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Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 67 (1997)No.
1. (January), Pages 212-224
Diagenetic Mineralogy, Geochemistry, and Dynamics of Mesozoic Arkoses,
Hartford Rift Basin, Connecticut, U.S.A.
Enrique Merino (1), Jean-Pierre Girard (2*), Michael T.
May (1**), Vishnu Ranganathan (1)
The main event in the diagenetic history of the Hartford basin was the
arrival of the rift-associated "heat wave" to its arkosic fill, in the
form of both high heat flow and diabase dikes, sills, and basalt flows
about 187 ± 3 Ma. Pre-heat-wave (or pre-basalt) diagenetic minerals
are widespread throughout the basin, and include hematite cement, quartz
and albite overgrowths, and minor euhedral rutile. This widespread distribution
suggests that they grew in a regime of generally downward-migrating meteoric
water. The silica, aluminum, and sodium needed to make these cements were
released probably by tropical weathering of the top of the arkose itself,
a few hundred meters overhead. The heat wave of which basalts and dikes
were part suddenly heated the basin, and radically determ ned the subsequent
diagenesis of the arkoses: (1) it drove pore-water convection through sills
and dikes and arkoses, erasing the earlier meteoric-water regime; (2) it
caused quick growth of post-basalt diagenetic minerals (chert and mosaic
albite cement, illite and chlorite cement, and fibrous laumontite cement
with bipyramidal quartz euhedra) mostly at localized occurrences, the "updrafts"
of pore-water convection cells; (3) it modified the texture of both quartz
and albite cements from overgrowth to microcrystalline; and (4) the dikes,
sills, and basalt flows probably provided much of the magnesium, ferrous
iron, potassium, and copper needed to make the illite + chlorite cement
and copper sulfide nodules found in the arkoses. The radiogenic age of
the illite cement from one locality i 180 (± 10) Ma, close to the
age of the basalts and dikes. This temporal proximity warrants linking
the growth of illite to the sudden heating and the igneous intrusion. High
18O content of the diagenetic illite suggests an origin from
relatively heavy water that could have been produced in surficial evaporative
environments of the rift basin, and that later sank. Combined petrographic,
K/Ar, geochemical, fission-track, and dynamic-modeling evidence and interpretations
lead to a generalized time-temperature profile for the basin.
Because of their distinctive tectono-thermal origin, rift basins probably
have a distinctive diagenetic history. The diagenesis and dynamics sketched
here for the Hartford rift basin, including some of the unusual mineralogical
and textural details in its arkoses, may apply to other rift basins.
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