SEISMIC EXPRESSION OF CAMP-RELATED IGNEOUS SHEETS AND THEIR ROLE IN CONSTRAINING THE AGE OF SYNRIFT SALT IN THE EARLY MESOZOIC ORPHEUS RIFT BASIN, OFFSHORE SOUTHEASTERN CANADA

A short-lived (< 1 Myr) but widespread magmatic event, associated with the Central Atlantic Magmatic Province (CAMP), occurred in latest Triassic/earliest Jurassic time during the development of the eastern North American (ENAM) rift system. CAMP-related igneous flows and intrusions are present within many of the exposed ENAM rift basins, yet little information is available on their subsurface abundance and geometry. We have used a dense-grid of 2D seismic-reflection data to identify potential CAMP-related intrusions within the eastern Orpheus rift basin, a major ENAM rift basin in offshore southeastern Canada that developed during the Late Triassic to Early Jurassic. Many reflections within the basin have distinctive seismic characteristics typical of igneous sheets. Namely, they have very high amplitudes and terminate abruptly. Although they are mostly parallel to bedding, many locally cut across bedding, climbing to younger stratigraphic levels. Some are present along basement-involved faults, whereas others bifurcate, forming complex arrays. These potential igneous sheets are truncated by the Early Jurassic breakup unconformity, indicating that they are likely related to CAMP, not a younger magmatic event. CAMP-related activity, with its short duration, provides a time marker that can constrain the age of the synrift rocks within the Orpheus rift basin. Several kilometers of synrift salt, called the Argo salt, accumulated within the subsiding basin during rifting. Well data from the western Orpheus rift basin indicate that the salt there is Early Jurassic in age. The age of the salt in the eastern Orpheus rift basin, however, is poorly constrained and potentially ranges from Late Triassic to Early Jurassic in age. In our study area, CAMP-related igneous sheets intruded all of the Argo salt. Therefore, the synrift salt in the eastern Orpheus basin predates CAMP-related activity and, thus, accumulated mostly in the Late Triassic. Thus, salt deposition likely began in the Late Triassic in the eastern Orpheus rift basin and expanded to the west by the Early Jurassic. Our study demonstrates the importance of recognizing CAMP-related igneous sheets in the subsurface to better understand the development of the ENAM rift basins.