96 



The Canadian Field-Naturalist 



[Vol. XXXIII 



THE CLIMATIC INTERPRETATION OF TWO EARLY ORDOVICIAN 



MUD-CRACK HORIZONS.'^ 



By E. M. Kindle. 



A mud-crack horizon which has not been pre- 

 viously reported occurs in the Grenville section 

 on the Ottawa river. This horizon which is ex- 

 posed on the north bank of the river immediately 

 above the Canadian Northern railroad bridge is in 

 the upper part of the Bcekmantown formation. Its 

 relationship to the associated beds is indicated in the 

 section below which was studied by the writer in 

 company with Dr. M. E. Wilson. 



Section above C.N.R. bridge al Hawkesbury. 



a. Sandstone with coarse sand and fine gravel 

 in upper part and fine sand in lower. Num- 

 erous vertical worm tube impressions (Base 

 of Chazy) 2 



b. Thin bedded limey shale (top of Beekman- 

 town) 2 



c. Dark grey fine grained limestone with 

 botryoidal fracture 8' 



d. Coarse textured grey limestone full of small 

 fossils 1 6 



e. Thin bedded shaly limestone 3' 



/. Heavy bedded grey limestone and covered 10' 

 g. Thin bedded grey argillaceous and mag- 



nesian limestone with mud-crack throughout 

 the upper 4' Resembles sandstone when 

 weathered 6 



Between a and b of this section there is probably 

 a disconformity. All of the Ottawa valley sections 

 show a rather abrupt change in lithology at this 

 horizon. The change in fauna is equally marked. 



The very sharp and clearly defined character of 

 the fossil mud-crack in bed g of this section is its 

 most noteworthy feature. The mud-crack polygons 

 exhibit a rather unusual and significant feature in 

 their upturned margins. Many examples of this 

 mud-crack show the unwarped margins of the poly- 

 gons rising above the centre as much as !4 inch. 

 Associated with these is a surface structure suggest- 

 ing raindrop impressions. 



It has been shown experimentally' that this type 

 of mud-crack results from the dessication of fresh 

 water mud and that flat or slightly downwarped 

 polygons develop from saline mud. Since mud- 

 crack with upwarped margins is produced only in 

 fresh or brackish water muds we must conclude that 

 this mud-crack horizon represents intertidal mud- 

 flats which were covered at high tide by relatively 

 fresh waters comparable perhaps with those of the 

 upper Baltic sea. The reappearance of a marine 

 fauna in the section a few feet above the mud- 

 crack horizon appears to indicate the return of 

 normal marine conditions. The relatively fresh 

 or slightly brackish water conditions under which 

 these mud-cracks were formed point toward their 

 development in lagoons near a shore which contri- 

 buted an abundance of river water to partially land- 

 locked arms of the sea. Such a land must have had 

 a moist climate or at least not an arid one. 



Another mud-crack horizon occurs about 100 

 feet higher in the Ontario Ordovician section al 

 Kingston in the Pamelia limestone. Cushing- has 

 reported this horizon in New York and the writer 

 has described its peculiar features at Kingston. '' 

 Attention is directed to it here because it suggests 

 climatic conditions near the close of Pamelia sedi- 

 mentation just the opposite of those indicated by the 

 Grenville mud-crack. The flat polygons of the 

 Pamelia mud-crack horizon show features which 

 have been interpreted^ as the product of a highly 

 saline condition of the calcareous mud in which they 

 were developed. Sea water would be likely to de- 

 velop the high degree of salinity represented by the 

 Kingston mud-crack only in an arid climate. 



It seems therefore that a relatively arid climate 

 prevailed during late Pamelia time in the lands 

 adjacent to the Ontario sea. This arid climate suc- 

 ceeded a cycle of moist climate in late Chazy time 

 if the inference which has been drawn from the 

 character of the mud-crack is correct. 



Published with the permission of the Director 

 of the Canadian Geological Survey. 



1 Kindle, E. M. Some factors "affecting the de- 

 ve]i)i)m<iit of mud-cracks .Toiirn. Geo!., vol 2,5 

 1!117. pp. 140-H2. " ' 



Separation of salt and saline water and mud 

 Bull. Geol. Soc. Amer., vol. 29, pp. 479-483, 1918. 



-Bull. N.Y. State Mus. Nat. Hist. No. 145, p. 76. 

 1917, pp. 13.5-144. 



3The Ordovician Limestones of the Kingston 

 Area. Rept. of the Ontario Bureau of Mines, vol. 

 25, pt. 3, p. 8, 1916. 



4Kindle, E. M. Some factors affecting the de- 

 velopment of mud-cracks. Journ. Geol., vol. 25. 

 1917, pp. 140-142. 



