Mark

This gospel was the first one written as a collection of sermon notes from Peter. The main point of Peter's preaching was that Jesus was the Son of God.

Mr 1:1 ¶ The beginning of the gospel of Jesus Christ, the Son of God;

His focus was based on the revelation that the Father had given to him concerning the identity of Jesus before his death:

Mt 16:16 And Simon Peter answered and said, Thou art the Christ, the Son of the living God.

Mt 16:17 And Jesus answered and said unto him, Blessed art thou, Simon Barjona: for flesh and blood hath not revealed [it] unto thee, but my Father which is in heaven.

Peter, familiar with scripture, had correlated the things Jesus did with the prophecies of scripture to arrive at his conclusion. His revelation from the Father, through the guiding of his memory scripture, was a subtle as the process God used to write prophecies of Christ in puns, riddles and double entendre. the same Spirit of prophecy that produces scripture guided him to interpret it.

2Pe 1:20 Knowing this first, that no prophecy of the scripture is of any private interpretation.

Although the 'sermon notes' don't always include reference to the scriptures being interpreted in light of Christ, Peter and his Hebrew audience would have known them well.

Rather than look for an outside source (such as Q) for Mark, perhaps a closer look at the scriptures available to Peter in is order.

Mr 1:1 How Peter inspired John.