After an animal dies its soft tissues normally decay or are eaten, often spreading the bones of the animal over a wide geographical area. Preservation of the tar pit fossils occurs when the bones become saturated, through the bone pores, with asphalt- which inhibits decay. The saturated bones were then buried beneath water-borne sediments, probably when the season changed and winter brought rain which filled the topographic lows with rivers, depositing sand and silt on top of  the asphalt pools and its victims.
    The cycle repeated itself each summer and over thousands of years masses of bone accumulated. If ten animals were trapped every ten years over intervals of 30,000 years it would account for the numbers of animals preserved in the tar pits.
    This unique combination of sedimentation and asphalt impregnation led to the preservation of unchanged, original and virtually intact fossil material.
    The asphalt itself has been found to have a great influence on the actual taphonomic processes of certain chemical compounds; enhancing the preservation potential of amino acids that occur in the collagen of bones and of thr chitin biopolymer that occurs in the cuticle of many insects.

Below is a picture of a sabretooth, held at Bristol University (Dept. of Earth Sciences), showing the dark colouration, typical of asphalt preservation.