To clarify some things - there were two "Richfields". Richfield of New York was split off from Richfield Oil Company of California back in the 1930s and became a wholly owned subsidiary of Sinclair. In 1938 Richfield (NY) became a jobber only company, operating only through marketers. The brand expanded from the northeast into North and South Carolina in the late 1930s and eventually into Florida in the 1950s. In June 1964 the Richfield brand was discontinued and remaining jobbers were offered the Sinclair brand, which many took on, while others took the opportunity to switch brands entirely. One Pennsylvania jobber had an unusual contract and forced Sinclair to continue to use the Richfield name for several months, but other than that single operation, Richfield of NY ceased to exist in the summer of 1964.
In the meantime, Richfield of California marketed in the west, and was a fully integrated oil company. They merged with Atlantic Refining in January 1966 and experimentation began with the Arco brand within a year or so in VERY LIMITED testing. Officially both Atlantic in the east and Richfield in the west was converted to the ARco brand beginning in 1969, to be completed by late 1970.
In this same era, Atlantic Richfield merged with Sinclair, and in an effort to win justice department approval of the merger, Sinclair marketing from Maine to Florida, which would have included any former Richfield NY jobbers still with Sinclair, was sold to British Petroleum, itself involved in a merger with Sohio. Sinclair stations from Maine to Florida were converted to BP in 1970. In 1972 BP sold marketing in Florida, Georgia, South Carolina and most of North Carolina to American Petrofina, resulting in a three year Sinclair to BP to Fina conversion. Fina marketing was sold off piecemeal in the 1990s and jobbers were lost to attrition. Elsewhere Sinclair marketing, after some areas converted to a transitional Arco-Sinclair image, was sold off to independent owners and remains in operation today.
Former Atlantic marketing in Florida, Georgia, South Carolina and again, most of North Carolina, which had rebranded Arco in 1969-1970, was abandoned in 1972 and sold off piecemeal. Arco operated the balance of the former Atlantic marketing, from Virginia north to New England, through the 1980s, eventually closing company stations and dropping jobbers, except in New York and Pennsylvania, which was spun off to become a new independent Atlantic Refining and Marketing in the 1980s (1985, I think). This was eventually sold to Sunoco and rebranded. Hope this helps.