Name:

“A” street outcrop

County:

clayton

Company/

Owner:

state of iowa

IDALS Reg. #:

none

IDOT Code #

none

Loc. (Q's):

nw nw se se

Sec.

22

T

95n

R.

3w

 

Quad:

prairie du chien (e45)

Elevation:

674 at top; 640 at base

 

Loc. Rmrks:

Outcrop on north side of “A” Street, downtown McGregor, Iowa;  landscaped into “Bluffside Garden” during 1997 by the city.

 

Descrip. By:

R.McKay with R.Anderson

Date(s):

11/13 & 11/18/1997

 

Descrip. Rmrks:

photos taken

 

Previous Descrips. & Other Remarks:

no previous descriptions in files

TFA = tangential or trough foreset dip azimuth; TAA = trough axis dip azimuth; PFA = planar xs foreset azimuth

Unit

Description

ft.

m.

1

Sandstone, appears as massive unit but internally is highly cross-stratified; divided into two subunits; top of unit 1 marked , in vertical profile, by a moderate slope change to a recessive unit 2.

Unit 1a (0-168 cm), f-m with only trace c at base, minor c throughout main part and moderate c in upper 20 cm, very friable & weakly cemented.  Top of 1a is marked by moderately prominent low-inclined bedding plane (dips se at 5 degrees) and coarser grain size above.  Most of unit is truncated trough cross-strata with sets typically 50-100 cm in width & 15-20 cm maximum thickness, but some sets to 150 cm wide.  Troughs are best visible in upper half of 1a.  A discontinuous 1 cm thick green clay layer is present along the base at north end of outcrop (near new garage) and rare, isolated green clay clasts are present in the lower meter also at north end.  At 163 cm near cellar excavation is a 6 cm thick by 45 cm long lense of poorly sorted ss with grain size from silt to vc & trace granule to 3 mm, and common 1-3 mm green clay & tan siltstn intraclasts.

Unit 1b (117 cm), f-c, minor vc & rare qtzte granule to 3 mm, sparsely intraclastic (clasts are tan dolomitic sltstn & <5mm). Trough cross-strata more prominent & larger than in unit 1a, dominated by truncated trough sets 10-40 cm thick & up to 2.0 m wide.  Unit 1 TAA = 214, 208, 184, 210, 222, 171, 207, 179, 197, 227, 244, 225; TFA=217, 104, 241.  Samples at base, 50, 90,  100, 150, 163, 173, 220, 275, & 283 cm.  All samples are extremely lightly cemented with silt-size dolomite rhombs that detach from quartz grains very easily.

9.35 ft

2.85 m

2

Sandstone, coarser and more intraclastic than unit below; slightly recessive profile; overlain by prominently horizontally stratified unit above.  Unit thickness varies laterally from 95 cm thick at west end of outcrop to 26 cm thick at far east end by nearest building courtyard.  On west end unit composed of 3 normal-graded & faintly low-angle planar/horizontally stratified to nonstratified sets that grade from m-vc, with minor 5 mm qtz granules at base to f-c, minor vc in upper part of sets.  Lower portions of sets contain qtzte granules & are highly intraclastic; intraclasts are dolomitic silty-vf ss to 5 mm with rounded edges; coarser grained at east end with intraclasts to 7 mm.  sample.

0.85-3.12 ft

0.26-0.95 m

3

Sandstone, with 3 subunits.

3a (0-28 cm) outcrop expression as a slightly protruding profile especially behind park sign, vf-m with rare c-vc, consists of 2-4 sets of trough cross-strata with minor silty flasers near top; thins laterally to 0 cm at east end of outcrop.

3b (60-125cm) generally recessive; sandstone, f-c with common silty ss intraclasts to 6 mm and minor ss intraclasts of coarse ss to 2 cm, dominated by low-angle planar strata, maximum thickness at east end.

3c (190 cm) at west end the lower 40 cm is recessive while remainder is prominent ledge-former;  best cross-stratification is visible on west end by new garage while east end appears massive.  Sandstone,  f-c, minor vc, minor ss intraclasts up to 1 cm thick by 8 cm long; west end composed of 6 to 8 sets of trough to broad low-angle tangential cross-strata with set thickness between 8 and 40 cm; TFA= 330, 340, 295, 15, 189, 284, 355, TAA=219

9.12 ft

2.78 m

 

Offset section back to the east end of outcrop to the tree-rimmed ledge within alcove area.

 

 

4

Sandstone, light greenish white, (visually prominent recessive marker bed), sharp erosional lower contact, lowermost 3 to 5cm is discontinuous, unstratified and poorly sorted, vf-vc with minor qtz pebbles to 6 mm, and f ss intraclasts to 8 mm.  Above that is somewhat laterally persistent 5-10 cm thick planar to low-angle tangential xs set with minor amount of deformed to overturned foreset laminae, PFA=98 (frames 16, 17).  Remainder of unit is thin-bedded and lenticular to flaser-bedded vf-c with 10-20 % green clay drapes, laminae, and flasers; microlenses & flasers noted in sample.  Includes two well-developed form-symmetrical ripple lenses in middle (frames 18, 19, 20).

1.61 ft

0.49 m

 

 

 

 

5

Sandstone, light greenish white, visually prominent whitish bed that is slightly protrusive; f-m, minor c, no dolmite crystal cement, green clay intraclasts at base, small scale lenticular ripple trough-xs with green clay & vf ss to silty flasers, drapes & discontinuos laminae, grades in upper part to wavy to crinkly to horizontally interlaminated ss and clay with clay laminae to maximum of 5 mm.  Laminae slightly deformed by loading of overlying unit.  Sample. 

0.56-0.92 ft

0.17-0.28 m

6

Sandstone, f-c, trace vc & granule to 4 mm, generally nondolomitic but moderate amount of dolomite crystals are concentrated in Skolithos tubes within bioturbated zones.  In center alcove area unit consists of two planar to tangential xs sets of subequal thickness that both pinch out laterally to the east (PFA=50o on lower set & 60o on upper set). Sets have highly intraclastic bases with common intraclasts 3-5 cm and some to 8 cm; intraclasts are weakly cemented tannish vf silty/clayey ss;  lower set contains several isolated Skolithos tubes penetrating foreset laminae (marks zone of foreset reactivation).  Laterally these two sets and the interset boundary grades to meter plus wide lenses with bioturbated tops and strongly bioturbated interlense zones.  The more highly bioturbated zones are marked by a highly vuggy, recessive profile and an abundance of tannish-orange Skolithos burrows.  Sample.  Photos=2

2.26-4.1 ft

0.69 m-1.25 m

7

Sandstone, f-m, lessor c, 8 to 10 sets of xs up to 15 cm thick with interset bioturbated zones and remnant wavy to horizontal laminae.  XS sets are commonly intraclastic.

4.49 ft

1.37 m

8

Dolostone, very sandy to dolomitic sandstone,  appears on outcrop as three to four 10-12 cm thick beds; difficult to access.  This is the top of the accessible section.  *Note* Samples for conodonts from this unit were sent Feb. 24, 1999 to Dr. James F. Miller, Southwest Missouri State.  Samples were actually collected from a more accessible point behind Mrs. Moe’s house at 124 Main St.  He found Eoconodontus notchpeakensis and Proconodontus muelleri in all 3 samples from this dolomite unit (conodont data in emails from J.F. Miller on May 4, 5, & 10, 1999).

1.41 ft

0.43 m