In Julius Caesar, there is the use of supersitition to foreshadow. What are the strange phenomena and omens which are reported by Casca in Act I Sc.3?

In Shakespeare's Julius Caesar Act 1, superstition and omens reflect the political turmoil in Rome.

Caesar himself introduces superstition in the play when he arranges for Antony to touch Calpurnia while he is running the race (scene 2).  The superstition is that the touch may cure her inability to have children.  This establishes that Rome in the time of the play is a superstitious place.

The Soothsayer then warns Caesar to beware of the ides of march (scene 2).  This is used as foreshadowing.

Finally, in scene 3, Casca describes omens of a major sort:

...never till tonight, never till now,

Did I go through a tempest dropping fire,

Either there is a civil strife in heaven,

Or else the world, too saucy with the gods,

Incenses them to send destruction.

He continues:

A common slave--you know him well by sight--

Held up his left hand, which did flame and burn

Like twenty torches joined, and yet his hand,

Not sensible of fire, remained unscorched.

Besides--I ha' not since put up my sword--

Against the Capitol I met a lion,...

Elizabethans believed that the cosmos and human affairs on earth were connected, were related.  The state of one was reflected in the other. 

These unnatural occurrences reflect the political turmoil in Rome.