:
419:
384:
351:
24:
610:
592:
577:
553:
359:
347:
616:
Although not useful to industry, it does contain rare and diverse
544:
499:
495:
415:
372:
363:
617:
584:
388:
366:. It is equivalent to the all but the basal formations of the
502:
at the end of the Ordovician, which lowered global sea level.
471:
403:
323:
355:
583:
and is often stripped and piled as waste in open-pit
418:). It does not contain many fossils, although large
620:, some of which are only known in Illinois such as
49:. Unsourced material may be challenged and removed.
16:Assemblage of geological formations in central USA
654:
429:The type section for the Scales Formation is in
124:Paleozoic Stratigraphy of the Upper Midwest, USA
539:in Illinois. In Illinois it crops out only at
304:
379:, which were being raised at the end of the
626:, a small unclassified fossil named after
591:in Wisconsin, where it was once mined for
448:
311:
297:
109:Learn how and when to remove this message
563:
509:
481:
452:
439:
322:
587:. The main exception is at the Neda's
655:
477:
609:The Maquoketa group is also a major
47:adding citations to reliable sources
18:
394:
13:
383:. To a lesser degree some of the
14:
694:
630:and found in the basal Brainard.
505:
668:Geologic formations of Wisconsin
559:
23:
375:sediments are derived from the
34:needs additional citations for
1:
633:
334:is an assemblage of several
7:
431:Jo Daviess County, Illinois
10:
699:
410:(rock form of the mineral
541:Kankakee River State Park
516:Kankakee River State Park
342:in age and named for the
623:Tentaculites oswegoensis
568:Tentaculites oswegoensis
449:Fort Atkinson Formation
346:in Iowa. It exists in
569:
519:
491:
458:
445:
387:were derived from the
327:
567:
513:
485:
456:
443:
326:
283:Mount Simon Sandstone
175:Platteville Limestone
435:Fayette County, Iowa
414:which is related to
276:Eau Claire Formation
269:Galesville Sandstone
43:improve this article
678:Ordovician Illinois
663:Ordovician Missouri
486:Waubonsie Creek in
336:geologic formations
259:Franconia Formation
191:St. Peter Sandstone
159:Wise Lake Formation
683:Ordovician Indiana
570:
520:
492:
478:Brainard Formation
459:
446:
391:to the southwest.
328:
249:Munising Formation
466:fossils, such as
321:
320:
264:Ironton Sandstone
244:Potsdam Sandstone
224:Shakopee Dolomite
199:Knox Unconformity
153:Dubuque Formation
129:Dates approximate
119:
118:
111:
93:
58:"Maquoketa Group"
690:
628:Oswego, Illinois
597:decorative stone
537:Du Page counties
488:Oswego, Illinois
395:Scales Formation
368:Cincinnati Group
340:Upper Ordovician
313:
306:
299:
234:Jordan Sandstone
219:Everton Dolomite
121:
120:
114:
107:
103:
100:
94:
92:
51:
27:
19:
698:
697:
693:
692:
691:
689:
688:
687:
673:Ordovician Iowa
653:
652:
636:
562:
508:
480:
451:
397:
389:Ozark highlands
344:Maquoketa River
332:Maquoketa Group
317:
254:Davis Formation
239:Potosi Dolomite
229:Oneota Dolomite
207:Knox Supergroup
135:Maquoketa Group
115:
104:
98:
95:
52:
50:
40:
28:
17:
12:
11:
5:
696:
686:
685:
680:
675:
670:
665:
635:
632:
561:
558:
507:
506:Neda Formation
504:
479:
476:
450:
447:
396:
393:
319:
318:
316:
315:
308:
301:
293:
290:
289:
288:
287:
286:
285:
280:
279:
278:
273:
272:
271:
266:
261:
241:
236:
231:
226:
221:
213:
212:
210:(~497-~470 Ma)
203:
202:
201:(~470-~459 Ma)
195:
194:
193:(~459–~455 Ma)
187:
186:
183:Glenwood Shale
179:
178:
171:
170:
169:
168:
162:
161:(449.4–447 Ma)
156:
147:
146:
139:
138:
131:
130:
126:
125:
117:
116:
99:September 2009
31:
29:
22:
15:
9:
6:
4:
3:
2:
695:
684:
681:
679:
676:
674:
671:
669:
666:
664:
661:
660:
658:
651:
648:
645:
642:
639:
631:
629:
625:
624:
619:
614:
612:
607:
604:
602:
598:
594:
590:
586:
582:
579:
575:
574:track ballast
566:
560:Economic uses
557:
555:
549:
546:
542:
538:
534:
530:
526:
517:
512:
503:
501:
497:
489:
484:
475:
473:
470:and abundant
469:
465:
455:
442:
438:
436:
432:
427:
425:
421:
417:
413:
409:
405:
402:
392:
390:
386:
382:
378:
374:
369:
365:
361:
357:
353:
349:
345:
341:
337:
333:
325:
314:
309:
307:
302:
300:
295:
294:
292:
291:
284:
281:
277:
274:
270:
267:
265:
262:
260:
257:
256:
255:
252:
251:
250:
247:
246:
245:
242:
240:
237:
235:
232:
230:
227:
225:
222:
220:
217:
216:
215:
214:
211:
208:
205:
204:
200:
197:
196:
192:
189:
188:
184:
181:
180:
176:
173:
172:
166:
165:Decorah Shale
163:
160:
157:
154:
151:
150:
149:
148:
144:
141:
140:
136:
133:
132:
128:
127:
123:
122:
113:
110:
102:
91:
88:
84:
81:
77:
74:
70:
67:
63:
60: –
59:
55:
54:Find sources:
48:
44:
38:
37:
32:This article
30:
26:
21:
20:
649:
646:
643:
640:
637:
621:
615:
608:
605:
589:type section
571:
550:
521:
493:
464:invertebrate
460:
428:
398:
377:Appalachians
331:
329:
209:
177:(455–454 Ma)
167:(454–452 Ma)
155:(447–446 Ma)
145:(454–446 Ma)
143:Galena Group
137:(446–440 Ma)
134:
105:
96:
86:
79:
72:
65:
53:
41:Please help
36:verification
33:
468:brachiopods
424:cephalopods
657:Categories
634:References
422:and large
420:trilobites
381:Ordovician
69:newspapers
601:oil shale
581:aggregate
401:dolomitic
385:sediments
352:Wisconsin
185:(~455 Ma)
611:aquitard
593:iron ore
585:quarries
578:concrete
554:hematite
412:dolomite
408:dolomite
360:Illinois
348:Missouri
338:. It is
618:fossils
545:oolites
500:ice age
496:fossils
416:calcite
373:clastic
364:Indiana
83:scholar
472:corals
362:, and
85:
78:
71:
64:
56:
404:shale
90:JSTOR
76:books
535:and
533:Will
529:Cook
525:Lake
356:Iowa
330:The
62:news
576:or
514:At
45:by
659::
603:.
531:,
527:,
358:,
354:,
350:,
312:e
305:t
298:v
112:)
106:(
101:)
97:(
87:·
80:·
73:·
66:·
39:.
Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
