Atlanta-based Cousins Properties, Austin-based Riverside Resources and Texas firm Ironwood Real Estate have formed a joint venture to develop 300 Colorado into a new, $175-million office tower in downtown Austin. The joint venture has leased all of the office space in the 300,000-square-foot building to Parsley Energy for 12 years. The Austin-based oil and gas company currently operates out Colorado Tower across the street. Construction is expected to begin at 300 Colorado in December, with a targeted completion date in December 2020. The building height, number of stories and other architectural features have yet to be determined.