ὡς δὲ ὁ φρουρῶν τὸ μαντεῖον Πύθων ὄφις ἐκώλυεν αὐτὸν παρελθεῖν ἐπὶ τὸ χάσμα,

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ὡς δὲ ὁ φρουρῶν τὸ μαντεῖον Πύθων ὄφις ἐκώλυεν αὐτὸν παρελθεῖν ἐπὶ τὸ χάσμα,

ἐπὶ it's usually followed by a genitive or a dative so why is τὸ χάσμα in accusative?

 

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Because epi can also be followed by an accusative:

http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0058:entry=e)pi/