20 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 445 



The value of a comparison between the spore assemblage of the Davenport 

 Limestone Member and Devonian spore assemblages described from other regions 

 is diminished by the small number of samples available. This assemblage also 

 represents a restricted environment extending over a small geographic area during 

 a geologically short time. A large number of Middle Devonian spore genera com- 

 monly reported by other workers are apparently lacking in this Illinois coal. These 

 include Leiotriletes, Cyclogranisporites , Verrucosisporites, Ste nozonotri lete s , 

 Densosporites , Geminospora, Aurora spora , Calyptosporites , and Grandispora . 

 Also missing are many of the genera described in the most recent publications 

 that may or may not prove to be of widespread occurrence as more data become 

 available. 



The Davenport assemblage is characterized by a predominance of small 

 species of Apiculatasporites , which is followed in abundance by Retusotriletes 

 and Spinozonotriletes . The assemblage differs considerably, even from the slight- 

 ly younger Cedar Valley coal assemblage (Sanders, 1967) and the slightly younger 

 Solon Member assemblage (Norton, 1967), both from Iowa. The only land plant 

 spores common to the Cedar Valley and Davenport assemblages are Rhabdosporites 

 langi, Punctatisporites atavus (Naumova) Sanders, 1967=(?) Punctatisporites sp., 

 and possibly a species of Ancyrospora. The Iowa flora contains many more acri- 

 tarchs (65 percent) than the Illinois flora, but no Tasmanites were observed in the 

 former. The algal (?) fossils described and illustrated by Sanders (1967, pi. 3, 

 figs. 1 and 2) are similar to those found in the Davenport coal. 



CONCLUSIONS 



According to Collinson et al. (1967, p. 936), during Middle Devonian time 

 normal saline marine water of the Illinois Basin to the south and the very saline 

 water of the basins in Michigan and Iowa were mostly separated by the Sangamon 

 and Kankakee Arches. Evaporites and sandy carbonates were laid down in the 

 Wapsipinicon Limestone to the north. During accumulation of the Davenport Lime- 

 stone Member, toward the end of Wapsipinicon deposition, the sea had transgressed 

 to a major extent onto the Sangamon Arch. Considerable evidence for this exists 

 (James, 1968) and will be presented in another publication by Collinson and James. 

 Near the strand line, along the northeastern margin of the Sangamon Arch, a di- 

 verse flora developed. The hydrophilous plants that contributed to the formation 

 of the coal must have had very thin cuticles, as none could be positively identi- 

 fied. The plants were apparently small and delicate, as the vitrinite lenses rare- 

 ly exceed 500 \i in length and 50 jjl in thickness. Assuming a high compaction ratio 

 of 20 to 1, the thickness of the stems probably did not exceed a few millimeters. 



The botanic remains that accumulated in this local environment were trans- 

 ported a short distance out to sea and were deposited under reducing saprophytic 

 conditions, as indicated by the abundance of fine-grained syngenetic pyrite aggre- 

 gates and the absence of oxygen-rich fusinite and other inertinite macerals. The 

 fine lamination of the coaly bands suggests slow sedimentation in quiet water. 

 Poorly sorted coal bands point to short periods of reworking and rapid resedimenta- 

 tion. The energy was sufficient to eliminate most of the spores, either by physical 

 destruction or by winnowing. 



Marine plant microfossils ( Tasmanites , Leiosphaeridia , and Veryhachium ) 

 probably were incorporated in the coal as the plant material was floated out to sea 



