CORRELATING LOWER JURASSIC LAKE CYCLES USING X-RAY FLUORESCENCE SPECTROMETRY, PORTLAND FORMATION, HARTFORD BASIN, CONNECTICUT

Strata from the Lower Jurassic Portland Formation (Hartford Rift Basin, Connecticut) contain cycles of alternating black lacustrine shale and reddish-brown playa siltstone that reflect deposition under fluctuating climatic conditions. Correlation of lake beds is difficult because of the similarity of facies within cycles and the discontinuous nature of exposures. This study examines lake cycles preserved in the Park River Tunnel cores (FD-19T, FD-23T, and FD-30T) from Hartford, CT. The correlation of the lake cycles among cores can be best understood by combining conventional lithostratigraphy and cyclostratigraphy with geochemical trends collected using handheld X-ray Fluorescence (XRF) and Gamma-Ray Spectrometry (GR).

Cores 19T and 23T contain three lake beds that have been correlated to each other based on geometric relations and lithologic similarities in both lake and playa beds, in particular, the distribution of laminated black shale, sandstone beds, and facies containing pedogenic features. Core 30T penetrates two lake beds at its base that are tentatively correlated to the two youngest beds in 19T and 23T (R. Steinen), requiring a fault between 19T/23T and 30T to explain apparent bed offset. The above mentioned techniques were used to test that correlation and structural interpretation. XRF data were collected from the youngest two lake beds in each core. Of the fifteen elements collected, K, Ca, Fe, Rb, and Mo were most useful for correlation. The XRF data suggests that the lake beds in cores 19T and 23T correlate well. However, elemental abundance trends in core 30T do not match those in 19T and 23T, suggesting they are different strata. GR and facies data support this interpretation. These results suggest a fault is not necessary between cores 19T/23T and 30T, and that the lake beds in core 30T are younger than those in 19T and 23T. This conclusion has implications for ongoing geochemical and paleoclimatic studies of the strata, where an understanding of the proper temporal distribution of lakes is critical to interpreting their nature and evolution.