The book of Mark is generally accepted as having been written before the other three gospels. Though the gospel of Thomas is not and should not be considered scripture, it is an interesting document which shows evidence of having been written later than Mark and familiarity with the hermeneutic tools of John.
There are two prominent declarations in Mark: Peter's proclamation that Jesus is the Messiah, the Son of God , and the centurion confession that Jesus is the Son of God . The first is the confession from the Jewish world, and the second from the secular world.
This gospel was the first one written and was probably a collection of sermon notes from Peter. The main point of Peter's preaching was that Jesus was the Son of God, and the evidences in the Old Testament.
- Mr 1:1 ¶ The beginning of the gospel of Jesus Christ, the Son of God;
Mark records the evidences that the Father revealed to Peter prior to his declaration. The focus of Peter's preaching was 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 of scripture, was subtle. It was the same process God used to write prophecies of Christ in puns, riddles, metaphor and word play. The same Spirit of prophecy that produced the scripture guided him to interpret it. 
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.
The core teaching built upon by gospel writers
Marks gospel was widely circulated. It became the core teaching of the gospel. With time, as the apostles learned to read the 'mystery which was hidden from the beginning' their gospel supplemented Mark with additional insights they had gained as they studied to show themselves approved.
Mark started with John the Baptist's testimony, and Matthew pushed the beginning of the story back to Abraham as he saw the hidden shadows of Christ. Luke pushed the beginning of the story back to Adam, seeing Christ in the scriptures preceding Abraham, and John pushed it all the way back to Ge 1:1 . John had the greatest insights into the mystery, having the most time to study before writing his gospel. John sees the Word hidden in the first word of the verse.
Marks narrative demonstrates the least insight into the mystery, and has many direct references to fulfilled scripture. The differences in the material in the gospels is instructive in the methods they used to interpret the scriptures. And can be studied to learn their methods. Their exegesis can be reproduced when seen in the light of a learning process over time as they shared new insights.
Mark and Peter
There is no substantial reason to question the unbroken tradition that Mark was intimately involved with Peter's ministry, and the contents of his gospel should be considered the teaching of Peter. Others have observed parallels between Peter's preaching in Acts and Mark's record. 
Various authors speculate that events in the Roman empire or event he death of Peter were the triggers causing Mark to write his gospel. It might be suggested that the cause for the writing was the entrance of gentiles into the church. It had been decided that the gentiles did not need to know or participate in many of the distinctive Hebraic practices. However, a basic knowledge of the events which gave evidence of the gospel needed to be recorded for the gentile church.
As such, the gospel should be reverse translated to Hebrew to best understand Peter's teaching, and then how it was transmitted to the gentile church by Mark in Greek. Peter was a Jew. He studied scripture in Hebrew, was taught by Jesus in Hebrew and Aramaic, and preached primarily among Jews in Hebrew. Mark wished to transmit these teachings in Greek to the Greek church.
As Mark records Peter's teaching, a rudimentary understanding of scripture interpretation is displayed. The events noted were important in Peter's mind bring him to the conclusion that Jesus was the Messiah and Son of the Living God. The Hebrew listener would likely identify the allusions to scripture without needing them quoted, nor the parallels enumerated in detail. A sophisticated understanding of the mystery is rarely displayed, suggesting an early date of writing.
- Mr 8:29 And he saith unto them, But whom say ye that I am? And Peter answereth and saith unto him, Thou art the Christ.
- Mr 15:39 And when the centurion, which stood over against him, saw that he so cried out, and gave up the ghost, he said, Truly this man was the Son of God.
- 2Pe 1:20 Knowing this first, that no prophecy of the scripture is of any private interpretation.
- The Q document theory was created by Greeks to try to understand why the gospels differ and why they are the same. They theorize that the gospel writers used a common text as their source. The gospel writers did not write in a vacuum, requiring a source text to copy from. Matthew has the book of Mark. Luke had Mark and Matthew, and John had the other three.
- 1Co 2:7 But we speak the wisdom of God in a mystery, [even] the hidden [wisdom], which God ordained before the world unto our glory:
- Ge 1:1 ¶ In the beginning God created the heaven and the earth.