Written Description of the Strata Exposed at the McGregor Quarry
description based on two separate measured sections, one by Norlene Emerson (Univ. Wisconsin) and the other by Brian Witzke, Greg Ludvigson, and Chris Schneider (7/6/2000).
Abbreviations: pkst (packstone), wkst (wackestone), indet. (indeterminate), lt (light), calc. (calcareous, calcitic).
DUNLEITH FORMATION
Beecher Member
Unit. 32. Dolomitic limestone to calcitic dolomite, skeletal pkst and wkst-pkst, includes brachiopodal pkst in upper ledges; unit displayed in weathered ledges 8-24 cm thick, base of units marks change to more dolomitic lithologies, varying degrees of weathering have produced oxidized and porous fabrics; thin argillaceous partings at 13, 28, and 52 cm above base of unit; calcite void fills present in lower beds, porosity fillings of poikilotopic calcite cements common in upper 40-50 cm. Crinoid debris and indet. brachiopods noted. Maximum exposed thickness 77 cm (2.5 ft).
DECORAH FORMATION
Ion Member (= Buckhorn and St. James members of Illinois classification)
Unit 31. Limestone, dolomitic in part, skeletal pkst and wkst-pkst, part argillaceous (wavy bedding streaks), forms ledge capped by shaley reentrant, 11 cm thick lt.gray calc. shale at top; additional shaley break 23 cm above base of unit; crinoid and brachiopod debris present, part burrow mottled. 57 cm (1.87 ft).
Unit 30. Shale and shaley limestone,lt gray to green-gray, calc., more calc. in lower half; forms recessive unit, top 28 cm is the shaliest and most recessive portion; some interbedded limestone beds include skeletal pkst lenses (28-32 cm below top); skeletal grains scattered to common in shale, especially below upper recessive interval; scattered brachiopods (orthids, Sowerbyella), scattered to common trepostome bryozoans include subhemispherical Prasopora (1.5-2.5 cm diameter), shaley burrows noted in pkst. 65 cm (2.13 ft).
Unit 29. Limestone, argillaceous, slightly dolomitic, wkst-pkst, primarily a fine skeletal pkst in upper 17 cm, includes lenses of brachiopod-bryozoan debris lower 15; wavy argillaceous to shaley partings especially in lower half; unit forms limestone ledges; green-gray argillaceous burrow mottles scattered to common; fossils include scattered large trepostome bryozoans (including hemispherical Prasopora), brachiopods (whole valves and broken debris includes orthids, dalmanellids, rhynchonellids), crinoid debris. 67 cm (2.2 ft).
Unit 28. Limestone ledges, interbedded with green-gray shale: basal 18 cm is limestone ledge, pkst. with broken fossil hash at top (brachiopods include strophomenids, orthids, dalmanellids; horn corals); shaley reeentrant 12-30 cm above base, contains thin lenses of burrowed pkst.; 30-40 cm above base is argillaceous wkst-pkst, burrowed, containing brachiopod pkst at top; 40-48 cm above base is limestone bed, brachiopod-rich pkst. in thin stringers (sowerbyellids, orthids), bryozoans (including subspherical Prasopora); 48-55 cm above base is argillaceous limestone bed, wkst to skeletal mudstone, scattered brachiopods (Sowerbyella) and large trepostome bryozoans; 55-72 cm above base is a shaley recessive interval, calcareous shale, green-gray to lt gray, contains thin discontinuous shaley limestone beds (2-5 cm thick), scattered to common skeletal debris; top 15 cm includes an argillaceous limestone bed (wkst-pkst) at base with 6 cm shale at top, brachiopods and bryozoans noted (including subspherical Prasopora). Total thickness 87 cm (2.85 ft).
Unit 27. Interbedded limestone and shale; lower 30 cm is limestone dominated with shaley partings, nodular bedded in part, thin pkst stringers internally, basal 6 cm is abraded-grain pkst, argillaceous horizontal burrows; upper part is shalier, interbedded pkst stringers and lenses, abraded-grain hash to whole-shell, common argillaceous burrow mottles, recessive especially upward; fossils include crinoid debris, brachiopods (Sowerbyella, Hesperorthis, strophomenids, dalmanellids, orthids, rhynchonellids), bryozoans (large encrusting trepostomes, flat-branching forms), solitary rugose corals. 1.0 m (3.3 ft).
