THORNTON — The city of Thornton has been sued because of new oil and gas rules authorized by the city council in August. The Colorado Oil & Gas Association and the Colorado Oil and Gas Conservation Commission jointly sued the city Tuesday to challenge the rules, according to a report in the Denver Business Journal. The two groups and an assistant attorney general had warned Thornton that the rules interfere with the state’s authority to regulate the oil and gas industry. The suit re-starts the legal battle in Colorado over which level of government has authority to regulate oil and gas. The Colorado Supreme Court a year ago threw out drilling bans that Fort Collins and Longmont had imposed. The Thornton suit challenges new setback standards, flowlines, insurance coverage and other issues, according to the report.