https://phys.org/news/2018-05-mars-harbor-life-billion-years.html

A new study that sheds light on where fossils might be preserved could aid the search for traces of tiny creatures—known as microbes—on Mars, which it is thought may have supported primitive life forms around four billion years ago.

A team of scientists has determined that sedimentary rocks made of compacted mud or clay are the most likely to contain fossils. These rocks are rich in iron and a mineral called silica, which helps preserve fossils.