Aurora Syncline

Location

Southern Kane to northwestern Will County (B-7)

References

Cady 1920, Willman and Payne 1942, McGinnis 1966, Graese 1988, Graese et al. 1988

Description

Cady (1920) described the "Aurora-Pawpaw syncline" or "Pawpaw-Aurora syncline" as trending approximately east-west from Lee County across southern De Kalb and northern Kendall Counties. Definition of this feature is vague on Cady's structure map of the top of the St. Peter Sandstone (Ordovician). The map has a contour interval of 200 feet (61 m) and was based on five or fewer control points per county in the area of concern. As described, the syncline would obliquely cross the Sandwich Fault Zone, the existence of which was unknown to Cady.

Willman and Payne (1942) applied the name Aurora Syncline to a structure that trends northeast from T37N, R6E, Kendall County into Kane County. Their structure maps did not cover Kane County and show only slight downwarping in Kendall County. Treworgy (1981), who listed the Aurora Syncline but did not show it on her map, considered its existence dubious.

Graese (1988) and Graese et al. (1988) resurrected the name Aurora Syncline for a syncline revealed by contouring the elevation of the top of the Galena Group (Champlainian) and Ancell Group (St. Peter Sandstone and Glenwood Formation; Blackriveran). The syncline trends eastward across southern Kane County, then turns southeastward into northwestern Will County. It is 2 to 6 miles (3-10 km) wide and has maximum relief of a little more than 100 feet (30 m) on both contouring surfaces. According to Graese (1988), the Galena Group consists mainly of limestone within the Aurora Syncline and of dolomite elsewhere in northeastern Illinois. Graese deduced that the syncline was downwarped during Galena deposition, lowering the sea floor below the zone of freshwater and saltwater mixing where dolomitization takes place.

The Aurora Syncline is also indicated, but not named, on a map of the Precambrian basement surface by McGinnis (1966). The map, based on geophysical and borehole data, indicates a syncline extending slightly south of east from T40N, R2E, Ogle County to northwestern Will County. McGinnis mapped the syncline within a graben bordered by the Sandwich Fault Zone on the southwest and an unnamed fault zone on the northeast (see Unnamed Fault Zone).