Don't know what it is, but it looks like it already took over. Perhaps you need to clear the area and start over.

Here is a link that might be useful: {{gwi:1342664}}

Maybe Burclover or Black Medic?

Here is a link that might be useful: burclover

Looks like Black Medic is your weed. You could have taken your picture directly from the Weed Alert site.

Here is a link that might be useful: Black medic picture