Unit 26. Limestone ledges, interbedded shaley partings, argillaceous wkst to pkst, part with broken skeletal grains, pkst in stringers and lenses, interbedded shaley partings, nodular bedded in part (includes argillaceous mudstones), horizontal burrow fabrics present; fossils include crinoid debris, brachiopods (Sowerbyella, orthids, strophomenids, rhynchonellids), bryozoans (flat-branching and small forms). 70 cm (2.3 ft).
Unit 25. Limestone, argillaceous, wkst to pkst, some nodular-bedded mudstone to wkst in middle to upper parts, fragmental skeletal stringers present; part shaley, top 20 cm is shaley recessive; part burrowed; very fossiliferous, including crinoid debris (some coarse), brachiopods (Sowerbyella, orthids), bryozoans (trepostomes, small flat-branching forms). 75 cm (2.45 ft).
Unit 24. Limestone, argillaceous wkst, part with burrowed pkst stringers; upper 15-20 cm is pkst, less argillaceous; whole brachiopods (Sowerbyella, orthids), crinoid debris, bryozoans. 30 cm (1 ft).
Guttenberg Member (units 20-23 = Glenhaven Member, unit 19 = Garnavillo Member of Illinois classification)
Unit 23. Limestone ledges, wkst to pkst, part slightly argillaceous, 10 cm thick brachiopod packstone along upper surface; horizontal beds, dominantly fine skeletal pkst; some thallassinoid burrow mottles; fossils include crinoid debris, small bryozoans, brachiopods (Sowerbyella, orthids, strophomenids). 60 cm (1.95 ft).
Unit 22. Limestone ledges, wkst, pkst stringers and lenses, some mudstone; horizontal beds (6-30 cm thick), scattered to common burrows; thinner bedded and slightly argillaceous in upper 20 cm; 51 cm above base of unit is a 5 cm light gray clay (possibly the Dickeyville K-bentonite); fossils include crinoid debris, brachiopods (strophomenids, orthids, Rhynchotrema), trilobite debris. 1.1 m (3.6 ft).
Unit 21. Limestone, wavy to nodular bedded (5-10 cm thick), mudstone to wkst with stringers of pkst; interbedded with irregular brown shale layers (1-6 cm); thicker limestone bed 7-22 cm above base (wkst-pkst); slightly darker colored than unit below; fossils include crinoid debris, brachiopods (Rafinesquina, others), small bryozoans, rare horn corals. 94 cm (3.1 ft).
Unit 20. Limestone, wavy to nodular bedded, mudstone to wkst, discontinuous stringers of brachiopod-rich pkst (more common upward, absent in lower 17-20 cm); upper 30 cm with fine skeletal pkst lenses; wavy limestone beds intercalated with irregular wispy brown shale layers (1-4 cm); prominent overhang at base; fossils include articulated crinoid stems (especially in lower part), crinoid debris, rare gastropods, trilobite fragments, brachiopods (Sowerbyella, Strophomena, Rafinesquina). 1.5 m (4.9 ft).
Unit 19. Limestone, thin to nodular bedded, interbedded with green-gray shale, mudstone to wkst, burrowed (some pkst-filled), phosphate-enriched (apatite grains present); shaley in upper part with lenses of skeletal pkst (includes trepostome bryozoans); top 3 cm is yellow-orange bentonite layer (Elkport K-bentonite). 29 cm (11 in).
Spechts Ferry Shale (Member)
Unit 18. Shale, green-gray, calc.; interbedded in part with scattered brachiopod coquinas and pkst lenses of comminuted skeletal debris; thicker pkst lenses to 10 cm thick (discontinuous megaripple bedforms at 25 cm and 1.3 m above base); 11-48 cm below top limestone interval (discontinuous nodular mudst, brachiopod wkst-pkst lenses); approximately 30 cm above base is 3 cm thick bentonite (Millbrig K-bentonite); scattered horizontal burrows; shale becomes brown-gray near base; fossils dominated by brachiopods (Pionodema, Strophomena, Rafinesquina), some pkst lenses contain trilobite material, bryozoans (small stick-like forms, trepostomes). 2.3 m (7.5 ft). (unit 18 = Glencoe Member of Illinois classification).
