Major shareholders of Harley Marine Services (Harley) have put their combined 60 percent stake in the U.S. marine transportation firm up for sale in a deal that could be worth at least $100 million, according to a report by the Wall Street Journal.

Australian investment bank Macquarie Group, who is also running the sales process, and California-based private equity firm Laurel Crown Partners aim to complete the sale by July.

Harley's Chairman and founder, Harley Franco, who owns the remaining 40%, said he intends to keep his stake in the firm which has a reported value of $350 million including debt.

Macquarie and Laurel Crown Partners acquired their stake in the business in 2008.

Harley says it owns a fleet of 36 tugs and 48 barges, and its services include bunker and oil transportation at hubs ranging from Houston and Galveston to Lake Charles and New Orleans.