INSIGHTS INTO CORRELATION OF CYCLIC SILICICLASTIC AND VOLCANIC STRATA FROM TWO CORE TRANSECTS WITHIN THE JURASSIC HARTFORD RIFT BASIN

Two core transects drilled within the city of Hartford in the 1970’s and 2010’s penetrate lower Jurassic basalt and cyclic alluvial/lacustrine sedimentary rocks of the Hartford rift basin, including some strata not well exposed in outcrop (i.e., the lower Portland Formation). However, high frequency cyclicity of similar facies, as well as the abundance of normal faults in the area, create challenges for correlation over even short distances. Investigation of these cores over the last few years has yielded insights through which correlation can be accomplished with confidence.

The Hartford Basin basalt flows, if properly identified, are the most reliable units for correlation. Geochemical fingerprinting and recognition of mineralogic differences among basalt units increase confidence in properly identifying individual flows. For example, the Hampden Basalt (youngest unit), is enriched in Fe and certain rare earth elements, and has abundant plagioclase phenocrysts in comparison to the older Holyoke Basalt.

Sedimentary facies contain reddish-brown alluvial mudrock/sandstone that alternates with black lacustrine shale. Although cyclicity is easy to recognize, details among the cycles vary and distinctive marker beds are few, making it challenging to confidently correlate the cycles between adjacent cores, especially considering the abundance of normal faults. An examination of consecutive cores through the lower Portland Fm reveals certain facies to be most useful for regional correlation. These include: (1) zones with multiple cross-bedded sandstone beds (reflecting periods of increased sediment influx into the central basin), (2) reddish-brown mudstone intervals that contain evidence of pedogenic alteration (such as carbonate nodules), and (3) black shale beds. Though shale beds are lithologically similar, their characteristics are variable which makes it is difficult to correlate them with confidence between adjacent cores. Minor and trace element profiles through these beds permit more accurate correlation. The best correlation strategy includes proper identification of basalt units, the correlation of paleosols and zones with abundant sandstone beds, and the geochemical fingerprinting of shale layers.