Unit 17. Limestone-dominated unit; basal 3-4 cm is yellow-orange bentonite, feldspathized in part (Deicke K-bentonite); lower limestone 6-7 cm thick, lensoidal wkst-mudstone, shaley upward, locally dolomitic (with brachiopod molds, Rafinesquina, Protozyga); upper limestone 7-9 cm thick pkst, with mudstone-filled burrows, comminuted brachiopod and crinoid debris, purplish-brown color; upper surface is carbonate hardground (up to 2 cm relief), pyritic, Trypanites borings on surface, truncated nautiloid noted on hardground surface. Unit thickness 20 cm (8 in). (Unit 17 = Carimona Member of Minnesota classification).
PLATTEVILLE FORMATION
McGregor Member (dolomitic strata of units 13-16 included within the “Grand Detour Formation” of the Illinois classification; wavy-bedded limestones of units 8-12 included within the Mifflin Member (Formation) in the Illinois-Minnesota classification).
Unit 16. Limestone, part dolomitic, mudstone to fine skeletal wkst, argillaceous partings; top half of unit is sparse skeletal mudstone with scattered burrows; uppermost portion with thin pkst stringers. 30 cm (1 ft). (Uppermost interval of dense mudstones may represent the Quimbys Mill Member of the Illinois-Wisconsin classification).
Unit 15. Calcitic dolomite to dolomitic limestone, fine to medium crystalline in part; argillaceous to shaley partings every 2 to 10 cm; basal portion includes fine skeletal debris, wkst-pkst burrow fillings, trepostome bryozoan noted; upper portion includes fine skeletal wkst-pkst with argillaceous partings and green horizontal burrow fills, prominent bedding break at top. 80 cm (2.6 ft).
Unit 14. Dolomitic limestone, thin bedded with argillaceous to shaley streaks every 1-4 cm, wkst-pkst stringers (including brachiopods) in middle part; top 13 cm is less argillaceous, more coarsely crystalline wkst-pkst, very calcitic, with crinoid debris. 66 cm (2.16 ft).
Unit 13. Dolomite, calcitic, finely crystalline, irregular wavy to fracture bedding, argillaceous streaks, becomes more argillaceous upward (especially in upper 32 cm); scattered dolomitized skeletal stringers in middle part; horizontal burrow fabrics scattered through, some Chondrites-type burrows. 76 cm (2.5 ft).
Unit 12. Limestone, skeletal mudstone to wkst; upper 19 cm includes pkst; wavy beds separated by argillaceous partings (similar to below), thicker shale parting (1 cm) at base; well-preserved fossils include brachiopods (Hesperorthis, other orthids, Strophomena), large ostracodes (Eoleperditia). 75 cm (2.45 ft).
Unit 11. Limestone, dominantly skeletal mudstone to wkst, some pkst stringers; upper 93 cm includes fine abraded pkst intermixed with mudstone-wkst, some fine pkst beds (87-93 and 37-46 cm below top); unit displayed in wavy beds 2-6 cm thick, separated by shaley to argillaceous partings, slightly less argillaceous than unit below; possible darkened hardground surfaces 56 cm above base and at top of unit; scattered horizontal and subhorizontal burrows (some to 2 cm diameter); well-preserved fossils include brachiopods (orthids, Strophomena), crinoid debris, trilobite material, large ostracodes (Eoleperditia), small branching and encrusting bryozoans, gastropods. 1.43 m (4.7 ft).
Unit 10. Limestone, skeletal mudstone to wkst, scattered wkst-pkst stringers; wavy to nodular bedding throughout, beds 1-6 cm thick, 8-10 cm at bottom; beds separated by shaley partings, calc. gray to green-gray shales; scattered burrows, mostly small horizontal to subhorizontal burrows (1 mm diameter, some Chondrites-like), additional horizontal burrow forms (2-4 mm diameter); well-preserved fossils include brachiopods (orthids, strophomenids), trilobite material, large ostracodes (Eoleperditia), gastropod molds locally concentrated within some beds but not along bed surfaces (planispiral, low- and high-spired forms), small bryozoans locally in skeletal stringers. 1.91 m (6.25 ft).
