APATITE AND ZIRCON FISSION-TRACK EVIDENCE FOR 
CRETACEOUS TILTING OF THE HARTFORD BASIN 
SEDIMENTS AND ADJACENT CRYSTALLINE ROCKS OF THE 
BRONSON HILL ANTICLINORIUM, MASSACHUSETTS AND 
CONNECTICUT.

	RODEN-TICE, Mary K., Center for Earth and Environmental 
		Science,  State University of New York at Plattsburgh, 
		Plattsburgh, New York 12901
	WINTSCH, Robert P., Department of Geological Sciences, 
		Indiana University, Bloomington, IN 47405

Apatite fission-track (FT) ages for 15 samples of Hartford basin 
sedimentary rocks range from 135 ± 14 Ma for the New Haven 
Formation at the basal unconformity at New Haven to 174 ± 11 Ma in 
the Portland Formation near Durham, Connecticut.  Zircon FT ages 
also increase from 167 ± 30 Ma in the New Haven to 238 ± 26 Ma in 
the Portland Formation.  These data suggest a general trend of 
increasing apatite and zircon FT ages from west to east across the 
basin.  The large uncertainty in the age measurements (± 10% of the 
age) throws a certain degree of inconsistency into the age gradient.  
However, the significance of the age gradient is that the basal 
sediments of the New Haven Arkose passed through the closure 
temperature for apatite FT retention (~100°C) approximately 20 Ma 
later than the younger Portland Formation sediments.  Both the apatite 
and zircon FT data suggest that the sediments in the Hartford basin 
were tilted later than the early Cretaceous.
	Even stronger evidence for post-early Cretaceous tilting is 
seen in the discontinuity of apatite FT ages across the border fault 
between the Hartford basin sedimentary rocks and the crystalline 
rocks of the Bronson Hill anticlinorium in both Connecticut and 
Massachusetts. Thirteen samples of Late Proterozoic to Late 
Ordovician gneisses and volcanics yield apatite FT ages ranging from 
98 ± 8 to 137 ± 16 Ma from north to south along the Bronson Hill 
anticlinorium.  These apatite FT ages are distinctly younger than the 
majority of the apatite FT ages for Portland Formation samples in the 
eastern Hartford basin, 153 ± 12 to 174 ± 11 Ma.  If there were no 
Cretaceous movement across the border fault, the apatite FT ages 
should be the same on either side of the fault.  The difference in 
apatite FT ages across the border fault strongly indicates post-early 
Cretaceous displacement.
	In the Massachusetts section of the Bronson Hill 
anticlinorium, there is also a suggestion of both increasing apatite and 
zircon FT ages to the east.  A sample of the Fourmile Gneiss on the 
border fault yields apatite and zircon FT ages of 106 ± 17 Ma and 147 
± 13 Ma, respectively and a sample of Monson Gneiss from Quabbin 
Reservoir yields apatite and zircon FT ages of 144 ± 9 Ma and 196 ± 
19 Ma, respectively.  This FT age gradient supports our post-early 
Cretaceous tilting hypothesis.