DENVER — Mark Crumpacker is stepping down as chief marketing and strategy officer of Chipotle Mexican Grill, Inc., according to a March 13 filing with the Securities and Exchange Commission. His departure follows a transition in leadership at the top, as former Taco Bell head Brian Niccol recently replaced founder M. Steven Ells as chief executive officer.

The Denver-based company has struggled to recover from a foodborne illness outbreak in 2015 that included 55 cases of E. coli O26 across 11 states followed by a norovirus outbreak at a Boston-based restaurant that reportedly sickened 80 individuals.

Turnaround efforts have been met with tepid response. Introductions of chorizo and queso, operational improvements and a new loyalty program, among other initiatives, have not been enough to restore customer traffic at the chain to pre-food safety crisis levels.

Mr. Crumpacker joined Chipotle in 2009. In 2016, he was placed on a three-month leave after he was arrested and charged for cocaine possession. Following his return to the company he apologized during an earnings call "for the recent events in my personal life."

"I am sorry that I caused a distraction for the company, and I wanted you all to know that I have put this behind me, learned from it and returned to my role in early September excited and with a renewed focus and determination to help drive Chipotle's recovery," he said on Oct. 25, 2016. "I want to thank you all for your support and forgiveness, which has meant a great deal to me."