Unit 9. Limestone,skeletal mudstone to wkst; basal 11 cm is a single bed, above is wavy-bedded 4-8 cm thick, separated by argillaceous partings; shaley parting at top; well-preserved fossils include brachiopods (Hesperorthis, other orthids, Strophomena), small bryozoans, trilobite material, large ostracodes (Eoleperditia), crinoid debris, gastropods (as above). 33 cm (13 in).
Unit 8. Dolomitic limestone, dominantly skeletal wkst, fine crystalline, in 5 beds separated by argillaceous partings, some finely-laminated argillaceous partings; irregular gray burrow mottling; top 10-12 cm marked by green-gray shaley interbeds and thin dolomitic limestones with calcitic skeletal stringers (brachiopods, crinoid debris), irregular stylolitic surface at base; base of unit is an irregular surface, probably a hardground. 43 cm (1.4 ft).
Pecatonica Member
Unit 7. Dolomite, fine skeletal molds, finely crystalline but some crinoidal material is dolomitized to coarse crystals; massive unit minor bedding breaks at 67 cm above base and 14 cm below top, locally overhanging at base; some secondary calcite void fills in fractures and as porosity-fillings by poikilotopic cements; skeletal material includes brachiopods and crinoid debris, fine molds are indeterminate fossils. 1.6 m (5.25 ft). (Unit 7 corresponds to the New Glarus Member of the Illinois classification).
Unit 6. Dolomite, argillaceous, skeletal molds, very fine to finely crystalline, burrow mottles, argillaceous content generally decreases upward; lower 72 cm is argillaceous to shaley (forms recessive interval) in beds 4-17 cm separated by argillaceous to shaley partings, skeletal molds, indeterminate small darker dolomitized grains, some skeletal stringers (brachiopods, crinoid and trilobite debris), horizontal burrows; interval 24-65 cm below top is less argillaceous, thicker bedded, more skeletal moldic (include pkst stringers), marked by shaley bedding break above, skeletal molds (crinoid debris, brachiopods), burrows (1-10 mm diameter), large burrows below shaley break; upper 24 cm is fine skeletal moldic, prominent shaley bedding break at top. 1.37 m (4.5 ft).
Unit 5. Dolomite, argillaceous, very finely crystalline, skeletal moldic, irregular wispy argillaceous partings (spaced 3-6 cm) display nodular to wavy-bedded aspect to unit; shaley partings at base, finer argillaceous partings in top 6 cm; fossil molds include brachiopods (orthids), crinoid debris, gastropods, nautiloid (noted 22 cm above base). 60 cm (2 ft).
Unit 4. Dolomite, very finely crystalline, skeletal-moldic porosity, some burrow fabrics, part slightly argillaceous, argillaceous partings in upper 13 cm; part with calcite void fillings; fossil molds include brachiopods, crinoid debris, bryozoans. 90 cm (2.95 ft). (Units 4-6 correspond to the Dane Member of the Illinois classification).
Unit 3. Dolomite, very finely crystalline, skeletal molds, burrow fabrics, small void spaces impart significant porosity; some embedded quartz sand present; prominent bedding break at top. Top of unit forms quarry floor; only 18 cm in upper part of unit was accessed. Unit mostly covered, total thickness estimated at 1.1 m (3.6 ft). (Unit 3 corresponds to Chana Member of Illinois classification).
GLENWOOD SHALE
Unit 2. Shale, green-gray, silty, noncalcareous; lower part of shale interval exposed along roadway into quarry, immediately above sandstone. Estimated total thickness 1.3 m (4.25 ft).
ST. PETER SANDSTONE
Unit 1. Sandstone, very fine- to fine-grained quartz sand, local iron-oxide cements in upper part, possible low-angle crossbeds in part; exposed along and on quarry roadway and in the adjoining ravines. Total thickness not accessed.
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from
Witzke, B.W., Ludvigson, G.L., and. Emerson, N.R., 2000, Stop 9: Platteville and Decorah Formations at the McGregor Quarry. in Anderson, R.R (ed.), The Natural History of Pikes Peak State Park, Clayton County, Iowa: Geological Society of Iowa Guidebook 70, p. 118-121.