This week, supervisors moved to create a new safeguard to further minimize the potential threat of offshore oil and gas exploration off the Marin coast. Developing onshore facilities that support offshore operations is already prohibited by the county, yet under the terms of an ordinance considered on Tuesday that will likely be approved next week, those facilities would now have to be approved by the majority of Marin voters. Similar restrictions exist in the counties to the north and south. Richard Charter, a Sonoma County resident and a senior fellow for the Ocean Foundation, approached supervisors about the need for such an ordinance in Marin, which he said had left open a gap. Mr. Charter fears the Trump administration will press forward with its five-year oil and gas drilling plan that proposes to open California’s coast to drilling; a draft plan was released in 2018 and has yet to be finalized. The waters off Marin are protected from drilling within the boundaries of the Greater Farallones, Cordell Bank and Monterey Bay National Marine Sanctuaries, though the administration has indicated that some of those protections could be revisited, at least in the areas recently added to those sanctuaries. New drilling leases have been banned in California waters within three miles offshore since 1969, though 27 previously established oil platforms continue to operate off the state’s coast, primarily in Southern California. Counties throughout the state banned the onshore development of support structures needed for offshore drilling as early as the 1980s, after then-Secretary of the Interior James Watt attempted to open the entire California coast to offshore drilling. Although Marin banned onshore development in 1980, its position against drilling is now stronger. Under the new ordinance, the majority of voters in Marin would have to consent to the development of an onshore facility in a general election. “This provides near-total local control,” Mr. Charter said of the ordinance. “Marin voters voting to support offshore drilling is not something I can imagine in my lifetime. This is a fail-safe mechanism.” What is the oil and gas potential off the coast? A map developed in the 1980s and based on seismic testing from the oil industry highlighted several areas of interest off Marin, including beyond the seaward side of the marine sanctuaries’ boundaries. New technological improvements, which allow drilling to take place in deeper waters and extend further below the ocean floor, have expanded those possibilities. Morgan Patton, the executive director of the Environmental Action Committee of West Marin, supports the new ordinance. “The waters off Marin County are globally significant and provide a productive marine ecosystem, abundant wildlife, and valuable fisheries,” she wrote. “The priceless biodiversity and thriving marine ecosystems of northern California waters are uniquely vulnerable to the impacts of an oil spill or fracking.”